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A study of manganese (III) oxidation of hindered phenols Poh, Bo Long 1972

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11252 A STUDY OF MANGANESE(III) OXIDATION OF HINDERED PHENOLS by BO LONG POH B.Sc, University of British Columbia, 1969 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY In the Department of Chemistry We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA March, 1972 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia , I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and s tudy . I f u r t h e r agree t h a t pe rmiss ion fo r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . I t i s understood that copying o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l ga in s h a l l not be a l lowed without my w r i t t e n p e r m i s s i o n . Department of O ^ W / Q / The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada Date % /97^ i i ABSTRACT The complex of manganese(ill) with trans-1,2-diaminocyclohexane-III tetraacetic acid was prepared and its formula shown to be KMn CyDTA(HgO), where CyDTA is an abbreviation for trans-1,2-diaminocyclohexanetetraacetic acid anion. A mechanistic study of the oxidation of 2,4,6-tri-t-butylphenol with this complex was carried out in methanol under three conditions! (1) in the presence of acid; (2) in the presence of base? (3) in the absence of acid and base. Under acidic conditions, the oxidant is HMn'^CyDTA^O) and the rate-determining step of the reaction involves a hydrogen atom abstraction from the 0-H group of 2,4,6-tri-t-butylphenol by the oxidant. A kinetic isotope effect of 4.8 was obtained at 25° . The activation parameters are AH = 1 5 . 7 Kcal.mole" andAS » -10.4 e.u. The small effect observed for substitution in the para position is consistent with the radical character of the reaction. A 1 value of 0.39 was obtained from a Hammett plot of log k_/k_„ versus (T. J P III 2— Under basic conditions, the oxidant is Mn CyDTA(OH) and there are two competitive reactionst one that involves electron-transfer from TTT ? 2,4,6-trl-t-butylphenoxy anion to Mn CyDTA(OH) " and another that involves hydrogen atom abstraction from 2,4,6-tri-t-butylphenol by III 2— —1 Mn CyDTA(OH) The activation parameters are AH = 10.2 Kcal.mole , * * —\ AS = -23 e.u. for the former reaction; AH = 1 0 . 2 Kcal.mole , i i i AS = -29 e.u. for the latter reaction. A kinetic isotope effect of 1.9 was obtained for the latter reaction at 25°. The rate of electron-transfer TTT from 2,4,6-tri-t-butylphenoxy anion to Mn CyDTA(OH) is 104- times faster than the rate of hydrogen atom abstraction from 2,4,6-tri-t-butyl-TTT 2— phenol by Mn CyDTA(OH) ~. The effect on reaction rate for substitution in the para position is large, a £ value of 2.3 being obtained from a Hammett plot of log k^/k^ versus <J^~ (k^ is the rate constant for the reaction between 2,4,6-tri-t-butylphenol and Mn^CyDTACOH)2-). Under neutral conditions, the oxidant is MnIIICyDTA(H20)~ and the rate-determining step of the reaction involves hydrogen atom abstraction from the C-H group of 2,4,6-tri-t-butylphenol by the oxidant. A kinetic isotope effect of 5-9 was obtained at 25°. The activation parameters are AH = 10.9 Kcal.mole" and AS = -39 e.u. The small effect observed for substitution in the para position is consistent with the radical character of the reaction, a Q value of -0.07 being obtained from a Hammett plot of log kja^kjaH versus cT. The compound 3i5-6^-t-butyl-4-hy&roxybenzaldehyde, however, has an anomalously high rate. This anomaly is also observed under acidic conditions. TTT P— The oxidising power of Mn CyDTA(OH) " is 360 times greater than that of MnIIICyDTA(H20)"" with respect to hydrogen atom abstraction from 2,4,6-tri-t-butylphenol. This difference in oxidising strength suggests that an 'inner-sphere'-type mechanism.is involved in the reactions. The iv difference in oxidising strength between Mri CyDTA(OH) " and III — Mn CyDTA(H20) is even greater in 95 % dimethyl sulfoxide solvent -5.4 x 10 7 . Oxidation of 2,4,6-tri-t-butylphenol with potassium ferrlcyanide in methanol in the presence of hydrochloric acid is shown to be a reversible reaction. The lack of data on the ionization constants of H^Fe(GN)^ and HJ?e(CN)/- prevented a detailed study of the equilibria involved. My help cometh from the Lord. Psalm 121s2 i t is the gift of Gods not of works, lest any man should boast." Ephesians 2s 8,9 V TABLE OF CONTENTS Page ABSTRACT i i LIST OF TABLES xi LIST OF FIGURES xv ACKNOWLEDGMENT xx I. INTRODUCTION 1 (A) Generation of phenoxy radicals 2 (i) Alkaline potassium ferricyanide 2 (ii) Lead dioxide 2 ( i i i ) Silver oxide and mercuric oxide 3 (iv) Manganese dioxide 3 (v) Metals 3 (vi) Other methods 4 (B) Physical properties of phenoxy radicals 4 (i) ESR spectra of phenoxy radicals 6 (ii) Infrared spectra of phenoxy radicals 7 ( i i i ) Visible spectra of phenoxy radicals 8 (C) Chemical properties of phenoxy radicals 8 (i) Dimerisation 8 (ia) Carbon-oxygen dimerisation 8 (ib) Carbon-carbon dimerisation 9 vi Page (ii) Addition of other radicals 10 ( i i i ) Disproportionation 11 (iv) With acids 13 (v) With base 14-(vi) With phenols 15 II. AIM OF THE PRESENT RESEARCH 1? III. EXPERIMENTAL 19 (i) Instruments used 19 (ii) Chemicals and purifications 19 ( i i i ) Method used in kinetic study 2? (iv) Preparation of stock solutions 27 (v) Kinetic runs 29 (vi) Methods used in product analysis 30 IV. SOME RELATED WORK 3^  (i) Determination of extinction coefficient of 313'»5» 5' ~tetra-t-butyl-p-diphenoquinone in methanol 3^  (ii) Determination of extinction coefficient of bis-(1,3,5-tri-t-butyl-2,5-cyclohexadiene-4-one) peroxide in methanol 36 ( i i i ) Determination of extinction coefficient of 2,4,6-tri-t-butylphenoxyl in methanol 38 v i i Page (iv) Determination of the equilibrium constant between 2,4,6-tri-t-butylphenol and its anion in methanol 41 (v) Spectra of the Mn*** complex in methanol 44 (vi) Molecular formula and crystal structure of the Mn*** complex 47 (a) Brief remarks 47 (b) Karl Fischer titration 48 (c) Crystal structure of KMn***CyDTA(H20) 51 V. DECOMPOSITION OF THE Mn*** COMPLEX IN METHANOL IN THE PRESENCE OF BASE 55 (i) Products 55 (ii) Analysis of kinetic results 55 ( i i i ) Determination of rate constant 59 (iv) Activation parameters 60 (v) Discussion 61 VI. REACTION OF THE Mn*** COMPLEX WITH METHANOL (SOLVENT) IN THE PRESENCE OF ACID 64 (i) Products 64 (ii) Analysis of kinetic results 64 ( i i i ) Mechanism 68 (iv) Determination of rate constant ................ 70 v i i i Page (v) Reactions with deuterated methanols 71 (vi) Activation parameters 75 (vii) Discussion 77 TTT VII. OXIDATION OF 2,4,6-TRI-t-BUTYLPHENOL WITH THE Mn COMPLEX IN METHANOL IN THE PRESENCE OF ACID 82 (i) Product 82 (ii) Analysis of kinetic results 82 ( i i i ) Mechanism 90 (iv) Determination of rate constants 93 (v) Kinetic isotope effect 93 (vi) Activation parameters 94 (vii) Oxidation of phenol, "benzaldehyde and benzoic acid 96 (viii) Substituent effect 96 (lx) Discussion 100 I I I VIII. OXIDATION OF 2,4,6-TRI-t-BUTYLFHENOL WITH THE Mn COMPLEX IN METHANOL IN THE PRESENCE OF BASE 120 (i) Product 120 (ii) Analysis of kinetic results 120 ( i i i ) Mechanism 127 (iv) Determination of rate constants 130 (v) Kinetic isotope effect 135 ix Page (vi) Effect of cations on rate constants ......... 136 (vii) Effect of substituent on rate constant ... 138 (viii) Activation parameters 140 (ix) Oxidation of 2,4,6-tri--b-butylphenol with the III Mn complex in methanol in the presence of t-butylamine 142 (x) Discussion 147 TTT IX. OXIDATION OF 2,4,6-TRI-t-BUTYLPHENOL WITH THE Mn COMPLEX IN METHANOL 152 (i) Product 152 (ii) Analysis of kinetic results 152 ( i i i ) Mechanism 154 (iv) Determination of rate constant 157 (v) Kinetic isotope effect 157 (vi) Substituent effect 158 (vii) Activation parameters 161 (viii) Discussion 161 X. OXIDATION OF 2,4.6-TRI-t-BUTYLPHENOL WITH THE Mn 1 1 1 COMPLEX IN DIMETHYL SULFOXIDE (DMSO) IN THE PRESENCE OF BASE 164 (i) Product 164 (ii) Analysis of kinetic results ,. 164 X Page ( i i i ) Discussion 168 XI. OXIDATION OF 2,4,6-TRI-t-BUTYLPHENOL WITH POTASSIUM FERRICYANIDE IN METHANOL 1?1 A. Equilibrium measurements 1?2 a. Methods 172 b. Analysis of results 174 c. Discussion 177 XII. ATTEMPTS TO SYNTHESIZE 3.5,7-TRI-t-BUTYLTROPOLONE .. 179 (a) Reaction of 3»5»7-tribromotropolone with t-butyl lithium 179 (b) Reaction of tropolone with t-butyl alcohol 180 (c) Reaction of tropolone with t-butyl chloride 180 XIII. SUGGESTIONS FOR FURTHER WORK 181 REFERENCES I 8 3 xi LIST OF TABLES Table Page I 2,6-di-t-butylquinone methides (XVl) 12 II Absorbance and yield of 2,4,6-tri-t-butylphenoxyl for various concentrations of 2,4 ,6-tri-t-butyl-phenol used • 4-0 III Equilibrium constant for ionization of 2 , 4 , 6-tri-t-butylphenol in methanol 42 IV Individual bond lengths in KtoIIICyDTA(H20) ..... 53 V Bond angles in KMnIIICyDTA(H20) 54 VI Variation of with base concentration at 25° . . 57 VII Variation of with temperature 61 VIII Variation of k^ with hydrochloric acid concentration at 25° 66 IX IC calculated from equation (41) 71 X Variation of k^ with deuterium chloride concentration at 25° 72 XI Rates for oxidation of isotopic methanols at 25° 74 XII Isotope effects in oxidation of methanol at 25° 75 XIII Variation of k^  with temperature 77 XIV Dissociation constants of deuterioacids in deuterium oxide 80 x i i Table Page XV Variation of k with acid concentration at 25° 86 a XVI Variation of k with concentration of 2 , 4 , 6 -tri-t-butylphenol at 25° 88 XVII Variation of k with the Mn 1 1 1 complex St concentration at 25° 90 XVIII Variation of k^ with temperature 94 XIX Oxidation rates of phenol, benzaldehyde and benzoic acid at 25° • 96 XX Rates, 0 " , and 0~~ values of some 4-substituted-2,6-di--t>-butylphenols (under acidic conditions at 25°) 97 XXI Oxidation products from oxidation of some 4-substituted-2,6-di-t-butylphenols with the Mn 1 1 1 complex under acidic conditions in methanol .... 98 XXII Variation of k with concentration of 2 , 4 , 6 -tri- 1 -d. butylphenol at 25° (includes data from Table XVI) 101 2+ XXIII Comparison of rates of reduction by Cr of CoIII(NH^)^L complexes by 'inner-sphere* mechanism at 25° and ionic strength of 1.0 109 XXIV Variation of k Q with base concentration at 25° 123 XXV Variation of k g with concentration of 2 , 4 , 6 -tri-t-butylphenol at 25° 125 x i i i Table Page III XXVI Variation of k with concentration of the Mn e complex at 2 5 ° 127 XXVII Rates for oxidation of 2,4,6-tri-t-butylphenol with the Mn**"'' complex in methanol and methanol-Od in the presence of base at 25° 136 XXVIII Rates for oxidation of 2,4,6-tri-t-butylphenol III with the Mn complex in the presence of different hydroxides in methanol at 2 5 ° 137 XXIX Rates of oxidation of some 4-substituted-2,6-di-t-butylphenols with the Mn*** complex in methanol in the presence of a limited amount of base at 25° 140 XXX Variation of kg and k^ with temperature 142 XXXI Variation of observed rate with concentration of t-butylamine for the oxidation of 2,4 ,6-tri-t-III butylphenol with the Mn complex in methanol at 2 5 ° 147 XXXII Values for the relative rate of reduction of ML-H^ O1**' complexes compared to MLc0H^n ^ + 0 c J complexes ('inner-sphere' mechanism) 151 XXXIII Variation of k' with concentration of 2 , 4 , 6-tri-3a t-butylphenol at 25° 154 xiv Table Page XXXIV ( T i (T and rates for oxidation of some P P 4-substituted-2,6-di-t-butylphenols with the Mn*** complex in methanol at 25° 158 XXXV Variation of k_ with temperature 161 _ja XXXVI Values of k^b at 25° 168 XXXVII K at constant acid concentration at 25° P (determined by method (i)) 175 XXXVIII K at constant acid concentration at 25° P (determined by method (ii)) 176 XXXIX Variation of K with acid concentration at 25° 177 P XV LIST OF FIGURES Figure Page 1 Structure of potassium trans-1,2-diamino-cyclohexanetetraacetatomanganate(lll) 18 2 N. M. R. spectrum of 3,5-di-t-butyl-4-hydroxy-benzaldehyde in CCl^ 21 3 Set-up for analysis of carbon dioxide 31 4- Beer's law plot for 3 ,3 ' ,5 ,5 ' -tetra-t-butyl-p-diphenoquinone 35 5 Beer's law plot for bis-(l , 3 , 5-trl-t-butyl - 2 , 5 -cyclohexadlene-4-one) peroxide 37 6 Visible spectra of KMnIIICyDTA(H20) in methanol 4-5 7 Model in perspective of the sexadentate six-coordinate MnII3:CyDTA(H20r 52 8 Plot of log A M n versus time for the decomposition III of the Mn complex in methanol in the presence of base 56 9 Plot of k, versus base concentration 58 a 10 Plot of log k^/T versus 1/T for decomposition III of the Mn complex in methanol in the presence of base 62 11 Plot of log A ^ versus time for reaction between the Mn 1 1 1 complex and methanol in the presence of acid 65 xvi Figure Page 12 Observed rate constant, k^, versus acid concentration 67 13 Observed rate constant, k^, versus concentration of deuterium chloride ....................... 73 14 Plot of log kj/T versus l/T for reaction between the Mn*** complex and methanol in the presence of acid 76 1 5 Visible spectrum showing the absorption bands of 2,4,6-tri-t-butylphenoxyl (A) and the Mn*** complex (B) in methanol in the presence of acid 83 16 ESR spectrum of 2,4,6-tri-t-butylphenoxyl in methanol 84 17 Plot of log A M N versus time for oxidation of III 2,4,6-tri-t-butylphenol with the Mn complex in methanol in the presence of acid ......... 8 5 18 Observed rate constant, k , versus acid a concentration 87 19 Observed rate constant, k , versus concentration a of 2,4,6-tri-t-butylphenol. Part (i) ........ 8 9 20 Plot of log k^/T versus l/T for oxidation of III 2,4,6-tri-t-butylphenol with the Mn complex in methanol in the presence of acid ............ 95 xvii Figure Page 21 Observed rate constant, k , versus concentration of 2,4,6-tri-t-butylphenol. Part (ii) 102 22 Plot of log k 0/k o t I versus C T " 105 J JA P 23 Plot of log k^/k^jj versus (T 106 24 ESR spectrum of 2,6-di-t-butyl-4-benzoylphenoxyl in methanol 112 25 ESR spectrum of 2,6-di-t-butyl-4-acetylphenoxyl in methanol 113 26 ESR spectrum of 2,6-di-t-butyl-4-methylphenoxyl in methanol 114 27 ESR spectrum of 2,6-di-t-butyl-4-cyanophenoxyl in methanol 115 28 Visible spectrum showing absorption bands of 2,4,6-tri-t-butylphenoxyl (A) and the Mn 1 1 1 complex (B) in methanol in the presence of base 121 29 Plot of log A ^ versus time for oxidation of 2,4,6-tri-t-butylphenol with the Mn 1 1 1 complex in methanol in the presence of base 122 30 Observed rate constant, k , versus base concentration 124 31 Observed rate constant, k , versus concentration of 2,4,6-tri-t-butylphenol 126 32 Absorbance versus (1 - exp(-k t)) for 2 ,4,6 -tri-t-butylphenoxyl 132 x v i i i Figure Page 33 Rate constant, k^, versus concentration of base 133 34 Rate constant, kr, versus concentration of 2,4,6-tri-t-butylphenol 134 35 Plot of log kc/kc,H versus <T~ 141 36 Plot of log kg/T versus 1/T for oxidation of 2,4,6-tri-t-butylphenoxy anion with the Mn*11 complex under basic condition 143 37 Plot of log k^ /T versus 1/T for oxidation of III 2,4,6-tri-t-butylphenol with the Mn complex under basic condition 144 38 Plot of observed rate constant, k^, versus concentration of t-butylamine 146 39 Plot of log versus time for oxidation of III 2,4,6-tri-t-butylphenol with the Mn complex in methanol 153 40 Observed rate constant, k^a, versus concentration of 2,4,6-tri-t-butylphenol 155 41 Plot of log k ^ / k ^ versus 0^ 159 42 Plot of log k- /k0 „ versus cr" 160 3a 3aH p 43 Plot of log k^a/T versus 1/T for oxidation of 2,4,6-tri-t-butylphenol with the Mn*** complex in methanol •••• 162 xix Figure Page 44 Plot of log (p - x)/(b - x) versus time for oxidation of 2,4,6-tri-t-butylphenol with the III Mn complex in DMSO In the presence of base 16? XX ACKNOWLEDGMENT I wish to take this opportunity to express my sincere thanks to Professor R. Stewart for his excellent guidance throughout this research. His innumerable valuable comments and advice are most gratefully acknowledged. The various discussions with Dr. B. R. James have proved very helpful. His suggestions are very much appreciated. I am also indebted to« Mr. S. Rettig and Dr. J. Trotter for their information on the crystal structure of the manganese(ill) complex; Mr. P. Borda for doing the microanalysis of compounds; and Miss P. Watson for recording the nmr spectra of compounds. This study was completed under a Colombo Plan scholarship granted by the Canadian International Development Agency. I am therefore thankful to them for the part this financial support played throughout this study. My personal thanks should go to my wife Sien for her patience and understanding. Many of her thoughtful suggestions have directly or Indirectly contributed to the completion of this work. It is to her that this thesis is affectionately dedicated. 1 I. INTRODUCTION The chemistry of hindered phenols (phenols that have bulky ortho 1 2 substituents) has been studied rather extensively after Cook ' and It o Muller and co-workers made their independent discovery of the stable 2,4,6-tri-t-butylphenoxy radical (i) in 1953» There are three main reasons for this interest. Firstly, many hindered phenols form relatively stable phenoxy radicals which provide valuable information on the structure of the unpaired electron, its behaviour and properties. Secondly, they are convenient models for investigating reactivity and its relation to structure . Thirdly, both the phenols and their phenoxy radicals have been found to be important in the inhibition of autoxidation of organic substances. An excellent review article^ on the role of phenols and phenoxy radicals in autoxidation inhibition has been written by Ingold. Two good summaries of the chemistry of stable phenoxy radicals are given 6 7 by Attwicker and Mihailovic . 