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Deuterium isotope effects in organic cations Mocek, Michael M. 1959

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DEUTERIUM ISOTOPE EFFECTS IN ORGANIC CATIONS. by MICHAEL M. MOCEK B.A., University of British Columbia, 1957. A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Chemistry We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA December, 1959 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t 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 r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g 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 g r a n t e d by t h e 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 u n d e r s t o o d t h a t c o p y i n g or 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 g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department of Chemistry  The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8 , Canada. D a t e ftrf. i i ABSTRACT. The effect of c< -deuterium substitution upon the ionisation of five identically di-substituted benzhydrols was studied. It was found that the equilibrium isotope effect K^ /K^ , which varied from 1.03 to 1.35, was small i f the substituents were basic in character but was greater in the case of substituents with electron withdrawing properties. The ionisation of Michler's Hydrol was quantitatively examined at acidities up to J>0% fuming sulfuric acid, and upon the spectral and other evidence, the existence of two new carbonium ions is postulated. The isotope effect was also determined for the above entities and i t was found that the effect increases with the number of protonated nitrogens. The effect of five solvents upon the ionisation of Michler's Hydrol Blue was examined and a conclusion was reached that the carbonium ion is almost completely ionised in some of these solvents. i i i ACKNOWLEDGEMENTS Most sincere thanks are extended to the research director, Dr. R. Stewart, for his invaluable help and many suggestions during the course of this investigation. Appreciations are also due to Dr. R.B. Moodie for helpful discussions. TABLE OF CONTENTS Page INTRODUCTION 1 OBJECT OF RESEARCH 7 METHODS OF APPROACH 8 1. The HQ and C Q Acidity Functions 8 2. IR, UV and NMR Spectra 10 3. Evidence for Existence of Carbonium Ions 10 EXPERIMENTAL 12 1. Synthesis of 4,4'Identically Di-substituted Benzhydrols . 12 2. Determinations of Ionisation Constants 14 3. Isotope Effects 16 FURTHER INVESTIGATIONS ON MICHLER'S HYDROL 28 1. Species at 401 mn 28 2. Species at 435 mu 31 3. Evidence for Identity of VI 34 4. Evidence for True Equilibria 36 5. Differences in A of V and VI 37 INVESTIGATIONS OF ULTRAVIOLET SPECTRA 38 1. Spectrum of Michler's Hydrol 38 2. Spectra of Michler's Hydrol Blue 39 3. Spectrum of Michler's Hydrol Yellow . 42 4. Simultaneous Appearance of Michler's Hydrol Yellow and Green 43 5. Spectrum of Michler's Hydrol Green 44 6. Spectrum of Michler's Hydrol in 60$ Fuming Sulfuric Acid. 45 7. Conclusions Derived from Ultraviolet Spectra 45 V Page INFRARED SFECTRA 56 SOLVENT EFFECT UPON IONISATION OF MICHLER'S HYDROL BLUE 57 DISCUSSION AND CONCLUSIONS 61 SUGGESTIONS FOR FURTHER RESEARCH 67 BIBLIOGRAPHY 68 v i TABLES Page I Values of the Ratio of Equilibrium Constants of Michler's Hydrol Blue in Various Media 26 II Ionisation and Isotope Effect Data for Benzhydrols .... 27 III Temperature Variation of Optical Density of Michler's Hydrol Yellow 28 IV Apparent pKR++ Values for Michler's Hydrol Yellow in 73 - 96$ Sulfuric Acid 31 V Temperature Data for Michler's Hydrol Green 32 VI Decay of Michler's Hydrol Green with Time 33 VII pKRt+*. Values for Michler's Hydrol Green in Fuming Sulfuric Acid 34 VIII Ionisation and Isotope Effect Data for Michler's Hydrol 35 IX Absorption Maxima of Michler's Hydrol 38 X Absorption Maxima of Michler's Hydrol Blue at pH 6.0 . . 39 XI Absorption Maxima of Michler's Hydrol Blue at pH 5.0 .. 40 XII Absorption Maxima of Michler's Hydrol Blue at pH 4,0 .. 