2 (A) Generation of phenoxy radicals The most commonly used reagents to generate phenoxy radicals from phenols in high yields are inorganic oxidising agents which can undergo one-electron reduction. Some of these oxidising reagents are potassium ferricyanide, lead dioxide, silver oxide, mercuric oxide and manganese dioxide. These reactions are usually carried out in a heterogeneous system (e.g. solution of the phenol in "benzene, and the oxidising agent either in suspension or in an aqueous phase). (i) Alkaline potassium ferricyanide Potassium ferricyanide in base was f i r s t used to generate 2,4,6-tri-1-3 t-butylphenoxyl from its phenol . Although this reagent generally gives high yields of phenoxy radicals a large excess of reagent is needed to achieve a high yield. Mechanistic details of the oxidation process involved are s t i l l not well understood. The reactions of this reagent with 8 phenols have been reviewed by Thyagarajan . (ii) Lead dioxide Lead dioxide oxidises phenols to phenoxy radicals in quantitative yields only when this reagent is high in active oxygen and when the solutions are dilute in phenol (less than 0.04 molar)^. It is commonly used to generate phenoxy radicals in situ because the procedure involved is simple and oxidation occurs immediately (e.g. i t has been found in this 3 work that 2,4-,6-tri-t-butylphenoxyl can he generated immediately by syringing benzene which has been flushed with nitrogen into a mixture of the solid phenol and solid lead dioxide in an oxygen-free container which is stoppered with a rubber cap). ( i i i ) Silver oxide and mercuric oxide Silver oxide and mercuric oxide are inferior reagents to alkaline potassium ferricyanide and lead dioxide for the purpose of generating phenoxy radicals from phenols. Freshly prepared samples of silver oxide 10 must be used to obtain high yields of phenoxy radicals . (iv) Manganese dioxide Highly active manganese dioxide has been used to produce apparently 11 high yields of phenoxy radicals . It is not as commonly used as potassium ferricyanide and lead dioxide. (v) Metals Another method which gives quantitative yields of phenoxy radicals involves the reactions of 4-halo-2,5-cyclohexadienones (ll) with metals as shown in equation (1). ( i ) I I R - CI, Br M « Hg, Ag, Cu, Zn, Na-K This method is convenient because the metal halide by-product can be 12 removed by filtration . (vi) Other methods There are many other methods of generating phenoxy radicals, e.g. 13 14 flash photolysis ", electrochemical methods , thermal dissociation of phenoxy dimers etc. These methods cannot be adapted to preparative scale. However, a method like flash photolysis when used in conjunction with electron spin resonance (esr) spectroscopy enables very short-lived 13 phenoxy radicals like 2,6-di-t-butylphenoxyl ( I I I a) to be detected . (B) Physical properties of phenoxy radicals Phenoxy radicals ( i l l ) can be isolated in solid form only when R, R' and R" are bulky groups (e.g. I is a dark blue crystalline solid with a melting point of 9 6 - 9 8 ° ) . They are highly coloured both as solids and in solutions. Those phenoxy radicals which exist as dimers in the solid state are generally colourless. Some examples of phenoxy radicals which can be isolated as solid monomers are I and III b while III c, III d and III e are some of the phenoxy radicals which exist as solid dimers^. III R* = R" » -C(GH3)3 III a R = H III b R = -CH III c R - -S-CCCH^ III d R = III e R = -S-C-//~^  Phenoxy radicals, because of their unpaired electron, are paramagnetic. The Gouy method of measuring magnetic susceptibility can be used to detect them i f they are stable. This method was f i r s t used to prove that I was essentially monomeric^^. It is s t i l l quite useful for the quantitative determination of radical content. But i t suffers from three disadvantages namely (i) the sample of radical used must be stable; (ii) the sample of radical must be pure and ( i i i ) i t is difficult to account for the diamagnetlc part of susceptibility in the calculation of the magnetic moment of radicals*^. Even very short-lived phenoxy radicals can now be detected by the powerful technique known as electron spin resonance (esr) spectrometry. This technique which requires very low concentrations of radicals (as low as 10~^ molar), has been used to elucidate detailed structures of 6 phenoxy and other radicals. (i) ESR spectra of phenoxy radicals The hyperfine structure of esr spectra of phenoxy radicals provides information on the interaction of the magnetic moment of the unpaired electron with the magnetic moments of the nuclei covered by the molecular orbital of the unpaired electron. From the magnitude of the hyperfine splitting constants the spin density of the unpaired electron in the various parts of the molecule can be calculated. For example, the presence of a triplet of 1 « 2 i 1 in intensity ratio in the esr spectrum of I indicates resonance structures I and I a are both important. I l a l b I c I d The triplet is due to the Interaction of the unpaired electron with the two equivalent meta-hydrogen atoms. This explanation is supported by the observations of a doublet esr spectrum of 1 J 1 intensity ratio when one of the meta-hydrogen atoms is replaced by deuterium, and a singlet esr 17 spectrum when both the meta-hydrogen atoms are replaced by deuterium . The distribution of spin density of the unpaired electron on the various 18—20 atoms of I has been calculated and found to be as followsj 0 J6 ^ C± 4.5 C 2 - C 6 12.5 P, C 3 - C 5 5 $>% 0^ 25 These results indicate that the unpaired electron density is greater at the para- than at the meta- position. Attempts to correlate the magnitude of splitting constants with 4 different substituents in phenoxy radicals have been made , but the relation is not yet clear. Little attention has been paid to the determination of an accurate value of the gyromagnetic ratio (or g-factor) of the unpaired electron in phenoxy radicals because, apparently, l i t t l e structural information can be obtained from this parameter. Further information on the use of esr technique in the investigation ieoret:J 21,22 of phenoxy radicals is found in review art i c l e s ^ ' 2 0 . The the etical aspect of this technique is well described in several books (ii) Infrared s-pectra of phenoxy radicals Perhaps the most characteristic feature of the infrared spectra of -1 9 phenoxy radicals is the absence of the 0-H band in the 3700 cm region . The presence of the 1600 cm"''' band indicates the presence of a carbonyl function which is contributed by resonance structures I a and I b. 8 ( i i i ) Visible spectra of phenoxy radicals Unlike most phenols, phenoxy radicals show strong absorption bands In the visible region. The band in the 400 nm region is usually much stronger than that in the 600 nm region . The brilliant colour of most phenoxy radicals is attributed to the contribution of the conjugated keto forms and charge separation forms (see resonance structures I a to I d). (C) Chemical properties of phenoxy radicals (i) Dimerisation Dimerisation reactions of phenoxy radicals can be divided into two main groups - (ia) carbon-oxygen dimerisation and (ib) carbon-carbon dimerisation. (ia) Carbon-oxygen dimerisation Certain phenoxy radicals undergo carbon-oxygen dimerisation to form quinol ethers. For example, alkyl or aryl 2,6-di-t-butyl-p-hydroxybenzoates 23 24 (IV) when oxidised give the quinol ethers (v) as shown in equation (2) J* 9 Carbon-oxygen dimerisations, e.g. to form V, are reversible. Some 2.6-di-substituted phenols when oxidised undergo carbon-oxygen 25 26 dimerisations to form polyphenyl ethers . (ib) Carbon-carbon dimerisation 10 27-29 Many phenoxy radicals ' undergo carbon-carbon dimerisations in the following manner: IX Oxidation^ of 2,6-di-t-butylphenol with base and oxygen gives IX (R = t-Bu) whereas oxidation of the same compound with silver oxide gives both VIII and IX (R - t-Bu, R' - H). The formation of galvlnoxyl (Xi) from 2,6-di-t-butyl-4-formylphenoxyl 1 0 (x) has been reported^ although the mechanism of the reaction has not been investigated. Apparently the radical X has undergone carbon-carbon dimerisation with loss of a carbon unit. 0 Because of sterlc hindrance highly hindered phenoxy radicals such as 2,4,6-tri-t-butylphenoxyl (i) do not undergo carbon-carbon dimerisation. (ii) Addition of other radicals Even phenoxy radicals which are stable with respect to dimerisation add small radicals to themselves quite readily. For example, I forms a o 1 0 peroxide with oxygen (XII) and i t adds nitrogen dioxide to form X l l i ' . 0 I XII 11 XIII In the case of 2,6-di-t-butylphenoxyl carbon-carbon dimerisation must be very much faster than peroxide formation with oxygen because XIV has not been detected. XIV ( i i i ) Disproportionation Hindered phenoxy radicals which possess an d-hydrogen on the 4-substituent (XV) disproportionate to starting phenols and quinone methldes (XVI) as shown in equation ( 7 ) . 12 0 OH 0 (7) Some examples of 2,6-di-t-butylquinone methides are listed in Table I. TABLE I 2,6-di-t-butylquinone methides (XVI) XVI References R R' H H 32 H Me 33 H Ph 34 Me Ph 34 H OMe 34 13 The rate of disproportionation of some examples of XV has been s t u d i e d - ^ ' T h e kinetic results obtained cannot always be explained in terms of equation (7), e.g. the rate of disproportionation of XV when R » H, R' = Me was found to be first-order with respect to radical concentration^ and to depend on the solvent used. However, when R - R*= Me, or R » Me, R' «= Et, disproportionation was reported to give good second-order kinetics^, consistent with equation (7). Moreover, the rates of these second-order reactions were found to be insensitive to the 37 solvent used. Recently , the disproportionation of XV, R = R* = H, was reported to be second-order with respect to the radical concentration, 2 - 1 - 1 with a specific rate constant k d « 3.33 x 10 M sec in tetrahydrofuran 9 1 1 r\ and k„ «= 4.59 x 10 M" sec" in benzene at 24 . Further investigation of CL the mechanism of disproportionation of phenoxy radicals is needed, (iv) With acids Phenoxy radicals react with acids as illustrated in equations (8) and (9). Mechanistic details of such reactions are lacking. 14 (9) (v) With base Very l i t t l e is known about the reaction of phenoxy radicals with base. Since they can be obtained quantitatively by oxidation of phenols with alkaline potassium ferricyanide, they must be generally unreactive 2 9 towards base. (For example, I was obtained in nearly 100 $ yield ' b y the oxidation of the phenol with alkaline potassium ferricyanide in water-benzene mixture for two hours). It is quite surprising that galvinoxyl (XI), which is stable to air, reacts with base-^ in methanol quickly to form XVII. (vi) With phenols 15 Phenoxy radicals react with phenols to give a variety of products depending on the structural features of the radicals and the phenols and on the conditions used. Some reactions give mixed quinol ethers^ as shown in equation (11). Some produce other phenoxy radicals as shown in equation (12). The equilibrium constants-^ for equation (12) have been determined for R =» t-Bu and R =* Me. They are 51 • 7 and 210 respectively at The rate constant for the intermolecular hydrogen transfer between I and its phenol has been determined by nuclear magnetic resonance line-40 - 1 - 1 broadening technique . It was found to be 300 M sec ; the activation + - 1 energy is 1 0.5 Kcal.mole . + 0 (ID 17 II. AIM OF THE PRESENT RESEARCH Although oxidation of hindered phenols using oxidising agents which are capable of undergoing one-electron reduction has often been carried out no mechanistic studies have hitherto been made. This deficiency is mainly due to the fact that a l l such reactions have been done in heterogeneous media, thus making kinetic investigation using simple technique difficult. The main aim of this research was to study the mechanistic details of oxidation of 2,4,6-tri-t-butylphenol with potassium trans-1,2-diamino-cyclohexanetetraacetatomanganate(lll). The latter compound will be referred to as Mn*** complex throughout this thesis; its structure is shown in 41 Figure 1. This oxidant which has only recently been prepared was chosen for four reasons: (1) It Is a stable solid and can be stored in the dark for months without showing any sign of decomposition; (2) i t is a one-electron oxidant; (3) no Mn*** compound has hitherto been used to oxidise hindered phenols (presumably because most Mn*** compounds are unstable, especially in basic condition); (4) i t is soluble in non-aqueous solvents like methanol and dimethyl sulfoxide. The compound 2,4,6-tri-t-butylphenol was used as reductant because i t is known to give a very stable phenoxy radical, I. Oxidation of some other 4-substituted-2,6-di-t-butylphenols with the same oxidant was carried out to gain some information about the structure of the transition states of these processes. 18 0 Figure 1. Structure of potassium trans-l,2-di-aminocyclohexanetetra-acetatomanganate(ill). 19 Oxidation of 2,4,6-tri-t-butylphenol was also carried out with potassium ferricyanide so that the relative oxidising power of the Mn 1 1 1 complex and potassium ferricyanide could be compared, III. EXPERIMENTAL (i) Instruments used All rates were measured on a Bausch and Lomb Spectronic 505 spectrophotometer fitted with a constant temperature block. A Cary 16 spectrophotometer equipped with a constant temperature block was used to measure extinction coefficients. An E-3 EPR spectrometer was used to record electron spin resonance spectra. Melting points of compounds were measured with a 'Buchi' melting-point apparatus. A Beckman aquameter III model KF-2 was used to determine water content of the Mn complex by the Karl Fischer method. A l l syringes used were equipped with Chaney adapters calibrated at desired volumes. A l l straight line graphs were calculated by the method of least squares and plotted using a Hewlett-Packard plotter and calculator model 9125A. (ii) Chemicals and purifications 3t3' .5.5'-tetra-t-butyl-p-diphenoqulnone (XVTIl) was prepared by oxidation of XXXIV with alkaline potassium ferricyanide in a water-benzene mixture. The crude product was recrystallised from hot methanol. M.p. 246-247° (Lit. 246°) 2 9. * The identity of each phenol was checked by its nmr spectrum. 20 Bis-(1 ,3 . 5-tri-t-butyl-2,5-cyclohexadiene-4-one) peroxide (XII) was prepared by oxidation of XXIII with potassium ferricyanide in a water-2 benzene mixture in the presence of oxygen . The crude product was recrystallised from hot methanol. M.p. 148-149° (Lit. 148-149 0) 2. 315-dl-t-butyl-4-hydroxybenzaldehyde (XXI) was synthesised by oxidation 42 of XX with bromine in aqueous acetic acid . The crude product obtained was recrystallised twice from hot acetone. M.p. 189-190° (Lit. 189-190°)^ Since this compound is shown to have an anomalously high rate (see Sections VII and IX), its elemental analysis data and nmr spectrum (see Figure 2) are given to show its authenticity. Analysis - Foundt C 76.86 <f>; H 9.48 Cal'di C 76.82 %; H 9.39 2,6-di-t-butyl-4-bromophenol (XXIl) was synthesised by bromination of XXXIV in glacial acetic acid. Glacial acetic acid (30 ml.) was added to 5.1 g (25 mmoles) of XXXIV in a 50 ml. round-bottomed flask. The contents were stirred with a magnetic stirer and 1.4 ml. (25 mmoles) of bromine was added dropwise to the mixture, which was stirred for half an hour. The contents of the vessel were then poured into a beaker containing 70 ml, of water, whereupon a pale yellow solid precipitated immediately. The crude product was recrystallised from hot methanol. Yield 90 m.p. 83-83.5° (Lit. 8 3 - 8 3 . 5 ° ) 4 5 . I I o 22 2,6-di-t-butyl-4-chlorophenol (XXIV) was prepared^ by chlorination of XXXIV in carbon tetrachloride at 0°. The crude product was recrystallised from hot methanol and then vacuum sublimed. M.p. 74-76° (Lit. 75-76°; 72-709'4 5. 2,6-dl--b-butyl-4-iodophenol (XXV) was prepared^ by reacting iodine bromide with XXXIV in dioxane. The crude product was recrystallised from hot methanol and then vacuum sublimed. M.p. 77-79° (Lit. 76.5-78.5°)^, 2,6-dl-t-butyl-4-acetylphenol (XXVI) was prepared^ by reacting acetyl chloride with XXXIV in the presence of aluminium chloride at 0°. The crude product was recrystallised twice from hot ethanol. M.p. 150-151° (Lit. 147-148°; 150-151 0) 4 6' 4 7. 2.6-di-t-butyl-4-benzoylphenol (XXXII) was prepared^ by reacting benzoyl chloride with XXXIV in the presence of aluminium chloride at 0°. The crude product was recrystallised thrice from hot methanol. M.p. 124-124.5° (Lit. 124 -125 0 ) 4 6 2,6-dl-t-butyl-4-hydroxyphenol (XXIX) was prepared by reduction of XIX with lithium aluminium hydride. The crude product was recrystallised from hot methanol and then vacuum sublimed. M.p. 103-104° (Lit. 110-111°)-*. 2,6-dl-t-butyl-4-cyanophenol (XXXIII) was prepared according to the 42 method described by Cohen . The crude product was recrystallised from hot methanol. M.p. 145-146° (Lit. 147°) 4 2. 23 The following phenols were purchased from commercial sources. 2,4,6-trl-t-butylphenol (XXIII) was purchased from Aldrich Chemical Company. It was recrystallised from hot ethanol. M.p. 131-132° (Lit. I30-131°; 1 3 1 - 1 3 2 0 ) 4 8 , 1 0 . 2,o-di-t-hutylphenol (XXXIV) was purchased from Aldrich Chemical Company. It was purified "by recrystallisation from hot methanol, followed by vacuum sublimation. M.p. 37 .5° (Lit. 36-37°) 1 0 . 2,6-dl-t~butyl-4-methylphenol (XX) was obtained from Eastman Organic Chemicals. It was recrystallised from hot ethanol. M.p. 70-70 .5°. 3,5-dl-t-butyl-4-hydroxybenzoic acid (XXX) was purchased from Aldrich Chemical Company. " (Lit. 217-2180)49. It was recrystallised from hot methanol. M.p. 221-222° 2,6-dl-t-butyl-4-hydroxymethyrphenol (XXVIII) was obtained from K & K Laboratories. It was recrystallised from hot methanol. M.p. 142-143°. 2,6-di-t-butyl-4-dlmethylaminomethylphenol (XXXI) was obtained from K & K Laboratories. It was recrystallised from hot methanol. M.p. 92-94°. 3.5-di-t-butyl-4-hydroxybenzyl ether (XXVII) was obtained from K & K Laboratories. It was recrystallised from hot methanol. M.p. 137-137.5°. 1 HO-<^V> —HD02HD - a IIAXX H = H SiDOD - a IAXX MO = a IIIXXX I = a AXX H^9OOD » H IIXXX TD « a AIXX (CHD)N2HD - a IXXX na-q- » a l i r a HOOD - a XXX = a IIXX HO = a XIXX OHO = a IXX HOZHD - a IIIAXX £HD - a XX XIX +72 25 Tetramethylammonium hydroxide was obtained as 10 % and 25 # solutions in water from Eastman Organic Chemicals. It was used without further purification. Tetrapropylammonium hydroxide was obtained as 10 # solution in water from Eastman Organic Chemicals. It was used without further purification. Tetrabutylammonium hydroxide was obtained as 10 % solution in methanol from Eastman Organic Chemicals. It was used without further purification. Potassium hydroxide and sodium hydroxide were obtained as standard solutions from British Drug Houses. They were used without further purification. Lithium hydroxide solution was prepared by dissolving a weighed sample of the compound (reagent grade) in a known volume of distilled water. Its exact concentration was found by titration against standard acid. Acids (sulphuric, perchloric and hydrochloric) used were of reagent grade. They were standardised by titration against standard base. The following deuterated compounds were obtained from Merck, Sharp and Dohme of Canada Ltd. They were used without further purification. Deuterium oxide (99.7 atom $ purity). Methanol-O-d (99 atom # purity). Methanol-d., (99.5 atom $ purity). 26 Deuterium chloride (38 # w/w in Deuterium oxide). Sodium deuteroxide (40 # w/w in Deuterium oxide). t-Butylamine was obtained from Eastman Organic Chemicals. It was used without further purification. Methanol used was 'Baker Instra-Analyzed* grade obtained from J. T. Baker Chemical Company. It was used without further purification. Chromotropic acid, di-sodium salt (4,5-di-hydroxynapthalene-2,7-di-sulfonlc acid, di-sodium salt) was purchased from British Drug Houses and was used without further purification. Nitrogen gas was high purity nitrogen (L grade, from Liquid Air Company). Water was boiled distilled water which was cooled by flushing high purity nitrogen into i t . Potassium ferricyanide was obtained from Baker & Adamson Chemical Company. It was recrystallised from a saturated solution of i t in water. Potassium trans-1,2-diamlnocyclohexanetetraacetatomanganate(III) was 41 prepared according to the method used by Hamm and Suwyn . It was purified by recrystallisation from methanol-water. M.p. 201-203°. Elemental analysis indicates that this compound has one molecule of water per molecule of compound instead of 2.5 molecules as reported by Hamm and 27 Suwyn . (See the crystal structure of this compound in Figure 7 of Section IV-vi(c)). Analysis - Foundj C 36 .98 #j N 6.0 £1 H 4.46 Cal'dt C 37.00 #5 N 6 . I 7 ii H 4.40 % * Cal'ds G 34.92 it N 5.82 #5 H 4 .78 * Calculated from KMnIIIC1^H18N208(H20), M. V. - 454.0 ** Calculated from KMnIIIC1^H18N20g(2.5H20), M. W. - 481.0 ( i i i ) Method used in kinetic study Spectrophotometry offers an attractive method for studying the mechanistic details of oxidation of 2,4,6-tri-t-butylphenol with KMnIIICyDTA(H20) (CyDTA is used as an abbreviation for the ligand, trans-l,2-diaminocyclohexanetetraacetic acid anion) because the former does not absorb in the visible region whereas the latter has a broad peak in the 500 nm region. Therefore, the disappearance of the oxidant can be followed by monitoring the decrease in absorbance near 500 nm. Moreover, the production of 2,4,6-tri-t-butylphenoxyl can be followed because i t has a strong, sharp, peak at 400 nm which is distinct from the broad peak of the oxidant. (iv) Preparation of stock solutions A solution of 2,4,6-tri-t-butylphenol was prepared as followsi 28 a sample of this compound (0.1700 g) was dissolved in 25 ml. of methanol in a 25 ml. volumetric flask to give a concentration of 2.59 x 10 molar phenol solution. This phenol solution was transferred to a 25 ml. Brlenmeyer flask which was then sealed with a rubber cap. The solution was flushed with nitrogen for about twenty five minutes and then thermostated in a constant temperature water-bath at the desired temperature for half an hour before i t was used for a kinetic run. —2 III A 6.92 x 10 molar KMn CyDTA(HgO) stock solution was prepared by dissolving 15.7 mg of the compound in 0.5 ml. of distilled water in a small vial which was sealed with a rubber cap. The solution was flushed with nitrogen for about ten minutes. The vial was then wrapped with aluminium f o i l (to prevent slow decomposition of the compound by light) and thermostated at the desired temperature. Tetramethylammonium hydroxide solutions (10 $ and 25 #) were available in 100 ml. bottles. Several 3 ml. aliquots of the base were transferred into vials under a nitrogen atmosphere in a glove bag. These vials of tetramethylammonium hydroxide solutions were capped with rubber stoppers, and stored in a desiccator. When needed for an experiment, one vial of the base was thermostated at the desired temperature and transferred to the reaction mixture by means of a syringe. By this means, stock solutions of the base were not exposed to the atmosphere. 29 (v) Kinetic runs A 1 cm. cell containing 3 ml. of methanol was placed in the reference cell holder of a Bausch and Lomb Spectronic 505 spectrophotometer. Another matching i cm. cell capped with a silicone disc(a silicon disc can be pierced by a syringe several times without any leakage occurring) was placed in the sample cell holder. The cell holders were thermostated at the desired temperature. Two syringe needles were pierced into the silicone disc, one for the nitrogen inlet and the other for the outlet. Air in the sample cell was flushed out by nitrogen for about a minute. Nitrogen was kept running through the sample cell while 3 ml. of 2,4,6-tri-t-butylphenol stock solution was transferred into i t by means of a 5 ml* syringe equipped with a Ghaney adapter calibrated at 3 ml. volume. Nitrogen was bubbled through the sample cell for another minute. A 50 Jil. aliquot of 10 $ tetramethylammonium hydroxide solution was syringed into the sample cell by means of a 50 Jul. syringe equipped with a Chaney adapter calibrated at 50 Jul. volume. A baseline from 350 nm to 650 nm was recorded on the chart paper of the spectrophotometer. The reaction was III initiated by introducing 25 Jul, of the stock Mn complex solution into the sample cell by means of another 50 jul. syringe equipped with a Chaney adapter calibrated at 25 jul. volume. The sample cell was shaken briefly to mix the reactants well. Spectra from 350 nm to 650 nm were recorded at various time intervals. 30 A similar procedure was followed for reactions carried out under acidic conditions except that hydrochloric acid was used instead of base. Each kinetic run was done in duplicate. (vi) Methods used in -product analysis Products which give absorption spectra were identified by spectrophotometry, their absorption spectra being compared with those of authentic samples. Free radicals were analysed by esr spectrometry. A typical esr spectrum was recorded as follows* at the completion of the reaction, 0.05 ml. of the reaction mixture was syringed into an oxygen-free esr aqueous solution sample cell model E-248-1 (from Varian Associates) and an esr spectrum was quickly recorded. Carbon dioxide was analysed qualitatively by the phenolphthalein-sodium carbonate indicator method^. Two test-tubes (10 cm. in height and 1.5 cm. in diameter), one as a sample tube and the other as a 'control' tube were used. A drop of phenolphthalein-sodium carbonate indicator was placed at the flattened end of a 4 cm. glass rod whose other end was attached to a rubber cap which fitted the mouth of the test-tube. This glass rod was inserted into the 'control' test-tube (see Figure 3). Methanol (3 ml.) was syringed into the sample tube and 0.050 ml. of hydrochloric acid was then added. The reaction was initiated by introducing 31 -2 III 0.025 ml. of 3 x 10 M of the Mn complex into the sample tube. A similar glass rod with a drop of phenolphthalein-sodium carbonate indicator at the flattened end of i t was inserted into the sample tube carefully so that the drop of indicator would not f a l l into the reaction mixture. Carbon dioxide evolved would turn the pink indicator colourless. The 'control' tube was used to ensure that the colour change of the indicator was not due to carbon dioxide from the atmosphere. This method is not applicable to reactions done in the presence of base because any carbon dioxide evolved would react quickly with the base. 7t indicator test-tube reaction mixture sample tube 'control' Figure 3 Formaldehyde was analysed by a modified ' chronotropic acid' method^ . At the completion of a reaction, the 3 ml. of reaction 32 mixture in a 1 cm. cell was transferred into a 10 ml. volumetric falsk. A 1 ml. aliquot of 2 $ chromotroplc acid was added, followed by 6 ml. of concentrated sulphuric acid. A pink colour develops when formaldehyde is present. (No explanation has yet been given for the appearance of this pink colour). A 3 nil. aliquot of this solution was transferred into a 1 cm. cell and its absorbance at 580 nm taken. A known concentration of formaldehyde in 3 ul. of methanol was placed in another 10 ml. volumetric flask. To this solution was added 1 ml. of 2 $ chromotroplc acid, followed by 6 ml. of concentrated sulphuric acid. A pink colour developed immediately and its absorbance at 580 nm was recorded. Thus, the unknown concentration of formaldehyde in the reaction mixture could be calculated. Three 'controls* were used to check whether (i) the Mn 1 1 1 complex, ( i l ) tetramethylammonium hydroxide, and ( i i i ) 2,4,6-trl-t-butylphenol, interfere with this method of analysing for formaldehyde. The f i r s t III 'control' was done as followss a 3 ml, sample of the Mn complex in methanol (about 6 x 10*^ M) was placed in a 10 ml. volumetric flask. A 1 ml, aliquot of 2 $ chromotroplc acid followed by 6 ml. of concentrated sulphuric acid were added to i t . No pink colour was seen. In the second 'control', 0.05 ml. of 10 $ tetramethylammonium hydroxide was added to a 3 ml. standard solution of formaldehyde in methanol. This solution was left to stand for 25 minutes and then 1 ml. of 2 # chromotroplc acid followed by 6 ml. of concentrated sulphuric acid were added. A pink colour developed. In the third 'control', 3 ml* of methanol containing known concentrations of 2,4,6-tri-t-butylphenol and formaldehyde was 33 placed in a 10 ml. volumetric flask. This solution was left to stand for 20 minutes and then 2 ml. of distilled water was added to precipitate the phenol. To 3 ml. of the solution was added 1 ml. of 2 f> chromotropic acid followed by 6 ml. of concentrated sulphuric acid. No pink colour was seen. The results of the three 'controls' show that the Mn1** complex and tetramethylammonium hydroxide do not interfere with the 'chromotropic acid' method whereas 2,4,6-tri-t-butylphenol does interfere with this test. 3 * IV. SOME RELATED WORK (i) Determination of extinction coefficient of 3 .3 ' ,5 .5 ' -tetra-t-butyl-p- dlphenoquinone in methanol A 2 . 5 mg sample of 3 , 3 ' *5»5'-tetra-t-butyl-p-diphenoquinone (XVIII) was dissolved in 10 ml. of spectroscopic grade acetone, (it was found that this amount of XVIII dissolved in 25 ml. of methanol with difficulty. Therefore, acetone was used as solvent for practical convenience). A 0 .025 ml. aliquot of this stock solution was syringed into 3 ml. of methanol In a 1 cm. cell, and the absorbance at 419 nm was recorded. Absorbance readings at various other concentrations of the compound were recorded similarly. A plot of absorbance at 419 nm versus concentration of the compound yields a straight line passing through the origin, indicating Beer's law is obeyed- (see Figure 4). The slope of the plot gives a value of 79,000 for the extinction coefficient of XVIII at 419 nm in methanol. 0: .0 XVIII 36 (ii) Determination of extinction coefficient of bis-(l , 3 . 5-tri-t-butyl- 215-cyclohexadiene-4-one) peroxide in methanol A 6.4- mg sample of bis-(l,3,5-tri-t-butyl-2,5-cyclohexadiene-4-one) peroxide (XII) was dissolved in 5 ml, of methanol in a 5 ml, volumetric flask. A 0,025 ml. aliquot of this stock solution was syringed into 3 ml. of methanol in a 1 cm. cell, and the absorbance at 240 nm was recorded. Absorbance readings at various other concentrations of the compound were similarly recorded. A plot of absorbance at 240 nm versus concentration of the compound gives a straight line passing through the origin, indicating Beer's law is obeyed (see Figure 5)» From the slope, a value of 22,010 was obtained for the extinction coefficient of XII at 240 nm in methanol. (Compare with the value of 25,800 at 240 nm in benzene) . XII Concentration of 'bis-(i,3t5-tri--b-l)utyl-2,5-cyclohexadiene-4-one) peroxide x IO 5 H 38 ( i i i ) Determination of extinction coefficient of 2,4,6-tri-t-butylphenoxyl  in methanol It is difficult to prepare a pure sample of 2,4,6-tri-t-hutylphenoxyl (i) "because of its reactivity towards oxygen. The method used here to determine its extinction coefficient avoids the difficult step of isolating a pure solid sample. It involves the generation of I from its phenol using lead dioxide as the oxidant, and determining the concentration of I from the amount of XII that is formed. The procedure is as followst a 9.8 mg sample of XXIII was mixed with O.i g of lead dioxide in a 2 ml. centrifuge tube. The tube was sealed with a rubber cap and the contents flushed with nitrogen. A 1.5 ml, aliquot of methanol (which had been flushed with nitrogen) was syringed into the tube through the rubber cap and the contents of the tube were shaken for one minute. Then, the tube was placed in a centrifuge for two minutes to bring the solid lead dioxide to the bottom of the tube, A clear blue solution of I appeared above the lead dioxide layer. A 0.025 ml. aliquot of this clear blue solution was syringed into each oft (a) two 1 cm. cells each containing 3 ml. of oxygen-free methanol and capped with a silicone rubber disc, and (b) two test-tubes each containing 3 ml. of methanol saturated with oxygen. Absorbance readings at 400 nm for I in the two oxygen-free cells were recorded. The solutions in the two test-tubes containing oxygen were left to stand for a few hours to ensure that a l l of I was converted to XII and then the absorbance at 240 nm was recorded for each sample. (The absorbance 39 of 2,4,6-tri-t-butylphenol is at its minimum in this region. Therefore, any unreacted 2,4,6-tri-t-butylphenol would make a negligible contribution to the absorbance at 240 nm). Knowing the extinction coefficient of XII (see Section IV-ii), its concentration could be calculated, and doubling this concentration gives the concentration of I (see equation (13)). Thus, the extinction coefficient of I could be determined, assuming that the same quantity of I was formed in the two sets of experiments. It was found that a l l duplicate samples gave results in good agreement with one another. This procedure was followed for three other quantities of 2,4,6-tri-t-butylphenol (up to 23.8 mg) with the weight of lead dioxide used being kept at O.i g. The results are summarised in Table II. An average value of 1955 * 66 was obtained for the extinction coefficient of I at 400 nm in methanol. (Compare with the value of 2050 at 400 nm reported in benzene)^. 40 TABLE II Absorbance and yield of 2,4,6-trl-t-butylphenoxyl for various  concentrations of 2,4,6-trl-t-butylphenol used (PhOH)a(M) Absorhance(400nm) (FhQ.)P'c(M) ^ 0 n m 2.0? x 10"4 0.220 1.13 x l O " 4 19^3 55 3 . 0 9 x 10"4 0.327 1.60 x l O " 4 2049 52 4 .03 x 10"^ 0.386 2.04 x Mfk I896 51 5.00 x 10"4 0.471 2.44 x IO"4 1933 49 a PhOH - 2,4,6-tri-t-butylphenol b PhO» = 2,4,6-trl-t-butylphenoxyl c concentration of 2,4,6-tri-t-butylphenoxyl was calculated from that of XII d ^ ^onm w a s c a ^ c u l a ' t e < i t v dividing absorbance by concentration of I e yield calculated by dividing the values in column 3 by those in column 1 41 (iv) Determination of the equilibrium constant between 2,4 ,6-tri-t- butylphenol and its anion In methanol The equilibrium constant K (defined in equation (15)) for the equilibrium shown In equation (14) was determined spectrophotometrically as described below. PhOH + OH T PhO + HgO (14) K - (Ph0~)/(Ph0H)(0H~) (15) Abbreviations usedt PhOH « 2,4,6-tri-t-butylphenol PhO~ » 2,4,6-tri-t-butylphenoxy anion To a 3 ml. solution of 2,4,6-tri-t-butylphenol of known concentration in methanol in a 1 cm. cell was added 0.050 ml. of a standard base followed by 0.025 ml. of distilled water (water content in the ce l l was maintained at 2.5 $> by volume). The concentration of 2,4,6-tri-t-butylphenoxy anion formed was determined from its absorbance at 302 nm using an extinction coefficient value^ of 4400. (At this wavelength 2,4 ,6-tri-t-butylphenol does not have any absorbance). Knowing the concentration of 2,4,6-tri-t>-butylpb.enoxy anion, the final concentrations of base and 2,4,6-tri-t^butylphenol could be calculated as could K using equation (15). 42 This procedure was repeated for other concentrations of base and an average value of K was calculated. The value of K was also determined in methanol-O-d containing deuteroxide ion. The results obtained are shown in Table III. TABLE III Equilibrium constant for ionization of 2,4,6-tri-t-butylphenol in methanol at 25° Base Solvent Cone, of base(M) (PhO")(M) (PhOH)(M) TMAa CH„OH 3 0.89 x lo"2 0.54 x 10"^ 2.51 x lO" 2 0.241 1.77 x 10~2 1.09 x IO"4 2.50 x 10"2 0.246 2.66 x 10~2 1.68 x 10"4 2.50 x 10"2 0.253 Average K 0.247 NaOH CH^ OH 0.81 x 10"2 0.78 x 10"^ 4.05 x 10"2 0.238 3.21 x 10"2 3.10 x IO"4 4.02 x 10"2 0.240 Average K 0.239 NaOD CH^ OD 1.01 x 10"2 1.18 x 10"^ 2.55 x 10"2 0.458 2.03 x 10~2 2.27 x 10~4 2.54 x 10"2 0.440 a TMA «s Tetramethylammonium hydroxide Average K 0.449 43 The equilibrium isotope effect, ^Ap. is 0.53. This value is 57 within the usual range of equilibrium Isotope effects for reactions similar to that shown in equation (14), For example, the value of K^ /K^  for the equilibrium shown in equation (16) was found to be 0.70 by Swain eg and co-workers^ using a spectroscopic technique and 0.74 by Ballinger and Long"'0' using a conductivity method. CHgClCHgOH + 0H~ <c CH2C1CH20" + HgO (16) This equilibrium isotope effect has two major causes: (i) 0D~ is a stronger base than 0H"{ (ii) DgO is a weaker acid than HgO. These two factors shift the equilibrium shown in equation (14) more towards the right when the deuterated solvent is used. The fact that OD" is a stronger base than OH* has been shown by many workers"'7"0^'; e.g. the equilibrium constant for equation (17) was found to be 3.9^° and 3 . 2 ^ . The acid strength of DgO has also been shown to be weaker than that of H20 " . An equilibrium isotope effect -3, KJJAQ. on the autoprotolysis of water (see equation (18)) was found to be 9.08 at 25°. K c 2 OD" + HgO ^ 2 OH" + D20 (17) 44 (v) Spectra of the Mn*** complex in methanol III / \ 41 KMn CyDTACHgO) has "been reported to exist in two different forms in water. In acidic and neutral conditions, the pink species with III — a formula believed to be Mn CyDTA(H20) predominates, whereas in basic III 2-conditlon the yellow species with a formula believed to be Mn GyDTA(OH) predominates. The pK value for the equilibrium between these two species (equation (19)) was reported to be 8 . 1 ^ . K Mn***CyDTA(H2or Mn***CyDTA(0H)2" + H+ (19) In methanol a similar equilibrium exists. In the absence of base, TTT _ the pink species Mn CyDTA(H20) predominates (absorption maximum at 510 nm). When base is added the pink colour immediately changes to III O— yellow owing to the formation of Mn CyDTA(OH) (absorption maximum at 466 nm). When acid is added to the yellow solution the colour reverts back to pink immediately. The interconvertibility between these two species is also observed in aqueous medium. In this work, i t was found that another form of the complex exists in methanol in the presence of an excess of acid (see curve B of Figure 6). This third species is also pink in colour but its visible spectrum shows i t to be different from the other two species. Its absorption maximum in the visible is at 520 nm. Addition of an excess base immediately changes this species to the yellow species, and when acid is added to the resulting yellow solution, 0.30 Figure 6. Visible spectra of KMn CyDTACHgO) in methanol . Wavelength (nm) 46 the pink colour appears immediately. This third species is probably a TIT TTT protonated form of the Mn complex with formula HMn CyDTA(HgO). (Kinetic evidence presented in Sections VI and VII of this thesis indicates the presence of such a protonated form). Protonation probably occurs at a carboxylate arm of the ligand which becomes free from TTT coordination with the Mn ion. (The following compounds a l l have a free, protonated, carboxylate arms Cr I I IYH(H 20) 6 5' 6 6; Fe I I IYH(H 20) 6 5" 6 7{ Ga I I 3TH(H 20) 6 5~ 6 7; Ni I I IYH(H 20) 6 8J where Y is ethylenediaminetetraacetlc acid anion. In the unprotonated form, a l l these complexes have a l l the four carboxylate arms coordinated to the central metal ion). An attempt to measure the ionization constant of the protonated III form of the Mn complex by spectrophotometric method failed because of its rapid reaction with methanol (see Section Vl). No spectral change was observed when acid was added to an aqueous III solution of the Mn complex. 47 (vi) Molecular formula and crystal structure of the Mn*** complex (a) Brief remarks Few manganese(ill) compounds have been prepared and characterised owing to their tendency to disproportionate to form manganese(II) ion and manganese(IV) oxide (see equation (20)). 2 Mn*** + 2 H20 v Mn** + Mn02 + 4 H+ (20) 41 Hamm and Suwyn , using a method similar to that described by Yoshino and 69 co-workers , prepared a solid trans-l,2-diaminocyclohexanetetraacetic acid complex of manganese(ill) in I 9 6 6 . This complex was assigned a formula of KMn***CyDTA'2.5H20 on the basis of elemental analytical data (CyDTA is used as an abbreviation for the trans-1,2-diaminocyclohexanetetraacetic acid anion ligand). They found that this complex was stable at room temperature when i t was stored in the dark, and that i t existed in two forms in solution. In acidic solution, a pink species with a formula believed to be Mn CyDTA(HgO) predominated, and under basic conditions, the predominant XXX 2**" species was yellow in colour with a formula believed to be Mn CyDTA(OH) . The pKa value for the pink species, Mn^CyDTA^O)", was found to be 8.1. The standard potential for equation (21) was found to be 0.814 . Mn***CyDTA(H20)" + e~ , Mn**CyDTA(H20)2" (21) 48 III In this work, the Mn complex was prepared using the procedure described by Hamm and Suwyn. The crude product was then recrystallised from hot methanol-water solvent. It was noted that elemental analytical results indicated a formula of KMn^^CyDTA^O) and the melting point (decomposition point) of a recrystallised sample of the complex was higher than that of the crude product (201-203° and about 180°, respectively). The lack of information on the melting point and purification procedure in reference 41 prompted an investigation on the exact molecular formula and structure of this complex. A Karl Fischer titration to determine the water content of the complex and an X-ray single crystal diffraction study to determine the structure of the complex were carried out. The results (described below) support a formula of KMnIIICyDTA(H20). (b) Karl Fischer titration A Beckman Aquameter Model KF-2 automatic titrator was used in the determination of water. The Karl Fischer reagent (from Fisher Scientific Company) used was f i r s t standardised with reagent grade sodium tartrate dihydrate (NagC^H^Og^HgO) in anhydrous methanol. A typical titration was carried out as followst to a beaker was added some methanol which contained some water and titration with Karl Fischer reagent was carried out automatically until the methanol in the beaker was anhydrous. Then, a known weight of sodium tartrate dihydrate was added to this anhydrous methanol and an automatic titration was started again until the end-point 49 was reached. The volume of Karl Fischer reagent used was recorded. The number of millilitres of Karl Fischer reagent needed to titrate one milligram of water was calculated (the water content in a sample of sodium tartrate dihydrate could be calculated from its molecular formula). This procedure was repeated for two more weighed samples of sodium tartrate dihydrate and an average value of the results was taken. For the Mn*** complex, a weighed sample of this complex (100 mg) was fi r s t dissolved in 2 ml. of anhydrous dimethyl sulfoxide (DMSO)j then this solution was added to the anhydrous methanol in the beaker and an automatic titration was carried out. A 2 ml. aliquot of the same sample of DMSO was also titrated to correct for any water present in i t . (The Mn*** complex does not dissolve in methanol easily, therefore DMSO was used as solvent. The percentages of DMSO and methanol in the titration vessel were about 5 # III and 95 # respectively). The titration of the Mn complex was carried out in duplicate at two different times. A similar titration on a crude sample of the Mn*** complex was also carried out. This crude sample had a lower and indistinct decomposition point (about 170°). The crude sample of the Mn*** complex was found to contain 6.6 $ water, corresponding to 1.7 moles of water of crystallisation. This is close to the value expected for KMn***CyDTA'2.5^0 assuming that one molecule of water is rather firmly coordinated to the Mn*** ion. Only 0.6 # of water was detected in a recrystallised sample of the 50 Mn**1 complex, indicating that there is l i t t l e , i f any, water of crystallisation in the sample. Elemental analysis and crystal structure show that one water molecule is present in each molecule of the complex. This water molecule is probably coordinated quite firmly to the complex since i t is not released to the methanol solvent immediately. It was found that this water molecule was released to the methanol solvent in about thirty minutes. After a sample of the recrystallised complex was kept in anhydrous methanol in the titration vessel for thirty minutes water was released corresponding to 4.0 # of the weight of the complex, that is 1.0 mole of water per mole of complex. It is not certain whether III a methanol molecule replaces the water molecule in the Mn complex or whether the water is released as a result of the slow decomposition of the Mn*** complex in the Karl Fischer reagent mixture. The fact that the mono-aquo complex is obtained pure by recrystallisation from 60 # methanol and 40 # water mixed solvent suggests that water is the preferred ligand. Attempts to determine the equilibrium concentration of water and methanol in the complex in the presence of 97 % methanol-3 $> water (the mixture used in the kinetic studies) failed. Flash evaporation of such solutions produced a solid residue which was partly decomposed and gave variable amounts of water. Throughout this thesis i t will be assumed that water is the ligand present in the complex during the oxidation steps. 51 (c) Crystal structure of KMnIIICyDTA(H20) An X-ray single crystal structure study on KMnIIICyDTA(H20) was carried out in this department by Mr. S. Rettig and Dr. J. Trotter. The crystal was shown to be triclinic and contain two molecules of the complex per unit c e l l . The structure of the complex is shown in Figure 7 and the various bond lengths and bond angles are listed in Tables IV and V respectively. I l l The structure indicates a slightly distorted octahedron of Mn III ion with the four carboxylate groups bonded to the Mn ion in two ways (02-C8 - OQ-C^ - 1.258 * 0.008 A°: - 0 6-C l 2 - 1.232 * 0.008 A°). The cyclohexane ring is in a chair conformation and the angles in the ring are close to 109.5°. The two nitrogen atoms attached to the cyclohexane ring are in trans-di-equatorial conformation. The position of the water molecule seems to vary in the molecule, The major position is at a distance of 3*68 * 0.02 A° from the Mn 1 1 1 ion and the other position is 4.55 * 0.02 A° from the Mn**1 ion. These two values are much longer than a normal metal-water bond (about 2.1 A°), suggesting that the Mn*** ion Is six-coordinated. (Compare with Fe***EDTA (HgO)"* and Mn**EDTA(H20)2~ which contain seven-coordinated metal i o n s ) 7 0 ' 7 1 . However, Karl Fischer titrations (Section IV-vi(b)) indicated that this water molecule is bound In the complex. Thus, this III water molecule is probably coordinated to the Mn ion in solution. 53 TABLE IV Individual bond lengths in KMnIIICyDTA(H20) Bond Length (A°) Bond Length (A°) Bond length (A°) ( * ° ' 0 0 8 ) ( * 0.008 ) ( + 0.080 ) Mn-01 2.064 N r c i 1.495 1.012 Mn-0^  1.880 N r c 7 1.510 G2-H21 1.010 Mn-0^  1.893 N r c 9 1.487 G3~H31 0.986 Mn-0^  2.002 N 2 - C 2 1.503 G3"H32 0.869 Mn-Nj 2.249 V c i i 1.482 G4"H41 0.951 Mn-N2 2.278 V c l 3 1.470 G4"H42 0.969 °r ce 1.284 G r c 2 1.566 G5"H5l 1.150 °2-C8 1.258 c r G 6 1.543 C5"H52 1.084 °3 _ C10 1.323 c 2 - c 3 1.538 G6"H61 0.992 VC10 1.234 C3"C4 1.530 G6" H62 0.922 o5-c1 2 1.308 V c 5 1.556 G7"H?1 0.953 °6- Cl2 1.230 C5"G6 1.539 °7"H72 I.O85 VC14 1.303 G7"C8 1.521 G9"H91 1.041 °8-C14 1.257 G9~G10 1.508 G9"H92 1.010 G11"G12 1.540 GH"H111 O.893 C13"G14 1.530 C11"H112 1.007 G13~H131 1.181 G13"H132 0.907 Bond angle 54 TABLE V I I L KnnA angles in KMn CyDTA (Hg0_) degree Bond angle degree Bond angle degree C 0.2) C o-5) C°' 5 ) Oj^ MnO^  96.1 MnNjCj 108.4 C4 C 5 C 6 110.6 Oj^ MnO^  87.4 MnN1C? 103.8 C1 C 6 C 5 110.7 O.MnOo 129.3 MnN1C9 103.4 N1 C7 G8 111.2 OjMnNj 76.5 C1 N1 C7 113.5 °1G8°2 124.6 OjMnNg 152.3 C i N l C 9 114.8 °1 C8 C7 116.7 (UtaQ- 175-5 111.7 °2 C8 G7 118.7 j J OoMnO„ 87.9 MnN2C2 108.4 N1 C 9 G 10 114.7 j f O-MnN. 82.3 MnN^ 103.8 °3G10°4 123.2 j A 0„MnN2 93.4 MnN2Cl3 104.4 °3C10G9 116.6 OcMnO„ 92.0 C2 N2 C11 115.3 °4C10G9 120.2 J f OJtaN. 95.9 C2 N2 C 13 112.8 N2 G11 C12 114.7 0-MnN2 82.3 C11 N2 C13 111.0 °5G12°6 122.7 0?MnN1 153.5 N1 C1°2 110.3 °5C12C11 117.0 OJtalL 76.9 N1 G1 C 6 113.4 °6C12G11 120.3 f N.MnN, 79.1 N2 C2 C1 107.1 N2 C13 G14 110.0 MnOjCg 117.3 N 2C 2C 3 113.6 °7C14°8 124.3 Mn03C10 120.5 C1 C2 C 3 113.9 °7G14G13 117.2 Mn05Cl2 120.7 C2 C 3 C4 110.5 °8C14C13 118.5 MnO?C11+ 118.6 C 3 G4 C 5 108.7 55 V. DECOMPOSITION OF THE Mn 1 1 1 COMPLEX IN METHANOL* IN THE PRESENCE OF  BASE (l) Products A trace of formaldehyde (see Section IH-vi for analytical method) and manganese dioxide (see discussion at the end of this section) were produced from the decomposition of the Mn*** complex in methanol in the presence of base. (ii ) Analysis of kinetic results Decomposition of the Mn*** complex in methanol was carried out in the presence of excess tetramethylammonium hydroxide. Plots of logarithm of absorbance of the Mn*** complex versus time are linear up to about III 50 <jt> of reaction. The decomposition of the Mn complex was not followed beyond 50 $> of reaction because of the appearance of a broad absorption band extending from the J00 nm region into the 400 nm region that is due to manganese dioxide formed in the reaction. The decomposition of the Mn*** complex can be expressed by equation (22). Figure 8 is a typical plot made using equation (23). d(Mn***)/dt = kd(Mn***) (22) * 2.5 f> water (v/v) 0.67 Time (min.) 57 Integrating equation (22) gives equation (23). Log A ^ - log A Q - - kdt/2.303 (23) (Mn*11) » concentration of the Mn 1 1 1 complex =* pseudo-first order rate constant M^n 0 absorbance °f * n e Mn**"*' complex at 466 nm at time t A q = absorbance of the Mn*** complex at 466 nm at time zero III When the concentration of the Mn complex was kept constant, k^ was found to vary linearly with the concentration of tetramethylammonium hydroxide (see Table VI and Figure 9). Thus, k d is a base-dependent term. TABLE VI Variation of k^ with base concentration at 25° (TMA)*(M) k^sec" 1) 0.71 x IO"2 0.96 x 10~4 0.90 x IO"2 1.04 x IO"4 1.43 x 10~2 1.49 x l O " 4 1.78 x 10"2 1.80 x 10~4 2.65 x 10~2 2.77 x 10~4 * TMA <= Tetramethylammonium hydroxide (Mn***) « 5»4 x 10 4 M 59 The following stoichiometric reaction is postulated! k7 2 Mn 1 1 1 + 4 OH" > Mn11 + Mn02 + 2 H20 (24) III The rate of disappearance of the Mn complex is given by equation ( 2 5 ) . - d(Mn m)/dt = 2k ?(0H")(Mn m) (25) Since (OH") » (Mn 1 1 1), 21^(0^) is a constant. Equation (25) is identical to equation (22) with Z ^ i O H ) « k d (26) Integrating equation (25) gives equation (27). log A M n - log A 0 - - kdt/2.303 (27) ( i i i ) Determination of rate constant For each concentration of tetramethylammonium hydroxide, k d was evaluated from the slope of a plot of log A^ n versus time (see equation (27) and Figure 8). Then a plot of k d versus base concentration (see equation (26) and Figure 9 ) yielded the value of 21^ from its slope. The -3 -1 -1 value of kj was found to be 4.7 x 10 J M sec . 6 0 (iv) Activation parameters I I I The decomposition of the Mn complex in methanol in the presence of base was studied at other temperatures in order to determine the values of the activation enthalpy (AH ) and activation entropy (AS ). According to transition-state theory, these two parameters are related to the reaction rate constant by equation ( 2 8 ) . k - KTexp(-AH*/RT)exp(-^S*/R)/h ( 2 8 ) where k = reaction rate constant . - 2 4 - 1 K «* Boltzman constant = 3«30 x 1 0 cal.deg. T = absolute temperature h = Planck constant *= 1 . 5 8 4 x 1 0 ^  cal.sec. R= gas constant = 1 . 9 8 ? cal.deg."*1 mole""1 Taking the logarithm of both sides of equation ( 2 8 ) and substituting the appropriate values of K, h and R, equation ( 2 8 ) is transformed into equation ( 2 9 ) . log k/T - 10 .32 + A S * / 4 . 5 7 4 - 4 H * / 4 . 5 7 4 T (29) A plot of log k/T versus l/T yields a slope of AH / 4 . 5 7 4 and an intercept of 10 .32 + A S * / 4 . 5 7 4 . 61 Using the data in Table VII, log k^/T was plotted against l/T (see e 10). The values f< -15.0 e.u. respectively. Figur ). ound for^H and AS are 16.1 Kcal.mole"" and TABLE VII Variation of k^ with temperature Temperature(°C) k„(M~*sec~*) 25 (4.9 * 0.2) x 10"3 30 (8.2 * 0.3) x 10"3 35 (12.7 * 0.5) x 10"3 40 (19.4 + 0.6) x 10"3 (v) Discussion III The disappearance of the Mn complex in methanol in the presence of base is due to its decomposition and not to oxidation of methanol by the complex. There are two pieces of evidence supporting this view. Firstly, only a trace of formaldehyde was detected. If the Mn*** complex oxidised methanol under basic condition, then one mole of formaldehyde would be produced from every two moles of the complex (see Section Vl). Secondly, in aqueous and dimethyl sulfoxide media, similar broad bands extending from the 300 nm region into the 400 nm region were observed when 63 tetramethylammonium hydroxide was added to the Mn*** complex in these two media. In dimethyl sulfoxide solvent, brownish precipitates of manganese dioxide appeared immediately and the solution was golden yellow. The golden yellow colour observed in a l l three media is probably due to / 4i dissolved manganese dioxide. (Hamm and Suwyn have reported that the Mn*** complex decomposes into manganese dioxide in water of pH above 11). When the reaction mixture at the completion of a reaction was oxidised by lead dioxide no spectrum due to the Mn*** complex could be obtained, thus indicating the Mn** formed in the reaction mixture was not complexed to CyDTA ligands. The decomposition of the Mn*** complex was not followed beyond 50 $> of reaction because of the appearance of a broad absorption band extending from the 300 nm region into the 400 nm region that is due to manganese dioxide formed in the reaction. The value of obtained is probably smaller than the true value of because the decrease of the Mn*** complex peak at 466 nm with time is not corrected for absorbance contribution from the manganese dioxide formed in the reaction. The value of is sensitive to the extent of reaction followedi e.g. the value of k^ for 50 # reaction is slightly smaller than that for 40 % reaction. In order to have a consistent way of computing a l l the values of K,, only least squares plots (Figure 8) with a correlation coefficient of at least 0.999 are used. 64 T T T * VI. REACTION OF THE M n A U COMPLEX WITH METHANOL (SOLVENT) IN THE PRESENCE OF ACID (i) Products II Three products - Mn complex, formaldehyde and carbon dioxide were identified. One mole of formaldehyde was produced for every two moles of the Mn**1 complex used. (See Section I l l - v i for methods used in product analysis). (i i ) Analysis of kinetic results TTT Reaction of the Mn complex with methanol (solvent) was carried out with excess hydrochloric acid present. Plots of logarithm of A ^ versus time are linear up to greater than 80 $ of the reaction, indicating that III the rate of disappearance of the Mn complex can be expressed by equation ( 3 0 ) . Figure i l is a typical plot of equation ( 3 l ) » where (Mn***)^^ = total concentration of the Mn*** complex, k^ •» pseudo-first order rate constant. Integrating equation (30) gives equation (31). * 2.5 £ water (v/v) -0.4 -0.7 h -1 .0 c < hD -1 .3 O i-3 -1.6 Figure 11. Plot of log versus time for reaction between the Mn1*1 complex and methanol in the presence of acid. (HCl) - I.63 x 10" 2 H (m^) - 5.6 x 10" 4 M o III Temperature -1 .9 -2.2 T TTT 2.5 5.0 7.5 10.0 Time (min.) 12.5 15.0 20.0 66 l o g A M n - l o g A o - - I^t/2.303 (31) III where A ^ «= absorbance of the Mn complex at 520 nm at time t, A Q «= absorbance of the Mn 1 1 1 complex at 520 nm at time zero. When the concentration of the Mn*** complex was kept constant, k^ was found to increase with increase in the acid concentration (see Table VIII and Figure 12). The curved plot in Figure 12 suggests that the oxidant involved in the rate-determining step of the reaction is a protonated species. TABLE VIII Variation of k^ with hydrochloric acid concentration at 25° (Mn***) = 5.6 x 10"1* M (HC1)(M) kjsec" 1) 0.81 X IO"2 0.88 x 10~3 1.63 X IO - 2 1.54 x 10"3 4.07 X IO"2 2.46 x 10"*3 6.09 X io"2 2.93 x 10"3 10.15 X io"2 4.50 x IO""3 14.44 X 10- 2 5.40 x 10 - 3 38.40 X io"2 8.40 x IO"3 68 ( i i i ) Mechanism The following reactions are postulated to explain the kinetic results obtained. Mn1-1 + H+ . HMn1*1 , fast (32) P P HMn*o* + CH^ OH > 'CHgOH + Mn11 + H+ (33) \ HMn1*1 + 'CH-OH > HCHO + Mn** + H+ , fast (34) P 2 \ Mn*** + 'CHgOH » HCHO + Mn** + H+ , fast (35) Abbreviations usedi Mn*** » Mn*I*CyDTA(H20)" HMn*** - HMn***CyDTA(H20) Equation (33) shows the rate-determining step. The rate of disappearance XXX of the Mn complex is given by equation (36). - d(Mn***)tot/dt = 2k1(HMn**>*)(CH30H) (36) 69 where (Mn***)^^ = total concentration of the Mn*** complex - (HMn**)*) + (Mn*2*) The concentration of HMn***CyDTA(H? 0) can be expressed in terms of the total concentration of the Mn*** complex by using the relationship in equation (37). (HMn*5*)/(Mnp) = K1 (H+) (37) Substituting (Mn***) - (Mn***)tot- (HMn***) in equation (37) and rearranging, equation (38) is obtained. (HMn***) = K*(H+)(Mn***)tot/(l + IC (H+)) (38) Therefore, equation (36) can be expressed as followsj - d(Mn***)tot/dt - 2ki lC(H +)(Mn* I I) t o t/(l + K»(H+)) (39) where kj - k^CH^OH). Integrating equation (39) yields equation (40). l o g A M n - logA Q = - 2k^K'(H+)t/2.303(1 + K* (H+)) (40) 70 Equation (40) is identical to equation (31) with = 2k}KMH+)/(l + K»(H+)) (41) (iv) Determination of rate constant The value of was obtained from the slope of a plot of log A M n versus time (see equation (40) and equation (41)). This was done for each concentration of acid. In order to determine kj the value of K* must be known. Equation (37) could not be used to solve for K* because (Mn*-1) T T T and (HMnpo ) could not be determined. An indirect method was used to estimate the magnitude of K*. From a plot of k^ versus concentration of acid (see Figure 12), the maximum value of k^ was estimated to be in the -3 -1 III region of 8.4 x 10 J sec . At the maximum value of k^ a l l the Mn complex exists in the protonated form and equation (36) becomes equation (42). - d(HMn*oI)/dt - 2k1(HMn*oI)(GH30H) (42) Therefore, 2k1(CH30H) - (43) where k. = maximum value of Ic. . bmax 0 24 71 The solvent used was 97.5 f> in methanol (v/v) giving a value of -3 -1 .1 M for the concentration of methanol. Knowing k. (8.4 x 10 sec ) and (CH^ OH) (24.1 M), was calculated and found to be 1.74 x IO - 4 jf^sec" 1, The value of K* was calculated for each value of (HGl) using equation (41). The results are shown in Table IX. An average value of TC « 12 + 2 was obtained. TABLE IX I? calculated from equation (41) (HC1)00 0.81 x 10""2 14 1.63 x 10"2 14 4.07 x 10"2 10 6.09 x 10"2 9 10.15 x IO - 2 11 14.44 x 10"2 12 (v) Reactions with deuterated methanols Reactions of the Mn1*1 complex with methanol-d^, methanol-d^ and methanol-O-d in the presence of deuterium chloride (hydrogen chloride was used for reaction with methanol-d,,) were carried out to obtain some 72 information about the mechanism of the reaction between the Mn1*1 complex and methanol. In the cases of methanol-d^ and methanol-d^ the reactions were carried out at a single acid concentration. In the case of methanol-O-d the reaction was carried out at various deuterium chloride concentrations in order to obtain the value of the equilibrium constant K* (see equation (32)) . Table X summarises the results obtained for the reaction between III the Mn complex and methanol-O-d in the presence of deuterium chloride. Figure 13 is a plot of versus deuterium chloride concentration. From the maximum value of Ic^ , a value of = 1.39 x 10 M sec was calculated (see equation (43)) . The equilibrium constant K* was found to be 25 + 2 . TABLE X Variation of k^ with deuterium chloride concentration at 25° (Mn 1 1 1) - 5.7 x 10"4 M Solvent = CH^ OD OSD(H) Vsec-1) K M M " 1 ) * 2.05 x IO - 2 2.20 x 10~3 24 4.89 x IO"2 3.80 x 10"3 2? 9.78 x 10~2 6.16 x 10~3 — 13.04 x IO"2 6.64 x 10~3 — 19.60 x IO"2 6.75 x 10"3 — -4-1 -1 calculated from equation (41) using kj = 1.39 x 10 M sec . 9.0 Figure 13. Observed rate constant, k^ , versus concentration of deuterium chloride. 7.5 (Mn 1 1 1) - 5.7 x 10" 4 M 6.0 o CO w S 4.5 3.0 1.5 Solvent =• CH^ OD Temperature = 25c 0.0 1 12 15 (DCl) x 10* M 18 21 24 74 The values of the r a t e constant, k^, f o r the f o u r methanols are gi v e n i n Table XI and compared i n Table X I I . I t can be seen t h a t the k i n e t i c i s o t o p e e f f e c t s are s m a l l . TABLE XI Rates f o r o x i d a t i o n of i s o t o p l c methanols a t 25° ( M n 1 1 1 ) - 5.6 x IO" " * M (HC1) • (DC1) = 2 .05 x 10~ 2 M —1 —1 — i a Experiment A c i d Solvent k^(sec~ ) k^(M** sec" ) A HC1 CH-OH" 2.10 x IC-""3 1.64 x 10"J[ . 3 (1.74 x 10" V B DC1 CH„0D 2.20 x 1 0 _ 3 1.14 x 10'^ G DC1 CD^OD D HC1 CD^OH 3 " ( l ! 3 9 x l O " V 2.03 x IO""3 1.04 x I O - 4 1.68 x 10""3 1.38 x I O - 4 a c a l c u l a t e d from equation (41) u s i n g IC = 12 f o r CH^OH and CD^OH, and K* «= 25 f o r CD^OD and CH^OD. b c a l c u l a t e d from k. (see equation ( 4 3 ) ) . 75 TABLE XII Isotope effects in oxidation of methanol at 2 5 ° Experiments compared Compounds compared k^/k^p A, B CH^ OH, CH^ OD 1 . 3 * 0 . 1 a C, D CD^ OD, CD^ OH 1.3 A, D CH^ OH, CD^ OH 1.2 * 0 . 1 a B, C CH^ OD, CD^ OD 1.2 + 0 . 1 a a an average value of the "bracketed and unbracketed values of k^  (see Table XI) was used in the calculations. (vi) Activation parameters III Reaction of the Mn complex with methanol in the presence of acid was carried out at several temperatures in order to evaluate the values of the activation enthalpy (AH ) and activation entropy (AS ). The data used in the plot in Figure 14 were taken from Table XIII. The values found for # * —1 A H and A S are 15.9 Kcal.mole and -22.5 e.u. respectively. -5 .0 -5 .3 h -5.6 Figure 14. Plot of log k^/T versus l/T for reaction between the Mn 1 1 1 complex and methanol in the presence of acid. * . -1 AH =15.9 Kcal.mole AS « -22.5 e.u. correlation coefficient - -0.9981 i t bD -5.9 O »-3 Os -6.2 -6 .5 L -6 .8 ± 3.00 3.05 3.10 3.15 3.20. l/T ( K) x 103 3.25 3.30 3.35 3.40 77 TABLE XIII Variation of k^  with temperature Temperature( C) k^(M sec" ) 25.0 (1.7 * 0.0) x IO - 4 30.4 (2.7 * 0.1) x 10~4 35.8 (4.2 + 0.1) x 10~4 40.0 (6.7 + 0.1) x 10~4 45.0 (9.6 + 0.5) x IO - 4 (vii) Discussion The mechanism postulated for the reaction of the Mn 1 1 1 complex with methanol in the presence of acid involves the formation of hydroxymethyl radical as the rate-determining step. The hydroxymethyl radical immediately reacts with another molecule of the Mn*** complex "by transferring a hydrogen atom to i t to form formaldehyde. Thus, two moles III of the Mn complex should yield one mole of formaldehyde, and analysis for formaldehyde showed that one mole of formaldehyde was Indeed produced III for every two moles of the Mn complex consumed. The reaction postulated is further substantiated by the kinetic results obtained for the oxidation of 2,4,6-tri-t-butylphenol with the Mn*** complex in methanol in the 72 presence of acid (see Section VII). (Recently, Wells and Barnes reported 78 the results of a study of the oxidation of methanol "by Mn 1 1 1 ion in aqueous perchloric acid. They found that one mole of formaldehyde was TTT produced for every two moles of Mn consumed). Carbon dioxide was also observed as one of the reaction products. It is probably formed from some decomposition of the CyDTA ligands. This is a minor reaction compared with the oxidation of methanol by the Mn*** complex since the resulting Mn** in the reaction mixture could be oxidised back to more than half the original concentration of the Mn*** complex with lead dioxide (as evidenced by similar visible spectrum). Since the Mn*** complex disappears rather rapidly under the conditions used (methanol solvent with acid present) the actual concentration of the III II Mn complex obtained by oxidising the Mn in the reaction mixture with lead dioxide was probably higher than that observed. The kinetic isotope effects observed for CH^ OD, CD^ OD and CD^ OH (see Table XII) are too small to account for a transition state which involves either the cleavage of the 0-H bond or of the C-H bond as the rate-determining step in the reaction. Thus, i t is likely that electron transfer rather than hydrogen atom transfer from methanol to the Mn*** complex is involved in the oxidation process. Equation (33) can be divided into equations (33a) and (33b). The small kinetic isotope effect observed for CH^ OD is likely due to the replacement of the HgO ligand by III DgO ligand in the Mn complex. The replacement of CH^  by QB^ gives a 79 secondary isotope effect. HMn^o1 + CH^ OH > Mn11 + CH^ CH ( 3 3 a ) CH^ CH > 'CHgOH + H+ , fast ( 3 3 b ) Equation ( 3 2 ) is postulated as the pre-equilibrium which is set up 73-75 before oxidation occurs. Experimental work'-' '^ has shown that the value of K* is larger for deuterio acids because the zero-point energy of a deuterio acid is lower than that of the corresponding protio acid. Table XIV gives a l i s t of acids of different strengths and their respective ratios of ^ /K^. (K is equivalent to l/K* in equation ( 3 2 ) . See the definition of K in equation (44)). The ratio of J^/Kp was found to 73 increase approximately linearly with Increase in the pKjj values'-'. The K value of HMnIIICyDTA(H20) (= l/l? ) is 0.08 and its 'piy value is 1 . 1 . The ratio of Kjj/Kp for HMnIi:CCyDTA(H20) is 2 . 1 , which is of the expected magnitude according to the values shown in Table XIV. This observed isotope effect on the equilibrium constant K* supports the presence of the pre-equilibrium shown in equation ( 3 2 ) before the occurrence of the rate-determining reaction. T T T T T T The protonated form of the Mn complex, HMn CyDTA(HgO), is a more powerful oxidant than MnIIICyDTA(H20)" since in the absence of acid the reaction of the complex with methanol was found to be very much 80 TABLE XIV Dissociation constants of deuterioacids In deuterium oxide Acid V*D Oxonium ion -1.74 1.00 Chloroacetic acid 2.76 2.74 2,6-di-nitrophenol 3.58 2.84 2,4-di-nitrophenol 4.02 3.28 Benzoic acid 4.21 3.13 Acetic acid 4.74 3.33 2,5-di-nitrophenol 5.17 3.32 p-nitrophenol 7.24 3.61 o-nitrophenol 7.25 3.71 Bicarbonate ion 10.25 3.95 2-chloroethanol 14.31 5.00 HA + H20 ^ H^ O* + A"" (44) K = (H30+)(A")/(HA) 81 slower. A pseudo-first order rate constant of 2.24 x lo'-sec"1 at 25° was obtained. It is not certain whether this rate is for the oxidation of methanol or for the decomposition of the Mn1*1 complex because in the aqueous system4* the rate of decomposition of MnIIICyDTA(H20)- was —6 —1 o reported to be 6.8 x 10~ sec" at 25 . Hydroxymethyl radical was written as the reaction intermediate instead of methoxy radical because the C-H bond is weaker than the 0-H bond in methanol (C-H bond strength » 92 Kcal.mole 5 0-H bond strength —1 76 77 » 102 Kcal.mole ) . However, this does not rule out the possibility 9 8 — f i l of formation of some methoxy radicals. Several studies on hydrogen atom abstraction from methanol by methyl and trifluoromethyl radicals have shown that these radicals do attack the 0-H bond whereas the esr 82 8*5 spectrum of the radical ' generated by the reaction between methanol and t-butoxy radical indicates that only hydroxymethyl radical is produced. 82 VII. OXIDATION OF 2,4,6-TRI-t-BUTYLPHENOL WITH THE Mn COMPLEX IN  METHANOL* IN THE PRESENCE OF ACID (i) Product 2,4,6-Tri-t-butylphenoxyl was identified (see the absorption peak at 400 nm in Figure 15 and esr spectrum in Figure 16). (ii) Analysis of kinetic results Oxidation was carried out with excess 2,4,6-tri-t-butylphenol and hydrochloric acid present. Plots of logarithm of absorbance at 520 nm versus time are linear up to greater than 80 # of the reaction, indicating that the rate of disappearance of the Mn 1 1 1 complex can be expressed by equation (45). Figure 17 is a typical plot of equation (46). - d(Mn' III ) t o t/dt - k a(Mn^) (45) tot where (Mn' III ) = total concentration of the Mn III complex tot k = pseudo-first order rate constant. Integrating equation (45) gives equation (46), * 2.5 f> water (v/v) Figure 15. Visible spectrum showing the absorption bands of 2,4,6-tri-t-butylphenoxyl (A) and the Mn 1 1 1 complex (B) in methanol in the presence of acid. " T tH±tg - t + 0.4 -I) » 0.3 8 I o 0.2 550 Wavelength (nm) - 0 . 6 - 0 . 8 h - 1 . 0 h c ss - 1 . 2 o -1.4 - 1 . 6 Figure 17. Plot of log versus time for oxidation of 2t4,6-tri-t-'butylphenol III with the Mn complex in methanol in the presence of acid. (Mn111) - 5.6 x 10" 4 M (HG1) = 1.63 x 1 0 " M (PhOH) 5.10 x IO"2 M Temperature =25 CO - 1 . 8 12 Time (min.) 20 3? 86 log - log A 0 - - kt/2.303 (46) I I I where A ^ = absorbance of the Mn complex at 520 nm at time t A q « absorbance of the Mn*** complex at 520 nm at time zero. When the concentrations of 2,4,6-tri-t-butylphenol and the Mn*** complex were kept constant and the concentration of hydrochloric acid was varied, the values of k obtained were found to increase with increase in the acid concentration (see Table XV and Figure 18). The curved plot in Figure 18 suggests that the oxidant involved in the rate-determining step of the reaction is a protonated species (compare with Figure 12). TABLE XV Variation of k. with acid concentration at 25° a (Mn***) = 5.6 x IO"4 M (PhOH^S - 5.10 x lO""2 M (HCI)(M) k (sec - 1) a - — 1.63 x IO"*4 3.18 x 10"5 0.81 x 10"2 1.04 x 10~3 I.63 x 10"*2 1.74 x 10"*3 4.0? x 10""2 2.30 x 10~3 6.09 x 10"*2 2.79 x 10"*3 10.15 x IO - 2 4.45 x 10"3 14.44 x 10"2 5.16 x 10"3 * PhOH is used as an abbreviation for 2,4,6-tri-t-butylphenol. 88 The values of k were found to increase with increase in the concentration a of 2,4,6-tri-t-butylphenol when the concentrations of the acid and the Mn*11 complex were kept constant (see Table XVI and Figure 19). From the data in Table XVI, i t can be seen that doubling the concentration of 2,4,6-tri-t-butylphenol increases the value of k by less than 50 $>. St Therefore, k is likely to be a composite rate constant; at least one term in k is independent of the concentration of 2,4,6-tri-t-butylphenol, TABLE XVI Variation of k with concentration of 2,4,6-tri--b-butylphenol at 25° i111) - 5.6 x IO - 4 n (HC1) - I.63 x 10" 2 M (PhOH)*(M) k a(sec - 1) 1.57 x IO"2 1.17 x 10" 3 3.00 x io""2 1.50 x 10" 3 5.10 x 10" 2 1.74 x 10" 3 6.64 x IO"2 2.12 x 10""3 7.74 x 10~2 2.30 x 10**3 9.22 x 10~2 2.69 x IO"3 * PhOH is used as an abbreviation for 2,4,6-tri-t-butylphenol. Figure 19. Observed rate constant, k^ , versus concentration of 2,4,6-tri-t-butylphenol. Part (i). (Mn111) - 5.6 x IO"4 M (HC1) - I.63 x iO" 2 M Temperature =25° ITTo T0T5 "T2T0 1.5 3.0 4.5 7.5 9.0 Concentration of 2,4,6-tri-t-butylphenol x 10 M 90 With constant concentrations of 2,4,6-tri-t-butylphenol and III hydrochloric acid, but different concentrations of the Mn complex, the values of k & were found to be approximately constant (see Table XVII), III indicating that the rate of disappearance of the Mn complex is f i r s t order with respect to its concentration, thus supporting equation (45). TABLE XVII Variation of k& with the Mn*** complex concentration at 25° (PhOH)* - 5.10 x IO"2 M (HC1) - 1.63 x IO - 2 M (Mn***)(M) k (sec"1) 5.6 x Id"1* l . ? 4 x 10"3 6.9 x IO - 4 2.05 x 10"*3 7.7 x IO"4 2.19 x 10""3 9.0 x I0~h 2.10 x IO"3 17.2 x I0"k 2.30 x 10"*3 * PhOH is used as an abbreviation for 2,4,6-tri-t-butylphenol. ( i i i ) Mechanism The following reactions are postulated. K" Mn*** + H+ . HMn*** , fast (32) P P 91 *1 HMn1*1 + CH-OH » 'CH^ OH + Mn11 + H+ (33) HMn**1 + 'CHgOH > HCHO + Mn** + H+ , fast (34) k4 Mn*** + »CHo0H > HCHO + Mn*1 + H+ f fast (35) P 2 k3 HMn*** + PhOH > Mn** + H + PhO» (47) P k5 •CH20H + PhOH > CH^ OH + PhO' , fast (48) Abbreviations used: PhOH o 2,4,6-tri-t-butylphenol PhO» a 2,4,6-tri-t-butylphenoxyl Mn*-*= Mn***CyDTA(Ho0)" HMn*** - HMn***CyDTA(Ho0) p ^ Equations (33) and (47) are the rate-determining steps in the III disappearance of the Mn complex. The rate of disappearance of the Mn*** complex is given by equation (49). - d(Mn***)tot/dt » 2rk1(HMn*F)(CH30H) + k3(HMn***)(PhOH) (49) 92 The parameter r is to account for the competitive reactions in equations (3*0» (35) and (48), The maximum value of r is 1 and the minimum value of r is 0 . 5 . When r « 1 a l l the hydroxymethyl radicals react with the Mn*** complex. When r = 0 , 5 a l l the hydroxymethyl radicals react with 2,4,6-tri-t-butylphenol. Expressing (HMn*o*) in terms of (Mn***)tot, equation (49) is transformed into equation ( 5 0 ) . (See Section V I - i i i ) . - d(Mn***)tot/dt = rk t )(Mn***) t o t + ^ K" (H+)(PhOH)(Mn***)tot/(i+K' (H+)) (50) - 2k1(CH30H)K'(H+)/(l + K»(H+)) (41) Since (PhOH), (H+) » ( M n 1 1 1 ) ^ , k c - ^ IC (H+)(PhOH)/(l + IC (H+)) is a constant. Thus, equation (50) becomest - d O t a 1 1 1 ) ^ / * - (rk,, • k c ) ( M n m ) t o t (51) Equation (51) is identical to equation (45) with k a - r k b + k c ( 5 2 ) Integrating equation (51) gives equation (53)• l o g A M n - logA o = ( r ^ * k c)t / 2 . 3 0 3 ( 53) 93 (iv) Determination of rate constants The values of Ic^  and IC have already been determined in Section VI. A value of k was determined for each concentration of 2,4,6-tri-t-butyl-St phenol from the slope of a plot of log A M n versus time (equation (53))• Figure 17 is such a plot. A plot of k versus concentration of 2,4,6-tri-t-butylphenol (see equation (52)) gave a slope of k^K*(H+)/(l + K* (H+)) —2 —1 —1 —4 —1 = 1.80 x 10 M sec and an Intercept of rk^ = 9.0 x 10 sec (see Figure 19). Knowing K" and (H+) the value of k^ was calculated and found to be (1.1 j 0.1) x 10 _ 1 M"1sec~1 at 25°. At this concentration of acid (1.63 x 10"2 M), » 1.54 x 10 - 3 sec - 1 (from Table VIII) and this gives a value of r = O.58, This value of r close to 0.5 indicates that in the range of 2,4,6-tri-t-butylphenol concentration used (1.57 x 10 M to 9.22 x 10 M) the hydroxymethyl radicals react almost solely with 2,4,6-tri-t-butylphenol. This is understandable because the concentration of 2,4,6-tri-t-butylphenol is very much more than that of the Mn 1 1 1 complex (5.6 x IO - 4 M). (v) Kinetic isotope effect Oxidation of 2,4,6-tri-t-butylphenol with the Mn 1 1 1 complex in methanol-O-d and deuterium chloride was carried out at 25° to determine whether the 0-H bond of the phenol is cleaved in the rate-determining -2 -1 -1 step of the reaction. A value of 2.50 x 10 M sec was obtained fo: giving a kinetic isotope effect of 4.8, This value indicates that the 9 4 0-H bond of 2,4,6-tri-t-butylphenol is cleaved in the rate-determining step of the reaction. (vi) Activation parameters Oxidation of 2,4,6-tri-1>-butylphenol with the Mn 1 1 1 complex in the presence of hydrochloric acid in methanol was carried out at several temperatures in order to evaluate the values of the activation enthalpy (AH ) and activation entropy (AS ). The data used in the plot in Figure 20 were taken from Table XVIII. The values found for A H and A S are 15.7 Kcal.mole and -10.4 e.u. respectively. TABLE XVIII Variation of k„ with Temperature( C) kn(M~ sec" ) 25.0 (1.1 + 0.0) x 10' 30.4 (1.7 * 0.1) x 10' 35.8 (2.5 * 0.0) x 10' 40.0 (3.8 * 0.0) x 10" 45.0 (6.4 + 0.3) x 10' ,-1 ,-1 r l r i -1 - 2 . 0 - 2 . 3 -2.6 Figure 20. Plot of log k^ /T versus 1/T for oxidation of 2,4,6-tri-t-butylphenol with the Mn*11 complex in methanol in the presence of acid. AH* - 15.7 Kcal.mole""1 AS = - 1 0 . 4 e.u. Correlation coefficient = -0 .9948 o >1 •2.9 vO " 3 . 2 " 3 . 5 - 3 . 8 3.00 3.05 3.10 3.15 3.20 1/T (°K) x 1 0 3 3.25 3.30 3.35 3.40 96 (vii) Oxidations of phenol, benzaldehyde and benzoic acid III Oxidations of phenol, benzaldehyde and benzoic acid with the Mn complex in methanol in the presence of hydrochloric acid were carried out at 25° to determine the rates of oxidation of the unhindered OH, CHO and COOH functional groups. Each oxidation was carried out with excess acid and substrate. The values of k^  were obtained in a manner similar to that in Section VII-viii (below). The results obtained are shown in Table XIX. TABLE XIX Oxidation rates of phenol, benzaldehyde and benzoic acid at 25° (Mn111) - 6.0 x l O - 4 M (HC1) = I .63 x 10"*2M —1 —1 Substrate k^(M sec ) Phenol 42 _2 Benzaldehyde 1.1 x 10 Benzoic acid no reaction (viii) Substituent effect Oxidation of a number of 4-substituted-2,6-di-t-butylphenols with the Mn1*1 complex in methanol in the presence of hydrochloric acid was carried out at 25°. Oxidation was carried out with excess acid and phenol. The value of k^ for each phenol was determined from equation (52) with r = 0 .5 . The values of ^  are tabulated with the <Tp and c r " values In Table XX and the oxidation products are listed in Table XXI. 97 TABLE XX Rates, (T, and CT~ values of some 4-substituted-2,6-di-t-butylphenols (under acidic conditions at 25°) Compound Substituent ^(M^sec" 1) a cr - p - .SpL. log, k ^ u . XXIX OH > 84 -0.357 -0.357 > 2.9 XXIII t-Bu° 1.38 X 10""1 -0.197 -0.197 0.111 XX CH 3 2.90 X io' 1 -0.170 -0.170 0.433 XXVIII CH„OHd 2.17 X IO' 1 0.332 XXVII CHgOCHg-s^-OHe 0.80 X 10" 1 -0.126 XXXI 1.66 X IO" 1 0.191 XXXIV H 1.07 X IO" 1 0 0 0 XXIV CI 1.41 X 10" 1 +0.226 +0.226 0.120 XXII Br 1.45 X I O - 1 +0.232 +0.232 0.132 XXV I 1 .69 X 1 0 _ 1 +O.276 +O.276 0.199 XXXII C0C 6H 5 3.53 X IO' 1 +0.459 +0.459 0.515 XXX COOH 1.26 X IO" 1 +0.265 +0.728 0.071 XXVI COCH^ 5.19 X IO" 1 +0.516 +0.874 0.686 XXXIII CN 2 .50 X IO" 1 +0.628 +1.00 O.365 XXI CHO 20.7 +0.216 +1.126 2.286 a values taken from reference 841 b k is the rate constant for ] -2 2,6-di-t-butylphenol; c calculated for run with (HCl) = I . 6 3 x 10 M; d, e CT and C~ values are likely to be close to those of CH^  group (the value of k^ was corrected for a statistical factor of 2 for XXTOt -two equivalent OH groups); f the amino group is protonated, (HCl) «= 2(XXVIl). For a l l runsj (HCl) - i . 6 3 x 10_2M; (Mn 1 1 1) - 5.6 x lO ' V (PhOH) = 2 x 10"*2M (4.13 x 1 0 " \ used for XXI) 98 TABLE XXI Oxidation products* from oxidation of some 4-substituted-2.6-di-t- butylphenols with the Mn complex under acidic conditions in methanol Compound Product XXIX XIX XXIII XXXVIII : R - t-Bu XX XXXV XXVIII XXXVI, XVIII XXVII XVIII, XXXV XXXI XXXVI, XVIII XXXIV XXXVI, XVIII XXIV XXXVII R = CI, XVIII XXII XXXVII R » Br, XVIII XXV XVIII XXXII XXXVIII R « COCgH^  XXX XXXVI, XVIII XXVI XXXVIII R = COCH^  XXXIII XXXIII, XVIII XXI XXXVI, XVIII * XXXVIII detected "by esr spectrometry XIX detected "by spectrophotometry (strong absorption band at 320 nm) XVIII detected by spectrophotometry (strong absorption band at 419 nm) 99 * XXXVI and XXXVII detected indirectly - by oxidising them to XVIII XXXV isolated as a colourless solid, m.p. 97-98° (Lit. 98-99°)8 55 nmr spectrum in CCl^ chemical s h i f t (<S, ppm) number of H's This work 1 .43 3 .25 4.18 4.97 6.97 Reference 85 1.40 3.24 4.21 5.00 7.00 This work 18 3 2 1 2 Reference 85 18 3 2 1 2 OH OH R 0 XXXVI XXXVII XVIII 0 XIX OH CHgOCH, XXXV R XXXVIII 100 (ix) Discussion In the proposed mechanism, 2,4,6-tri-t-butylphenol and the Mn*** complex compete for the hydroxymethyl radicals. If this is true, a plot of k versus the concentration of 2,4,6-tri-t-butylphenol is expected to Or show a minimum value of k & at a certain concentration of the phenol because the two terms, rk^ and k , oppose each other (see equation (52)). As the concentration of 2,4,6-tri-t-butylphenol is increased k will c also increase because this term is proportional to the concentration of 2,4,6-tri-t-butylphenol. But rk^ will decrease since the hydroxymethyl radicals will more likely react with 2,4,6-tri-t-butylphenol. When the concentration of 2,4,6-tri-t-butylphenol is decreased, rk^ will increase and k Q decrease. Thus, a minimum value of k a is expected at a certain concentration of 2,4,6-tri-t-butylphenol. Figure 21 is such a plot and i t indeed shows the expected shape. Data used in Figure 21 were taken from Table XXII. At equal concentration of the Mn*** complex and 2,4,6-tri-t-butylphenol r was calculated and found to be 0,7. This value implies that hydroxymethyl radical reacts faster with 2,4,6-tri-t-butylphenol (by abstracting a hydrogen atom from 2,4,6-tri-t-butylphenol) than with the Mn*** complex (by transferring a hydrogen atom to the Mn*** complex). Using this value of r, k-/(kg + k^) was found to be 1.5. 101 TABLE XXII Variation of with concentration of 2,4.6-trl-t-butylphenol at 25°  (includes data from Table XVI) (Mn111) = 5.6 x 10~ 4 M (HCl) - I.63 x 10~2 M (PhOH)*(M) k^sec' 1) 3.4? x 10" 6 1.51 x IO"3 3.47 x 10" 5 1.38 x 10" 3 5.53 x 10~ 4 1.12 x 10" 3 5.29 x 10~3 1.04 x 1 0 _ 3 7.70 x 10~3 1.13 x IO"3 9.70 x 1 0 - 3 1.14 x IO'3 1.57 x IO"2 1.17 x 10" 3 3.00 x 10" 2 1.50 x 10~ 3 5.10 x 10~2 1.74 x 10~ 3 6.64 x IO - 2 2.12 x IO"3 7.74 x IO""2 2.30 x 10" 3 9.22 x 10~2 2.69 x 10~3 * PhOH ls used as an abbreviation for 2,4,6-tri-t-butylphenol. 6.0 5 . 0 4.0 Figure 21, Observed rate constant, k , versus concentration of 2,4,6-tri-t-butylphenol. Part ( i i ) . (Mn111) = 5.6 x I0~k H (HCl) - 1.63 x 10' Temperature ="25° -2 o 0) w o 3 , 0 X 0.0 1.5 3.0 4.5 6.0 7.5 9.0 T0T5" Concentration of 2,4,6-tri-t-butylphenol x 10 M T2.0 103 That the oxidant is not Mn***CyDTA(H20)"", but a different form of the Mn*11 complex, is indicated by the very slow oxidation of 2,4,6-tri-III t-butylphenol with the Mn complex in methanol when no acid is present (see Section IX). This more powerful Mn*** complex present in acidic medium is probably a protonated species as indicated by the reversible change of its visible spectrum in methanol (see Section IV-v), by the curved plots of observed rate constant versus acid concentration (see Figures 12 and 18), and by the equilibrium isotope effect (see Section Vl-vii). The effect on the oxidation rate of varying the 4-substituent (see III Table XX) rules out electron-transfer from hindered phenol to Mn complex. If electron-transfer were involved in the oxidation reaction the transition state would look like XXXIX and electron-withdrawing groups should retard the reaction. + HO Oxidant 0« • »H» •'Oxidant R XXXIX XXXX The similar oxidation rates of most 4-substituted-2,6-di-t-butylphenols 104 suggest that a radical intermediate (XXXX) is formed in the transition state. Both electron-withdrawing and electron-donating substituents will stabilise this transition state and their effects on the rate are usually small. A Hammett plot of log k0/k_„ versus (f~ correlates the substituent J j"- P effects as t e l l as that of log k 0/k 0„ versus <T (k 0 is the rate constant •i p } for a 4-substituted-2,6-di-t-butylphenol and k o u is the rate constant for 2,6-di-t-butylphenol). Figures 22 and 23 are such plots. It can be seen from Figures 22 and 23 that the slopes are practically zero (0.23 and 0,38 respectively), indicating the effects of substituents are very small. Two 4-substituted compounds, the 4-hydroxy and 4-formyl compounds, deviate considerably from the straight line plots In Figures 22 and 23. For the 4-hydroxy compound i t is the unhindered OH group that is oxidised III by the Mn complex, (it reacts 420 times faster than the hindered OH group - compare the rates of 2,6-di-t-butylphenol and phenol in Tables XIX and XX). The point for the 4-hydroxy substituent has thus been omitted from the plots. The rate of the 4-hydroxy substituent is too fast to be measurable, but k^ was estimated to be greater than 84 M'^sec"1. It is not clear why the 4-formyl compound has an anomalously high rate. The unhindered CHO group is about ten times less reactive than the hindered OH group (compare the rates of benzaldehyde and 2,6-di-t-butyl-phenol in Tables XIX and XX). It is therefore not likely that the CHO 3.0 2.4 Figure 22. Plot of log k0/k_„ versus <f" j P slope - Q. =» 0.23 CHO O 1.6 w cn cn •* 0.8 W) o H3 o.o u 0 C H 3 GOCzH. O C O C H . t-Bu O COOH -0.8 h -1 .6 J 1 I I 1 L -1 .2 -0 .9 -0.6 -0.3 0.3 0.6 0.9 1.2 3.0 107 group i s o x i d i s e d i n s t e a d of the OH group unless the £ value of the r e a c t i o n i s more negative than -3 .8 . A s i m i l a r o x i d a t i o n of the 4-formyl I I I compound w i t h the Mn complex I n methanol-O-d i n the presence of deuterium c h l o r i d e showed a k i n e t i c i s o t o p e e f f e c t of 4.9 (the value of k^ i n methanol-O-d was found t o he 4,2 M""*sec -*), thus i n d i c a t i n g t h a t i t i s the OH group t h a t i s o x i d i s e d by the Mn 1* 1 complex. (Compare t h i s k i n e t i c i s otope e f f e c t w i t h t h a t of 2 , 4 , 6 - t r i - t - b u t y l p h e n o l below). The observed k i n e t i c isotope e f f e c t (kou/k„_.) of 4,8 f o r the o x i d a t i o n of 2 , 4 , 6 - t r i - t - b u t y l p h e n o l i n d i c a t e s t h a t 0-H bond i s cleaved i n the rate-determining s t e p of the r e a c t i o n . Since the r a t e i s f i r s t order w i t h r e s p e c t t o the c o n c e n t r a t i o n of HMn'^CyDTA^O) and f i r s t order w i t h r e s p e c t t o the c o n c e n t r a t i o n of 2 , 4 , 6 - t r i - t - b u t y l p h e n o l , these two molecules form an a c t i v a t e d complex which i n v o l v e s the b r i d g i n g of the OH group of the hindered phenol and probably a l i g a n d of the Mn*1* complex. The OH group of 2 j 4 , 6 - t r i - t - b u t y l p h e n o l can be br i d g e d v i a the hydrogen atom t o e i t h e r the coordinated water molecule or a carbonyl group of HMn***CyDTA(H20). Water and carboxylate carbonyl l i g a n d s are w e l l known i n t h e i r f u n c t i o n as b r i d g i n g l i g a n d s i n 'inner-sphere' e l e c t r o n - t r a n s f e r r e a c t i o n s . (An ' inner-sphere' e l e c t r o n - t r a n s f e r r e a c t i o n i s one i n which the two r e a c t a n t s i n v o l v e d are b r i d g e d through a common atom or l i g a n d d u r i n g the e l e c t r o n - t r a n s f e r process and bond 86—88 b r e a k i n g occurs i n the r e a c t i o n ) . A v a i l a b l e data i n d i c a t e t h a t a coordinated water molecule i s b e t t e r than a non-conjugated carb o x y l a t e 108 carbonyl group as a bridging group in the ' inner-sphere* electron-transfer reactions (see Table XXIII). The three carbonyl groups of the three carboxylate ligands which are coordinated to the Mn*** ion and the carbonyl group of the free carboxylic arm are not likely to differ from the carboxylic ligands shown in Table XXIII in their abilities to bridge with a reactant in an electron-transfer reaction. Therefore, the coordinated water ligand of HMn***CyDTA(H2 0) is likely to be the ligand involved in the hydrogen atom transfer reaction. The reaction mechanism involved may be pictured as» (i) the formation of an activated complex between HMnIIICyDTA(H20) and 2,4,6-tri-t-butylphenol as represented by XXXXI. The hydrogen atom is then transferred from 2,4,6-tri-t-butylphenol to the coordinated water ligand to form intermediate XXXXII and 2,4,6-tri-t-butylphenoxyl in a rate-determining step, (ii) Intermediate XXXXII quickly loses a proton to give HMn^CyDTA^O)-. HMn CyDTA(H20) XXXXI + HMn**CyDTA(H_0+) XXXXII HMn**CyDTA(H20)" + H+ <e 109 In the activated complex, XXXXI, the oxygen atom of the phenol has radical character and such a structure is consistent with the small effect on the reaction rate exerted by 4-substituents. It was reported in Section VI that a kinetic isotope effect of 1.3 results from replacing the coordinated water ligand by deuterium oxide ligand. Taking this isotope effect contribution into account the kinetic isotope effect due to the 0-H bond cleavage is 3»7« TABLE XXIII Comparison of rates of reduction by Cr 2 4' of Co***(NH3)^L complexes  by 'inner-sphere' mechanism at 25° and ionic strength of 1.0 —1 —1 L k(M~ sec"" ) Reference HgO 0.56 89 CH3C00" O.35, 0.18 88, 90 CH3(CH2)2C00" 0.08 91 CH3CH=CHC00~ 0.18 91 (CH^CCOO" 0.009 92 C^COO" 0.15 88 HOgCCHgCHgCOO" 0.19 90 110 The oxidation of compound XXXI (2,6^di-t-butyl-4-dimethylamino-methylphenol) has an interesting feature. When the concentrations of acid and XXXI are equal the reaction with the Mn 1 1 1 complex is very slow. When acid is in excess the reaction occurs at a normal rate, that is, similar to those of the other 4-substituted-2,6-di-t-butylphen61s. In the absence of acid the reaction is very rapid (very much faster than the reaction III between 2,4,6-tri-t-butylphenol and the Mn complex in the absence of acid). These observations can be explained as follows. When the concentrations of XXXI and acid are equal a l l the acid is taken up by the amino group of the 4-substituent of XXXI. The reaction is then between MnIIICyDTA(H20) and the protonated form of XXXI (see equations (54) and (55)). Since MnII][GyDTA(H20)~ is a poor oxidant compared to HMnIIICyDTA(H20) the reaction observed is slow (see Section IX). OH OH XXXI XXXXIII XXXXIII + Mn CyDTA (HgO)" products (55) I l l When the concentration of acid is more than that of XXXI the TIT reaction is "between XXXI and HMn CyDTA(HgO) and the rate is of the right magnitude expected for (see Table XX). I l l — In the absence of acid XXXI removes a proton from Mn CyDTA(HgO) to form XXXXIII and MnIi:CCyDTA(0H)2~, the latter is a more powerful TTT -» oxidant than Mn CyDTA (HgO)"* and thus the reaction is rapid (see equations (56) and (57)). (Also see Sections VIII and IX for evidence showing MnI3:iCyDTA(OH)2" to be a stronger oxidant than MnmCyDTA(H20)-). MnIIICyDTA(H20)- + XXXI ^ MnI]CICyDTA(OH)2- + XXXXIII (56) MnmCyDTA(0H)2- + XXXI and/or XXXXIII > products (57) Oxidation of 4-substituted-2.6-di-t-butylphenols with the Mn complex in methanol in the presence of hydrochloric acid gave the corresponding 4-substituted-2,6-di-t-butylphenoxyls. The esr spectra of the following 4-substituted-2,6-di-t-butylphenoxyls were recorded! 4-t-Bu, 4-COCH^  and 4-C0C^ H^ . The esr spectra of 4-CH^  and 4-CN radicals were recorded under basic and neutral conditions but not under acidic condition because they reacted rapidly under the latter condition. The 115 116 other 4-suhstituted-2,6-di~t-butylphenoxyls reacted rapidly and they could not be detected by esr spectrometry. The probable reaction steps involved for the different 4-substituted-2,6-dl-t-butylphenols in the oxidation reactions are shown below. A l l substituents XXXVIII R =» H, CI, Br, I (see reference 45) 2 XXXVIII XXXVII XVIII 1 1 7 R = OH OH 0 XXXIII R » CH- (see reference 85) 0 0 OH CH. CH- CH„OCH. XXXV 118 CHgOCH. XXXV 120 VIII. OXIDATION OF 2,4,6-TRI-t-BUTYLFHENOL WITH THE Mn 1 1 1 COMPLEX IN METHANOL* IN THE PRESENCE OF BASE (i) Product 2,4,6-Tri-t-butylphenoxyl was formed quantitatively as seen from its absorbance at 400 nm (Figure 28), (ii) Analysis of kinetic results III Oxidation of 2,4,6-tri-t-butylphenol with the Mn complex was carried out with excess 2,4,6-tri-t-butylphenol and tetramethylammonium hydroxide present. Plots of logarithm of A^n versus time are linear up to more than 80 £ of reaction, indicating that the rate of disappearance of the Mn 1 1 1 complex can be expressed by equation (58). Figure 29 is a typical plot of equation (59). - d(«n m) t o t/dt = * , < M „ m ) t o t (58) Integrating equation (58) gives equation (59). log A M N - log A q = - kt/2.303 (59) III III where (Mn " total Mn complex concentration k - pseudo-first order rate constant T T T A„ => absorbance of the Mn complex at 486 nm at time t Mn A Q « absorbance of the Mn*11 complex at 486 nm at time zero. * 2,5 £ water (v/v) Wavelength (nm) -0.6 -1.8 I 1 I I I 1 I I I 0 2 k 6 8 10 12 14 16 Time (min.) 123 The value of k Q was found to increase slightly with an increase in the concentration of tetramethylammonium hydroxide while the concentrations of the Mn11* complex and 2,4,6-tri-t-butylphenol were kept constant (see Table XXIV). TABLE XXIV Variation of k. with base concentration at 25° e (Mn111) - 5.6 x IO"4 M (PhOH)* - 2.52 x lO" 2 M (TMA) (M) k>ec 1) 0.54 x 10"2 1.95 x 10~3 0.90 x 10~2 2.15 x 10"3 1.78 x 10~2 2.34 x 10~3 2.65 x 10"2 2.60 x 10"3 4.46 x 10"2 2.75 x iO" 3 * PhOH - 2,4,6-tri-t-butylphenol ** TMA = tetramethylammonium hydroxide III When the concentrations of the Mn complex and tetramethylammonium hydroxide were kept constant, k was found to increase linearly with the concentration of 2,4,6-tri-t-butylphenol (Table XXV and Figure 31). 6 Figure 30' Observed rate constant, versus base concentration. (Mn111) - 5.6 x 10" 4 H (PhOH) - 2.5 x 10"2 M Temperature =25° o <D CO m o X A! ro 1 _L 2 3 4 5 Concentration of tetramethylammonium hydroxide x 102 M 125 TABLE XXV Variation of k_ with concentration of 2,4,6-tri-t-butylphenol at 25° (Mn 1 1 1) « 5.6 x 10"4 M (TMA)* - 1.78 x IO - 2 M (PhOH) (M) ke ( s e c " 1 ) 0.43 x IO"2 0.68 x 10~3 0.87 x 10"2 0.94 x IO"*3 1.26 x 10"2 1.16 x 10~3 1.94 x 10"2 1.96 x 10"3 2.52 x 10"2 2.34 x IO"3 2.92 x IO"*2 2.65 x IO"3 * TMA • tetramethylammonium hydroxide ** PhOH - 2,4,6-tri-t-butylphenol When the concentrations of 2,4,6-tri-t-butylphenol and base were kept constant, and the concentration of the Mn*** complex was varied, k g was found to remain almost constant (see Table XXVI), indicating that the rate of disappearance of the Mn*** complex is indeed f i r s t order with respect to its own concentration. 127 TABLE XXVI Variation of k with concentration of the Mn*** complex at 25° e c  (TMA) - 1.78 x 10 M (PhOH) - 2.52 x 10 M (Mn***)(M) M s e c " 1 ) 4.5 x 10~4 2.30 x 10" 3 5.6 x IO**4 2.34 x IO"3 7.5 x l O - 4 2.20 x 10" 3 11.0 x l O " 4 2.48 x 10" 3 * TMA = t e t r a m e t h y l a m m o n i u m h y d r o x i d e ** PhOH - 2 , 4 , 6 - t r i - t - D u t y l p h e n o l ( i i i ) Mechanism The f o l l o w i n g r e a c t i o n s a r e c o n s i d e r e d . *2 Mn*** + 0H~ s Mn*** + rLO (60) P ^ y * K PhOH + 0H~ „ PhO" + H20 (14) 2 Mn*** + 4 OH" > Mn1* + Mn02 + 4 HgO (24) 128 *8 Mn*-- + PhO~ > Mn11 + PhO' (61) k 9 TTT 7 TT 4-MiT~ + PhOH > Mn A + PhO. + H (62) TTT J TT 4-Mn - + PhOH > Mn 1 + PhO' + H (63) P ^0 Mn1-1 + PhO" > Mn11 + PhO- (64) P Abbreviations used: Mn*-- = Mn***CyDTA(0H)2-Mn*-* - Mn***CyDTA(H„0)-P PhOH - 2,4,6-tri-t-butylphenol PhO- = 2,4,6-tri-t-butylphenoxy anion PhO« = 2,4,6-tri-t-butylphenoxyl Equation ( 6 3 ) can be ignored because the reaction between 2,4 , 6-tri-t-butylphenol and Mn***CyDTA(H20)"" is very slow (see Section IX). Equation (64) can also be ignored because in the presence of excess base TTT mm the concentration of Mn CyDTA(H20)~ is negligible. Therefore, the rate of disappearance of the Mn*** complex is given by equation ( 6 5 ) . 129 - d (Mn m) t o t/dt = 2k?(0H")(Mnp) + kg(PhO~)(Mn^) + k^ FhOHKMn*.*^ ) (65) In excess "base, the total Mn*** complex concentration, (Mn***)^^, is equal to (Mn***), and using the relationship in equation ( l5)i equation (65) can "be written in the form of equation (66), - d(Mn***)/dt - ( 2^(0"") + kgK(PhOH)(OH") + k^PhOH) )(Mn***) (66) Equation (66) is Identical to equation (58) with k g - 21^(0^) + kgK(PhOH)(OH") + k^PhOH) (67) Integrating equation (66) gives equation (68). log A M N - log A o - - kt/2.303 (68) The rate of formation of 2,4,6-tri-t-butylphenoxyl was followed at 400 nm. It is given by equation (69). d(PhO.)/dt= ( kgK(Ph0H)(0H") + ^ (PhOH) )(Mn***) (69) 130 Let k r * kgK(PhOH)(OH-) + k9(PhOH) (70) and from equation (68) (Mn*--) can be expressed as follows: (Mnjii) » (Mn^) Qexp(-k et) (71) where (Mn**-)Q is the i n i t i a l concentration of the Mn*** complex. Substituting equations (70) and (71) into equation (69), equation (72) is obtained. d(PhO')/dt = kr(Mn***)oexp(-kt) (72) Integrating equation (72) gives equation (73)» A . = £ k (Mn*--) (1 - exp(-k t))/k (73) pho. p r x y 'ov ^ v e e v where £^ = extinction coefficient of 2,4,6-tri-t-butylphenoxyl at 400 nm, ^pho» ~ a t s o r l 3 a n c e o f 2,4,6-tri-t-butylphenoxyl at 400 nm at time t. (iv) Determination of rate constants A value of k was obtained for each concentration of base from the e slope of a plot of equation (68) (see Figure 29). A plot of k g versus 131 base concentration gives a slope of 2k^ + kgK(PhOH) and an intercept of kg(FhOH) (see equation (67)). Figure 30 is such a plot. The values of K and have been determined in Sections IV and V respectively, (PhOH) is a known quantity, thus, kg and kg can be evaluated. The values of kg and -1 -1 -2 -1 -1 kg were found to be 2.0 M sec and 7.5 x 10 M sec respectively. The rate constant k g was also plotted against concentration of 2,4,6-tri-t-butylphenol (see equation (67) and Figure 31) "to check the values of kg and kg obtained above. From this plot, kg + kgK(0H~) and 2k7(0H") were found to be 8.3 x IO*"2 M _ 1sec~ 1 and 2.5 x 10*"4 sec - 1 respectively. Using the values of kg and kg obtained above, kg + kgK(0H~) — —2 —1 —1 and 2k,p(0H ) were calculated and found to be 8.4 x 10 M sec and -4 -1 1.7 x 10 sec respectively. These values agree quite well with those calculated above. For each run, the value of k g obtained from a plot of equation (70) was used to calculate the term l-exp(-ket) in equation (73)• Then, Ap h Q t was plotted against l-exp(-k t), and such a plot was found to be linear up to about 60 $ of reaction. Figure 32 is a typical plot of equation (73). From each such plot a value of k was calculated. Then a plot of k r r versus base concentration (see equation (70) and Figure 33) was done to compare the values of kg and kg obtained from this plot with the values obtained previously. The values of kg and kg obtained from this plot were -1 -1 -2 -1 -1 found to be 2.7 M sec and 6 x 10 M sec , respectively. Although 135 these values do not agree well with those obtained earlier they are of the correct order of magnitude. A plot of k versus concentration of 2,4,6-tri-t-butylphenol was also made (see Figure 3*0 • The slope of this plot gives a value of kg + k^K(0H ) -2 - i - i -2 » 7.2 x 10 M sec , which is slightly lower than the value of 8.4 x 10 M^sec""* obtained previously, (v) Kinetic Isotope effect III Oxidation of 2,4,6-tri-t-butylphenol with the Mn complex was carried out in methanol-O-d in the presence of different concentrations of sodium deuteroxide at 25°. The values of the different rate constants were obtained using equations (67) and (68), (The value of k^ was obtained III from the decomposition of the Mn complex in methanol-O-d in different concentrations of sodium deuteroxide at 25°). Oxidation of the same phenol with the same oxidant in methanol in the presence of different concentrations of sodium hydroxide was also carried out at 25° and the values of the rate constants were obtained for comparison. The results are summarised in Table XXVII. 136 TABLE XXVII Rates for oxidation of 2,4,6-tri-t-butylphenol with the Mn 1 1 1 complex  in methanol and methanol-O-d in the presence of base at 25° Solvent Base CH^ OH NaOH CH^ OD NaOD (vi) Effect of cations on rate constants III Oxidation of 2,4,6-tri-t-butylphenol with the Mn complex was done in different hydroxides to determine the effect of cations on the rate constants. For each hydroxide, oxidation was carried out at three different concentrations of base and the values of kg and k^ were * obtained using the procedure described above (Section VIII-v). The results are summarised in Table XXVIII. ;0(M" sec' ) k^M^sec" 1) k8H/Js8l) ^ 9 H ^ 9 D „-l -1 -10.0 7.5 9.7 x 10' 5.0 x 10' -2 -2 1.3 1.9 137 TABLE XXVIII III Rates for oxidation of 2,4,6-tri-t-butylphenol with the Mn complex  in the presence of different hydroxides in methanol at 25° Base -1 -1 kr, (M sec ) k^(M~*sec~*) KOH 10.9 14.8 x 10"2 74 NaOH 10.0 9.7 x 10"2 103 Li OH 9.7 6.4 x IO"2 151 (C3H?)4N0H 11.0 6.7 x 10~2 164 ( n- c4 H 9 V 0 H 7.6 7.0 x IO"2 109 (CH^NOH 2.0 7.5 x 10""2 27 average KgA^  = 104 £ XXX ~ * r Base concentration at 10 M{ Mn complex concentration at 10 M; -2 / -4 2,4,6-tri-t-butylphenol concentration at 10 M (about 10 M for ionised form). 138 (vii) Effect of substituent on rate constant Oxidation of a number of 4-substituted-2,6-di-t-butylphenols with the Mn1** complex was carried out at 25° to determine the substituent effect on the rate of oxidation. The concentration of tetramethylammonium III hydroxide was kept equal to that of the Mn complex so that the concentrations of the 4-substituted-2t6-di-t-butylphenoxy anions were negligible and equation (62) represents the only important reaction. (Those 4-substituted-2,6-di-t-butylphenols with pK values close to that ct of Mn***CyDTA(H20)"" were not studied. They were found to react too rapidly). The rate of disappearance of the Mn*** complex can be expressed by equation (?4). - d(Mn***)tot/dt - k9(PhOH)(Mn***)tot (74) III In the presence of a limited amount of base, (Mny—) is not equal to (Mn***)tot. But (Mn***) can be expressed in terms of (Mn***)tQt using equations (60) and (75). Thus, equation (?4) can be written as equation (76). (Mn***) t Q t = (Mn*i*) + (Mn**i) (75) - d(Mn***)tot/dt - K2(OH"*)k9(PhOH)(Mn***)tot/(l + ^(OH*)) (76) 139 The term K2(0H")/(1 + ^(OH -)) is a constant for a given concentration of base. Since equation (77). (PhOH) » (Mn***).^, equation (76) can be expressed as - * ( M n m ) t o t / d t - ^ ( M n 1 1 1 ) ^ (77) = K2(0H")k9(Ph0H)/(l + Kj(QH")) (78) Integrating equation (77) yields equation (79). l o g A ^ - logA o = - k^t/2.303 (79) where A M n « total absorbance of the Mn 1 1 1 complex at 500 nm at time t, A Q « total absorbance of the Mn1** complex at 500 nm at time zero, The value of k^ for each 4-substituted-2,6-di-t-butylphenol was obtained from a plot of equation (79). The value of ^ ( O f O k^Al + K^OH")) was obtained by dividing the value of k^a by the concentration of the phenol. In order to obtain the value of k ,^ the value of the term, K2(0H")k9/(l + K^ OH."")), for 2,4,6-tri-t-butylphenol was determined under identical conditions. Since the value of k^  for this compound has already been determined, the factor KgColO/O- + K^OH-)) could be calculated and thus, the values of k^  for the other compounds could be evaluated. 140 The results are shown in Table XXIX, The products obtained were identical to those obtained under acidic conditions (see Section VTI-viii). A Hammett plot of log k^/k^ versus cr~ gives a Rvalue (slope of the plot) of 2.3 (see Figure 35). TABLE XXIX Rates of oxidation of some 4-substituted-2,6«-di-t-butylphenols with the Mn 1 1 1 complex in methanol in the presence of a limited amount of o base at 25 Substituent 1 1 k9(M~ sec" ) log kg/k^ t-Bu 7.5 x 10"2 -0.197 -0.545 CH3 23.9 x 10"2 -0.170 -0.042 H 26.3 x 10"2 0 0 Br 75.3 x 10"2 +0.232 +0.457 CI 80.8 x 10"2 +0.226 +0.487 * k ^ is the rate constant for 2,6-di-t-butylphenol. (viii) Activation parameters Oxidation of 2,4,6-tri-t-butylphenol with the Mn 1 1 1 complex was studied at other temperatures so that the values of the activation 142 enthalpy and entropy could be determined. The data used in the plots in Figures 36 and 37 were taken from Table XXX. The values of AH and AS for kg and k^ were found to be 10 .2 Kcal.mole-* and -23 e.u.5 10.2 Kcal. -1 mole and -29 e.u. respectively. TABLE XXX Variation of kg and kg with temperature Temperature(°C) kg (M-1sec"1) k^CM'^sec"1) 25 2.0 7.5 x 10"2 30 2.9 11.4 x 10"2 35 3.6 13.6 x 10"2 40 4.9 18.6 x IO"2 (ix) Oxidation of 2,4.6-tri-t-butylphenol with the Mn complex in methanol in the presence of t-butylamine The kinetic results of the oxidation of 2,4,6-tri-t-butylphenol with the Mn 1 1 1 complex in methanol in the presence of strong bases and the interesting observations seen in 2,6-di-t-butyl-4-dimethylaminomethylphenol T T T P _ (see Section VH-ix), a l l indicate that Mn CyDTA (OH) is a strong oxidant for unionised phenols. To further support this finding, oxidation of 2,4,6-tri-t-butylphenol with the Mn*** complex in methanol in the presence of t-butylamine was carried out. Since t-butylamine is a weaker -1.8 -1 .9 -2.0 -2 .1 so o t-3 -2.2 -2.3 •2.4 3.00 Figure 36. Plot of log kg/T versus l/T for oxidation of 2,4,6-tri-t-butylphenoxy anion with the Mn*** complex under basic condition. * . -1 AH = 10.2 Kcal.mole AS = -23 e.u. Correlation coefficient » -0.9958 3.05 3.10 3.15 3.20 1/T (°K) x 103 3.25 _ 1 3.30 3.35 3.40 145 base than the hydroxides the ratio of the concentrations of MnII:CCyDTA(0H)2" and MnIi:ECyDTA(H20)" can be varied by the addition of different concentrations of t-butylamine while maintaining the concentration of 2,4,6-tri-t-butylphenol in its unionised form. A rate expression (equation (80)) for the disappearance of the Mn 1 1 1 complex can be derived in a procedure similar to that shown in Section VIII-vii. - d ( M n m) t o t/dt - k 9 b(PhOH)(Mn m) t o t (80) where k^ - K^t-BuM^k^t-Bufo^) + K^t-BuNH^)) (81) and - (Mny--)(t-BulH3)/(MnpII)(t-BuNH2) (82) A plot of the values of k ^ against the concentration of t-butylamine is expected to be a curve which flattens off when the concentration of t-butylamine reaches the point that MnIIIGyDTA(H20)~ is completely III 2— converted to Mn CyDTA(OH) . (Compare with Figure 13 of Section VI). Such a curved plot is indeed obtained (see Figure 38)• The plot in Figure 38 does not flatten but rises gradually because the reaction shown in equation (61) becomes important at high concentrations of t-butylamine. (The values used in the plot in Figure 38 were taken from Table XXXI). The value of k^ was obtained by extrapolating the linear portion of the -2 -1 -1 plot to zero concentration of t-butylamine, A value of 7.2 x 10 M sec 147 was obtained. This value agrees with those obtained earlier (see Table XXVIII). TABLE XXXI Variation of observed rate with concentration of t-butylamine for III the oxidation of 2,4,6-tri-t-butylphenol with the Mn complex in  methanol at 25° (Mn 1 1 1) - 6.0 x 10" 4 M (PhOH)* = 4.02 x l O - 2 M (t-BuNHj(M) Of 1 s e c " 1 ) 1.5 x 10*"3 3.04 x 10~2 7.4 x 10*"3 5.55 x IO"2 7.8 x 10" 2 7.74 x 10"2 15.5 x 10" 2 8.26 x IO*"2 30.5 x 10~2 9.31 x IO*"2 * PhOH is used as an abbreviation for 2,4,6-tri-t-butylphenol. (x) Discussion The kinetic results of the oxidation of 2,4,6-tri-t-butylphenol agree with a mechanism involving the oxidation of 2,4,6-tri-t-butylphenol TTT 2 and 2,4,6-tri-t-butylphenoxy anion by Mn CyDTA(OH) . The former is 148 probably a hydrogen atom transfer reaction whereas the latter is an electron-transfer reaction. The kinetic isotope effect of kgH/kgD « 1.3 is likely due to the replacement of the hydroxide ligand by a deuteroxide ligand. (Taube and co-workers 8 6' 8 0' 9 3 have found that CofNHj^CHgO)^ 2+ and Co(NH3)^(0H) are better oxidants in water than in deuterium oxide whether electron-transfer reactions occur via a 'bridged activated' complex or an ' outer-sphere' mechanism. (An ' outer-sphere' mechanism is one in which the two reactants maintain their own complete coordination shells in the activated complex, and the electron must tunnel through both of these shells. Thus, no bond is broken in this mechanism). The kinetic isotope effects observed by them were near two. No satisfactory reason has been given to account for the slower rates observed when a coordinated water (or hydroxide) ligand is replaced by a deuterium oxide (or deuteroxide) ligand). The kinetic isotope effect of kg^/k^p = 1.9 after accounting for the solvent contribution (1.3) is 1.5 which is rather small for a reaction which involves the cleavage of the 0-H bond of 2,4,6-tri-t-butylphenol in the rate-determining step. A possible XXX 2— reason for this small kinetic Isotope effect is to regard Mn CyDTA(OD) T T T 0 _ as a stronger oxidant than Mn CyDTA(OH) with respect to hydrogen atom abstraction. Deuteroxide is known to be a stronger base than hydroxide"'7 6 1, III Therefore, i f the hydroxy and deuteroxy ligands of the Mn complex are regarded as analogous to 0H~ and 0D~ respectively, and their abstractions of hydrogen atom from the 0-H group of 2,4,6-tri-t-butylphenol are also 149 regarded as analogous to proton abstractions by 0H~ and OD", then MnmCyDTA(0D)2" will be a stronger oxidant than MnmCyDTA(0H)2~. If the above assumptions are correct, then the true kinetic isotope effect of k^/k^jj is greater than the value observed. The above possible reason does not contradict with the fact that C O(N H-)_(H 00) 3 + and Co(NH0)c(OH)2+ are better oxidants in water than in deuterium oxide as observed by 86 89 93 Taube and co-workers ' because the reactions studied by them are electron-transfer reactions and not hydrogen atom transfer reactions. The kinetic isotope effect of kguAgj) ~ 1«3 i s probably true because an • electron-transfer reaction is involved. The ratio of kgAg gives the relative rate of electron-transfer from 2,4,6-tri-t-butylphenoxy anion versus hydrogen atom transfer from 2,4,6-tri-t-butylphenol, to MnmCyOTA(0H)2~. An average value of 104 is obtained (see Table XXVIII). As can be seen from Table XXVIII, cations seem to affect the values of the two rate constants, kg and k^. The trend in the metal cations indicates that the larger the cation the larger are the two rates. The values of kg and k^ for the tetraalkylammonium cations are too random for any generalisation to be drawn. On the whole, the effects are small. 94-97 Other w o r k e r s h a v e also reported the catalytic effect of cations on electron exchange reactions between two anions. In general, the larger the metal cation the larger is the rate of electron exchange between 150 two anions, but the change is usually small. The role of cations in an electron exchange reaction between two anions is s t i l l not well 94 understood. One proposal is that the electron is actually transferred by the cation which serves as a bridge between the two anions. In this mechanism the polarizability of the cation and its ability to complex with the anions should play a major role in affecting the reaction. The greater polarizability of the larger cations agrees with their generally greater catalytic effect and supports this mechanism. Electron-withdrawing substituents at the 4-position enhance the rate of oxidation (see Table XXIX). It is difficult to say whether this trend is due entirely to the effect of the 4-substituent on the rate of oxidation or is partially due to the ease of ionization of the 0-H group when the 4-substituent is an electron-withdrawing group. The fast rate of reactions of most other 4-substituted compounds prevented a more extensive investigation of 4-substituent effect on reaction rates. The larger rate of oxidation of 2,4,6-tri-t-butylphenol by MnmCyDTA(0H)2~ compared to its reaction with MnmCyDTA(H20)~ (see Section IX) suggests that an 'Inner-sphere'-type mechanism is involved. (The mechanism is not exactly 'inner-sphere' because the hydrogen atom from the phenol is not transferred to the coordination sphere of the oxidant). 98 99 / Taube and co-workers have suggested that large values of k^/k^ ^ are to be associated with ' inner-sphere' reactions whereas small values of this ratio are to be associated with ' outer-sphere' reactions. (The two rate constants, k_u and Ic. n, refer to the hydroxo- and aquo- complexes 151 respectively). The ratio of k^/k^ found in this work is about 3^0. Some values of relative rate between hydroxo- and aquo- complexes in 'inner-sphere' reactions are shown in Table XXXII. TABLE XXXII n+ Values for the relative rate of reduction of MLJ^O complexes  compared to ML^ OH^ "7"^ * complexes (' Inner-sphere' mechanism) MLr-H^ O Reductant -OH^H 0 Reference Co(NH 3) 5H 20 3 + C r 2 + 3.5 x 10 6 89 Cu" 3.8 x 10 5 100 101 FeOigO)3^ Cr 2 + 7.7 x 10 3 Fe 2 + 1.3 x 10 3 102 Cu(H 20) 2 + V3* 9.4 x 10 3 103 152 IX. OXIDATION OF 2,4,6-TRI-t-BUTYLFHENOL WITH THE Mn 1 1 1 COMPLEX IN  METHANOL* (i) Product 2,4,6-Tri-t-butylphenoxyl was produced quantitatively. (ii) Analysis of kinetic results Oxidation of 2,4,6-tri-t-butylphenol with the Mn**1 complex was carried out in the presence of an excess amount of 2,4,6-tri-t-butylphenol. Plots of logarithm of absorbance at 500 nm versus time have the features shown in Figure 39. The fi r s t 7 # of the reaction gives a curvature in such a plot. From the 7 $> to about 60 # of reaction the plot is linear. This linear portion is used in the evaluation of the rate constant. (See Section IX-iii for explanation of curvature in Figure 39). The linear portion of Figure 39 can be expressed by equation (83). - d ( M n m ) t o t / d t - V ^ t o t W Integrating equation (83) gives equation (84). log A M n - log A Q = - k3 a(Mn***) t o t (84) where A ^ = absorbance of the Mn*** complex at 500 nm at time t, III A = absorbance of the Mn complex at 500 nm at time zero, n * 2.5 $ water (v/v) -0.58 Time (min.) 154 The value of k' was found to increase with increase in the 3a concentration of 2,4,6-tri-t-butylphenol (see Table XXXIII and Figure 40). TABLE XXXIII Variation of k' with concentration of 2,4,6-tri-t-butylphenol at 25C Ja (Mn111) - 5.8 x IO"4 M (PhOH)*(M) k^fsec""1) 0 0.2 x 10**5 4.24 x IO**2 1.1 x 10"5 8.58 x 10**2 2.1 x 10**5 12.77 x 10"2 3.3 x 10**5 * PhOH is used as an abbreviation for 2,4,6-tri-t-butylphenol. ( i i i ) Mechanism The following reactions are considered. Mn*-11 , ^ ^ Mn*-1-1 + H+ (8 5) k Q Mn*** + PhOH > Mn** + PhO' + H+ (62) y 156 k3a TTT TT 4 -Mn - + PhOH > Mn 1 + pn0« + H (86) P Abbreviations usedi TTT TTT Mn - • Mn CyDTA(H-O) Mn1-- - MnI3:ICyDTA(OH)2" y PhOH = 2,4,6-tri-t-butylphenol PhO» =» 2,4,6-tri-t-butylphenoxyl The reaction shown in equation (62) was found to proceed quite rapidly (see Section VIII). Initially, the small concentration of MnI][ICyDTA(0H)2" present reacts with 2,4,6-tri-t-butylphenol via reaction (62). As the reaction proceeds, acid is produced and eventually the equilibrium shown in equation (85) lies far to the left; the only Important reaction then III 2— is that shown in equation (86). As Mn CyDTA(OH) is a more powerful oxidant than Mni:riCyDTA(H20)~, the i n i t i a l curvature in Figure 39 is expected. The linear portion of the plot in Figure 39 is due to reaction (86). The rate of disappearance of the Mn1*1 complex (after reaction (62) becomes unimportant) is given by equation (87). 157 - d(Mn^) t Q t/dt . l ^ C f t O H ) ^ 1 1 ) ^ (87) Since (Mn I I 3 :) t o t - (Mn*-1) and (PhOH) » (M n 1 1 1 ) ^ , equation (87) becomes equation (88) after integration. log - log A 0 - - k3a(PhOH)t/2.303 (88) Equation (88) is identical to equation (84) with k 3 a * k 3 a ( P h 0 H ) ( 8 9 ) (iv) Determination of rate constant A value of k*ja was obtained for each concentration of 2,4 ,6-tri-t-butylphenol from a plot of log Aj^ versus time (see equation (88) and Figure 39). Then a plot of k*^ versus the concentration of 2,4 ,6-tri-t-butylphenol was done to obtain an average value of k^ (see Figure 40). An average value of k^a = 2.4 x 10 M sec was obtained. (v) Kinetic isotope effect Oxidation of 2,4,6-tri-t-butylphenol with the Mn*** complex was carried out in methanol-C-d at 25°. The value of k^a obtained was 4.1 x 10 J M sec , giving a kinetic isotope effect of 5.9. Thus, the 0-H bond of 2,4,6-tri-t-butylphenol is broken in the rate-determining step of the reaction. 1 5 8 (vl) Substituent effect Oxidation of some other 4-substituted-2,6-di-t-butylphenols with the Mn*** complex in methanol was carried out at 25°. The values of for these hindered phenols were obtained using a procedure similar to that of 2,4,6-tri-t-butylphenol. The results are shown in Table XXXIV. The effect observed is small. Figures 41 and 42 are plots of log k^/k^jj versus and (T~ respectively. The products obtained are identical to those obtained under acidic conditions (see Section V l l - v i i i ) . TABLE XXXIV (Tp, cr" and rates for oxidation of some 4-substltuted-2,6-di-t-butyl- phenols with the Mn*** complex In methanol at 25° Substituent k^M-W 1) cr cr" l o g ^ / ^ H t-Bu 2.4 x IO"4 -0.197 -0.197 -0.412 CH^ 4 .7 x 10" 4 -0.170 -0.170 -0.120 H 6.2 x 10~ 4 0 0 0 Br 6.0 x IO - 4 +0.232 +0.232 -0.014 I 5.8 x IO - 4 +O.276 +0.276 -0.029 C0C 6H 5 2 .5 x 10~ 4 +0.459 +0.459 -0.394 CN 3.4 x 10" 4 +0.628 +1.000 -0.261 CH0 69.O x 10~ 4 +0.216 +1.126 +1.047 * k a^H is the rate constant for 2,6-di-t-butylphenol. 1.40 1 .05 0.70 Figure 41. Plot of log k ^ a A ^ a H versus slope = ^ - - 0 . 0 7 bo o H3 o.oo H O c h 3 -o .35 r t-Bu - 0 . 7 0 1.2 - 0 . 9 - 0 . 6 - 0 . 3 O 0H0 1.2 1.40 Figure 42. Plot of log k ^ a / k 5 a H versus cr"*. 1.05 slope = ^ = -0 .33 CHO O 0.70 r-c^0.35 o 1-3 0.00 |_ -0.35 U H 0 BrCtQl Qt-Bu O COC6H5 -0.70 •1.2 -0.9 - 0 . 6 - 0 . 3 J _ 0 cr" p 0.3 0.6 0.9 1.2 161 (vii) Activation parameters Oxidation of 2,4,6-tri-t-butylphenol with the Mn1** complex in methanol was carried out at three other temperatures in order to evaluate the values of activation enthalpy and entropy. The values of A H and A S were found to be 10.9 Kcal.mole * and -38•6 e.u. respectively. The data in Figure 43 were taken from Table XXXV. TABLE XXXV Variation of k^^ with temperature Temperature (°C) k^CM'^sec"1) 25 2.4 x 10" 4 35 3.9 x I 0 ~ k 45 7.7 x 10~k 50 10.7 x 10" 4 (viii) Discussion The kinetic isotope effect of 5*9 indicates that the oxidation of 2,4,6-tri-t-butylphenol with Mh^CyDTA^O) - goes via a hydrogen atom abstraction mechanism. The 0 - H bond of the reductant is cleaved in the rate-determining step of the reaction. An ' inner-sphere' mechanism is probably involved and the hydrogen atom of the reductant forms a bridge 1/T (°K) x 10 163 between the two reactants (see Section Vll-ix). The small substituent effect observed suggests the presence of an activated complex which has radical character in the 0-H bond of the reductant. The activated complex may be pictured as XXXXIV. Such an activated complex is stabilised by both electron-withdrawing and electron-donating groups and the effect is usually small. (Compare with Section Vll-ix). Again, the 4-CH0 compound has an anomalously high rate compared with the other hindered phenols. (A similar anomalously high rate for this compound was also observed in acidic condition (see Section VII-ix)). Elemental analysis, melting point and nmr spectrum (see page 20) show that the compound used is 3•5-di-t-buty1-4-hydroxybenzaldehyde. Mn CyDTA(H_0)~ H • •. .Q XXXXIV The relative oxidising power of the three oxidants, HMn CyDTA(HgO), MnIIICyDTA(H20)" and MnnICyDTA(0H)2"", with respect to 2,4,6-tri-t-butyl-phenol as the reductant is 458 t 1 i 312. 164 X. OXIDATION OF 2.4.6-TRI-t-BUTYLPHENOL WITH THE Mri COMPLEX IN  DIMETHYL SULFOXIDE (DMSO) IN THE PRESENCE OF BASE (i) Product 2,4,6-Tri-t-butylphenoxyl was produced quantitatively. (ii) Analysis of kinetic results Oxidation of 2,4,6-tri-t-butylphenol with the Mn 1 1 1 complex was carried out in 95 £ DMSO and 5 £ water mixed solvent (v/v) in the presence of tetramethylammonium hydroxide. The concentration of the base used was kept lower than that of the Mn*** complex for three reasonsJ (1) to ensure that a l l the base was used up in ionising the Mn*** complex to its yellow form, Mn***CyDTA(0H)2~; (2) to ensure that 2,4,6-tri-t-butylphenol remains unionised; ( 3 ) excess base tends to decompose the Mn*** complex and thereby complicates the kinetics involved. Oxidation was followed spectrophotometrically by monitoring the change in absorbance at 400 nm (due to the formation of 2,4,6-tri-t-butylphenoxyl). The absorbance of the Mn*** complex was too weak to be -4 detected in the narrow concentration range studied (less than 4 x 10 M). The following reactions are considered. 165 II Mn + PhOH y Mn' + PhO* + H (91) Mn11 + PhO- + H+ + PhOH (92) Abbreviations used8 Mn — y - Mn CyDTA (OH)' 2-- Mn CyDTA(H20)' PhOH = 2,4,6-tri-t-butylphenol PhO' = 2,4,6-tri-t-butylphenoxyl Reaction (92) can be ignored because in the absence of base no detectable reaction between 2,4,6-tri-t-butylphenol and MnIIICyDTA(H20) was observed in a few hours. Reaction (91) is the rate-determining step. Thus, the rate of formation of 2,4,6-tri-t-butylphenoxyl is given by equation (93)' Assuming a l l the base was used up in converting Mn CyDTA(HgO)"" to MnIIICyDTA(0H)2"*, the in i t i a l concentration of MnmCyDTA(OH)2~ would be equal to the concentration of base added (see Section X - i i i ) . Equation (93) d(PhO')/dt = k^ (Mn jS)(PhOH) (93) 166 can be written as equation (94). dx/dt - k^(b - x)(p - x) (94) where x = concentration of 2,4,6-tri-t-butylphenoxyl at time t, b = concentration of tetramethylammonium hydroxide added, p = concentration of 2,4,6-tri-t-butylphenol added. Integrating equation (94) gives* log (p - x)/(b - x) = k ^ t + log b/p (95) The value of x at time t was calculated from the absorbance at 400 nm using an extinction coefficient value of I989 (measured in this work). The value of the rate constant, k^, was determined from the slope of a plot of log (p - x)/(b - x) versus t. Figure 44 is a typical plot of equation (95)• The values of k^ obtained for the various concentrations of reactants are summarised in Table XXXVI. An average value of , + —1 —1 k'Q, - 450 ?0 M~ sec"" was obtained. 168 TABLE XXXVI Values of k^ at 25° (TMA)* - 1 x 10~4 M (Mn m)(M) (PhOH)**(M) k^OT^ec"1) 2.50 x 10~4 2.22 x 10~4 540 2.50 x iO"*4, 2.60 x 10~4 460 2.50 x l o ~ 4 3.07 x lO*"4 400 2.50 x lO**4 3.32 x IO*"4 470 3.25 x 10~4 2.22 x 10~4 520 2.09 x 10~4 2.22 x 10~4 410 1.70 x IO"4 2.22 x 10~4 36O average k ^ 450 _ 7 0 * TMA = tetramethylammonium hydroxide ** PhOH - 2,4,6-tri-t-butylphenol ( i i i ) Discussion In the derivation of equation (95) i t was assumed that the III 2— concentration of Mn CyDTA(OH) " was equal to that of the base added. 169 This assumption was based on the following facts: (1) the pK value of i n / \- 41 Mn CyDTA(H90) in water was found to be 8.1 and the extrapolated pK value of 2,4,6-tri-t-butylphenol in water 1 0 4 was estimated to be 12.2 (pK values of 2,4,6-tri-t-butylphenol in 50 # ethanol, methanol and DMSO were found to be 14.?5, 17.40 and 12.28 respectively) 1 0 4" 1 0 6. Therefore, Mn***CyDTA(H20)~ will ionise before 2,4,6-tri-t-butylphenol does in a limited concentration of baset (2) i t was found (in this work) that both the Mn*** complex and 2,4,6-tri-t-butylphenol were completely ionised in the presence of an equimolar amount of base in 95 # DMSO solvent. For example, when 1,0 x 10 ^  M of base was added to a solution -3 III of 1.0 x 10 J M of the Mn complex, the complex was quantitatively converted to its yellow form. It was observed that when the base was kept at a fixed limiting concentration, the amount of 2,4,6-tri-t-butylphenoxyl produced was the same for a l l concentrations of the Mn*** complex and 2,4,6-tri-t-butyl-phenol, in agreement with the reaction path proposed. Only a narrow range of concentration variation could be studied because oxidation occurred too rapidly at higher concentrations of reactants. Even in the narrow concentration range studied only the last 20 # of the reaction could be followed. Lower concentrations of reactants make spectrophotometric measurement impossible. Comparing the value of k ^ with that of k^ in Section VIII, i t can 170 be seen that the oxidation of 2,4,6-tri-t-butylphenol with MnmCyDTA(0H)2~ is about 6000 times faster in DMSO than in methanol. The substrate is a neutral molecule and the medium effect on its activity is probably small. The high rate in DMSO solvent is presumably due to the III 2 increase in reactivity of Mn CyDTA(OH) ". The value of the rate constant, k^, for the oxidation of 2 , 4 , 6-tri-t-butylphenol with MnIIICyDTA(H20)'" in 95 $ DMSO was found to be 1 x 1 0 " 5 M*1sec"1. Thus, MnmCyDTA(0H)2~ is 4 .5 x 1 0 7 times as powerful as MnI]CICyDTA(H20)" as an oxidant of 2,4,6-tri-t-butylphenol. This high ratio of k^/k^ (= 4 .5 x 10 ) suggests an ' inner-sphere' mechanism is involved in the oxidation reaction (see Section VIII-x). 171 XI. OXIDATION OF 2.4.6-TRI-t-BUTYLPHENOL WITH POTASSIUM FERRICYANIDE IN  METHANOL* Oxidation of 2,4,6-tri-t-butylphenol with potassium ferricyanide in methanol was carried out under three conditionss (1) in the presence of base; (2) without acid or base; (3) in the presence of acid. Unfortunately, no kinetic data could be obtained because, in the presence of base (sodium hydroxide), the potassium ferrocyanide formed in the reaction precipitated; there was no observable reaction in the absence of base and acid; and the reaction occurred instantaneously in the presence of hydrochloric acid. However, i t was observed that potassium ferricyanide was not consumed completely even when oxidation was carried out in the presence of a large excess of hydrochloric acid and 2,4,6-tri-t-butylphenol. This observation indicated a reversible reaction of the type shown in equation (96). Further investigations (as described below) confirm the presence of a reversible reaction. Ferricyanide + PhOH ^ Ferrocyanide + PhO« + H (96) ** these species may be protonated. PhOH » 2,4,6-tri-t-butylphenol PhO» » 2,4,6-tri-t-butylphenoxyl * 2.5 £ water (v/v) 172 A. Equilibrium measurements a. Methods Al l solutions used were flushed with high purity nitrogen and transferred with syringes equipped with Chaney adapters. The values of the equilibrium constant, (defined in equation (97)), were determined at 25° in two wayss (i) from reaction between 2,4,6-tri-t-butylphenol and potassium ferricyanide in the presence of hydrochloric acid; (ii) from reaction between 2,4,6-tri-t-butylphenoxyl and potassium ferrocyanide in the presence of hydrochloric acid. ( i ) . To a 3 nil* aliquot of a known concentration of 2,4,6-tri-t-butylphenol in methanol in a 1 cm. cell which was capped with a silicon rubber disc was added O.05O ml. of hydrochloric acid of known concentration followed by 0.025 ml* of potassium ferricyanide of known concentration. The cell was shaken briefly to mix the reactants well. This cell was then put in the sample cell holder of a Bausch and Lomb Spectronic 505 spectrophotometer and in the reference cell holder was placed a similar cell containing 3 ml* methanol. Spectra from 300 nm to 450 nm region were recorded at short time intervals for the f i r s t five minutes, (it was found that the peak- at 400 nm (due to formation of 2,4,6-trl-t-butyl-phenoxyl) reached a maximum value instantaneously and then decreased with time. The decrease in absorbance at 400 nm was rather rapid at high concentrations of acid. Therefore, extrapolations to zero time were 173 needed to obtain the instantaneous absorbance at 400 nm. The peak at 420 nm (due to ferricyanide) also decreased with time, but the instantaneous decrease was most drastic, A similar extrapolation to zero time method was used to obtain the instantaneous absorbance at 420 nm). A spectrum of an identical solution but without 2,4,6-tri-t-butylphenol was recorded to obtain the absorbances of potassium ferricyanide at 400 nm and 420 nm. The extinction coefficients of potassium ferricyanide at these two wavelengths in a given acid concentration were then calculated. Thus, for each reaction the amount of potassium ferricyanide left could be calculated from the absorbance at 420 nm and the amount of 2,4,6-tri-* t-butylphenoxyl formed could be calculated from the absorbance at 400 nm (after correcting for contribution from potassium ferricyanide) using an extinction coefficient of 1955 (see Section I V - i i i ) . * i n i t i a l concentrations calculated because absorbances at 400 nm and 420 nm decreased with time due to the formation of a fluffy blue solid which precipitated out. This fluffy blue solid was not sensitive to oxygen, i t was insoluble in water and aqueous acids, but decomposed by aqueous potassium hydroxide to a brown solid. This blue solid was probably prussian blue - KPe I I I(Fe I I(CN) 6) or KFe I I(Fe I I I(CN) 6). Prussian blue is insoluble in water and aqueous acids, but decomposes in aqueous potassium or sodium hydroxide to form brown ferric hydroxide. 174 ( i i ) . To a 3 ml. aliquot of methanol containing a known concentration of 2,4,6-tri-t-butylphenol and hydrochloric acid in a 1 cm. cell which was capped with a silicon rubber disc was added 0.050 ml. of a known concentration of 2,4,6-tri-t-butylphenoxyl (see Section I V - i i i for method used to generate 2,4,6-tri-t-butylphenoxyl) followed by 0.025 ml. of potassium ferrocyanide. The cell was shaken briefly to mix the reactants well. Spectra from 350 nm to 4 5 0 nm region were recorded at short time intervals in the f i r s t five minutes. The concentration of potassium ferricyanide formed and the concentration of 2,4,6-tri-t-butylphenoxyl left were calculated from the absorbances at 420 nm and 400 nm respectively b. Analysis of results It was found that the values of (see equation ( 9 7 ) ) were constant for a given concentration of acid (see Table XXXVII) and the values of Kp derived from the two methods agreed well (see Tables XXXVII and XXXVIII). But the values of were found to vary proportionately with acid concentration (see Table XXXIX). (see (1)). K P ( F e -1 1 ) ^ (PhO- )/(Fe i l l) + r t + (PhOH) (97) tot = total concentration of ferrocyanides (unprotonated + protonated forms), 175 ( F e 1 1 1 ) ^ ^ = total concentration of ferricyanides (unprotonated + protonated forms), (PhOH) • 2,4,6-tri--t-butylphenol, (PhO') « 2,4,6-tri-t-butylphenoxyl. TABLE XXXVII K at constant acid concentration at 25° (determined by method (i)) (PhOH)(M) (Ph0O(M) K ~P-1.00 x 10" 2 1.43 x IO" 4 5.53 x IO" 4 1.43 x 1 0 " 4 3.7 x 10" 3 1.97 x 10" 2 1.87 x 1 0 " 4 4.87 x l o " 4 1.87 x 10**4 3.7 x IO" 3 2.98 x 10" 2 2.24 x IO" 4 4.39 x 1 0 " 4 2.24 x 10**4 3.8 x IO" 3 4.95 x 10" 2 2.76 x 1 0 " 4 4.03 x 10**4 2.76 x IO**4 3.8 x IO" 3 4.95 x 10" 2 1.97 x 1 0 " 4 2.05 x IO**4 1.97 x IO" 4 3.9 x IO" 3 4.95 x 10" 2 2.44 x IO*"4 2.85 x 10""4 2.44 x IO" 4 4.2 x IO" 3 5.00 x 10" 2 1.13 x 10"*4 6.53 x 1 0 " 4 1.13 x 1 0 " 4 3.9 x IO" 3 average K = P 3.9 x IO"3 (HC1) - 1.63 x IO**2 M a put equal to (PhO«) 176 TABLE XXXVIII at constant acid concentration at 25° (determined by method (ii))  yFhOH)(M) (PhO KM) ( F e m ) l o i M ( F e 1 1 ) * ^ ^ 5.01 x IO"2 0.40 x 10"4 0.99 x lo" 4 5.51 x 10~4 4.4 x 10~3 5.01 x 10"2 0.17 x lO" 4 0.56 x l O - 4 5.83 x lO" 4 3.8 x 10"3 5.01 x 10"2 0.75 x l O - 4 2.10 x lo" 4 4.42 x 10"4 3.2 x 10"3 5.01 x 10"2 0.89 x 10"4 1.92 x l O " 4 4.63 x IO"4 4.3 x IO"3 (HCl) • 1.63 x 10"2 M average K = 3.9 x 10~3 P * calculated from the difference between the concentration of ferrocyanide added and the concentration of ferricyanide formed. 177 TABLE XXXIX* Variation of K with acid concentration at P (HCI)CM) K -P~ ( K / ( H + ) ) ( M ) 1.63 x l c f 3 1.9 x I 0 ~ k 1.2 x IO"1 i . 6 3 x i o ~ 2 3.9 x i o ~ 3 2.3 x IO"1 3.51 x i o " 2 1.5 x 10~2 4.3 x IO"1 5.09 x IO - 2 2.2 x 10" 2 4.3 x IO"1 6.55 x 10" 2 3.0 x 10~2 4.6 x IO""1 l . o i x i o " 1 5.0 x 10" 2 5.0 x IO"1 * determined "by method (i) c. Discussion The oxidant involved is likely to be a protonated form of ferricyanide because, in the absence of acid there is no noticeable reaction between ferricyanide and 2,4,6-tri-t-butylphenol. Ferrocyanide formed is likely to be protonated too because unprotonated ferrocyanide is not soluble in methanol. (Under basic conditions ferrocyanide precipitates immediately when 2,4,6-tri-t-butylphenol is oxidised with potassium ferricyanide in methanol). Oxidation of 2,4,6-tri-t-butylphenol with potassium ferricyanide under acidic conditions occurs instantaneously and that the reaction is 178 reversible is shown by the results obtained (see Tables XXXVII to XXXIX). In order to solve the various equilibria involved the values of the ionization constants of ferrocyanic acid (H^Fe(CN)g) and ferricyanic acid 107—110 (H3Fe(CN)^) must be known. Several attempts ' to measure these ionization constants have been made, but only the f i r s t two ionization constants of ferrocyanic acid were obtained (K^  «* (6.7* 0.3) x 10~^; + —3 110 K 2 = ( 6 _ 2 ) x l 0 - j . The values of the other ionization constants were 110 considered to be larger than 0,1 . The lack of data on the ionization constants of these two acids prevent a detailed analysis of the equilibrium shown in equation (96). 179 XII. ATTEMPTS TO SYNTHESIZE 3.5.7-TRI-t-BUTYLTROPOLONE Several attempts to synthesize 3i5»7-tri-t-butyltropolone (XXXXV) by various methods were made because i t was thought that the oxidation of this compound with the Mn1** complex would yield a radical analogous to 2,4,6-tri-t-butylphenoxyl (see equation (98)). Unfortunately, a l l attempts failed to give 3 t5»7_,tri-t-butyltropolone. The methods used are described below. 0. OH 0 O «0 0, (a) Reaction of 3.5.7-tribromotropolone with t-butyl lithium A sample of 2.5 g of 3»5t7-tribromotropolone (6 mmoles) was dissolved in JO ml. of dry benzene in a 250 ml. three-necked flask. (The compound 3i5»7-tribromotropolone was synthesized by bromination of 111 tropolone according to the method of Takase) . High purity nitrogen was bubbled into the benzene solution through one neck of the flask and a second neck with a glass tube dipping into a beaker of water formed the outlet. The third neck was stoppered with a rubber cap. The benzene solution was stirred with a bar magnet while nitrogen was bubbled In. oxidation (98) XXXXV 180 Small aliquots of t-butyl lithium were syringed into the reaction flask by means of a 5 ml. syringe. The total volume of t-butyl lithium added varied from 11 ml. (22 mmoles) to 20 ml. (40 mmoles). The reaction mixture was stirred at room temperature for 5 to 22 hours. Then, the reaction mixture was poured into 200 ml. of benzene-water ( 1 : 1 ) acidified with hydrochloric acid. The dark benzene layer was flash-evaporated to dryness, A dark solid was obtained that could not be recrystallised from acetone, alcohols or benzene. Analytical data and nmr spectrum showed that i t was not the desired compound. (b) Reaction of tropolone with t-butyl alcohol A 1 g sample of tropolone (from Aldrich Chemical Company) was dissolved in 10 ml. of t-butyl alcohol in a 50 ml. round-bottomed flask and 0.1 ml. of concentrated sulphuric acid was added to the solution. The reaction mixture was refluxed at about 150° for 24 hours. Starting material was recovered at the end of the reaction. (c) Reaction of tropolone with t-butyl chloride A 2 g sample of tropolone was dissolved in 20 ml, of glacial acetic acid containing 4,4 g of sodium acetate in a 50 ml, round-bottomed flask. Then, 6.6 ml. of t-butyl chloride was added to the mixture and the contents refluxed at about 110° for four hours. Sodium chloride and the starting material were obtained. 181 XIII. SUGGESTIONS FOR FURTHER WORK Further work on: (1) the determination of the ionization constants of ferrocyanic acid (H^Fe(CN)g) and ferricyanic acid (H^Fe(CN)^); (2) the synthesis of 5-substituted-3»7-di-t-butyltropolones, are needed. 107-110 (1) . Several attempts to measure the ionization constants of ferrocyanic and ferricyanic acids have been made, but only the f i r s t two ionization constants were obtained. The values of the other ionization 110 constants were considered larger than 0.1 .A titrimetric method was used in a l l these attempts and i t is not surprising that the values of the other ionization constants could not be determined, as they are larger than 0,1. A preliminary investigation (this work) indicates that a spectrophotometric method may yield the values of a l l the ionization constants (the absorbance of potassium ferricyanide at 420 nm in sulfuric acid or perchloric acid up to 10 M acid varies with the acidity of the medium and a study of the variation of absorbance with acidity of the medium may give the ionization constant values). A detailed investigation on the variation of absorbance of potassium ferricyanide and potassium ferrocyanide with acidity in both aqueous and non-aqueous (or mixed) solvents needs to be done and the results compared. Knowing the ionization constants, the equilibrium between 2,4,6-tri-t-butylphenol, ferricyanide and 2,4,6-tri-t-butylphenoxyl, ferrocyanide, can be studied in details. (2) . The series of 5-substituted-3,7-di-t-butyltropolones has not yet 182 been prepared. 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(Japan), 21* l 2 ° 7 (1964). 189 APPENDIX An alternative mechanism involving a chloro-complex formation III between the chloride ion and the Mn complex instead of protonation of the Mn*** complex (see equation (32)) was ruled out on two pieces of III evidence: (1) the spectrum of the Mn complex in methanol in the presence of hydrochloric acid is identical to that of the same complex in methanol in the presence of an equivalent amount of perchloric acid. Therefore, the spectrum B in Figure 6 is not that of a chloro-complex; (2) the rate of oxidation of methanol by the Mn*** complex is barely affected by the chloride ion concentration (see Table XXXX). TABLE XXXX * o Variation of with concentration of chloride ion at 25 (Mn***) - 5.9 x iO" 4 M (H+) - 1.63 x 10" 2 M ( C D ( M ) kjsec" 1) I .63 x 1 0 " 2 0 1.14 x 10" 3 1.30 x 10" 2 0.33 x lO" 2 1.30 x 10" 3 0.82 x 1 0 " 2 0.81 x lO" 2 1.52 x 10~ 3 0.33 x IO - 2 1.30 x 1 0 " 2 1.49 x 10" 3 0 1.63 x iO" 2 1.54 x 10" 3 190 * Variation of chloride ion concentration was obtained by mixing different volumes of 1 N HCl with different volumes of 1 N HCIO^ with the total volume of the mixture being maintained at 1 ml. (in the presence of I .63 x 10 M of HCl in water the value of Ic^  was found to be 2.96 x 10"^ sec"1 at 2 5 ° ) . 

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