41 XIII Absorption Maxima of Michler's Hydrol at pH 0,0 41 XIV Absorption Maxima of Michler's Hydrol Yellow in 81$ H2S04 42 XV Absorption Maxima of Michler's Hydrol Yellow in 96$ H2S04 43 XVI Absorption Maxima of Michler's Hydrol Yellow in 98$ H 2S0 4 43 XVII Absorption Maxima of Michler's Hydrol Green in 10$ Fuming Sulfuric Acid 44 XVIII Absorption Maxima of Michler's Hydrol Green in 30$ Fuming Sulfuric Acid 45 XIX The WFAB of Carbonium Ions and Their Parent Hydro-carbons 47 v i i Pa£§ XX Infrared Data for Benzhydrols 56 XXI Variation of Extinction Coefficients and Isotope Effect of Michler's Hydrol Blue with Solvents 60 v i i i FIGURES Page 1 Disappearance of Carbonium Ion with Time (k,4'-Dimethyl Benzhydrol) 17 2 Ionisation Equilibria of Michler's Hydrol 22 3 Ionisation of Michler's Hydrol 24 4 Spectrum of Michler's Hydrol 48 5 Spectrum of Michler's Hydrol Blue at pH 6.0 49 6 Spectrum of Michler's Hydrol Blue at pH 5.0 50 7 Spectrum of Michler's Hydrol Blue at pH 4.0 51 8 Spectrum of Michler's Hydrol at pH 0.0 52 9 Spectrum of Michler's Hydrol Yellow in % % HgSO^  53 10 Spectrum of Michler's Hydrol Green in 30% Fuming H2S0^ . . . 54 11 Simultaneous Appearance of Species V and VI 55 1 1 INTRODUCTION Shortly after the discovery of deuterium in 1932 ( l ) , Cremer and Polanyi (2) as well as Eyring and Sherman (3) predicted on the basis of theoretical considerations only, that deuterium and hydrogen should react at different rates. Their predictions have been verified experimentally by many investigators and the technique of using isotopes of elements other than hydrogen has been greatly extended since then. The so called primary kinetic deuterium isotope effect which operates through the cleavage of C-H or C-D bonds respectively is used today with great advantage for elucidating the mechanisms or organic reactions (4,5). The effect may be best described as being due primarily to the difference in the zero point energies of the C-H and C-D bonds. The theoretical basis for the deuterium isotope effect using the absolute theory of reaction rates was worked out by Biegeleisen (6) and the theoretical value for the ratio of the rate constants k H/k D a 6.9 at 25° may be obtained (4). In order to illustrate the usefulness of the deuterium isotope effect i t may suffice to quote one example of oxidation of isopropyl alcohol (and the corresponding <X -deuterio compound) with chromic acid (7,8)• The large isotope effect showed clearly that the hydrogen bond at the oC -carbon was cleaved in the rate determining step which led to the postulation of the following reaction mechanism: 2 CH.-CH-CH. + 2H + HGrO," 3 | 3 4 OH CH3-CH-CH3 + H 20 OCrO H 2 CH H <^A" u ..-C) -CH 3 | / + 3 CH -C-CH + HA + H CrO 3 3 2 3 Many other reactions of compounds containing deuterium have been successfully investigated to obtain their mechanisms (4,5). A different kind of isotope effect in which the C-H or C-D bonds remain intact during the reaction, appropriately termed the secondary deuterium isotope effect, has been recently described by several investigators. It is this secondary isotope effect \*hich was the object of this study and which shall be described in more detail below. Halevi and Nussim (9) have shown that the presence of the deuterium on the o< -carbon atom in phenyl acetic acid has a small but a significant effect upon the dissociation of the protium and deuterium compounds respectively: C/HC D fiOOH C,H_CD COO" + H + 6 5 2 6 5 2 The deuterio analogue of the phenyl acetic acid was the weaker acid 3 and the ratio of the ionisation constants found to be KpAj, B 0.895, Halevi interpreted the effect of the deuterium in terms of the difference of the inductive electron release which is supposedly-greater from the deuterium than from hydrogen (10). Streitwieser, Jagow, Fahey and Suzuki (ll) have studied the effect of c< and /3 deuterium substitution upon the rate of the acetolysis of the cyclopentyltosylates. They have attributed the effect of o<-deuterium substitution predominantly to the change of a tetrahedral C-H bending vibration to an out of plane bending deformation in the transition state: O T s " The effect of ^-deuterium substitution was ascribed to a change of the tetrahedral C-H bending to a vibration along a molecular orbital owing to the increased electron demand of the electron deficient transition state. The interpretation of the ft -effect is similar in principle to that put forward by Shiner (12,13,14) and Lewis, Boozer and Coppinger (15,16,17,18). Stewart and coworkers (19) who have examined the effect of deuterium substitution on the <* -carbon in benzhydrol and 4,4'-dimethoxybenzhydrol have pointed out that greater inductive electron release from deuterium, a difference in the electronic wave function for the isotopically substituted species and vibrational differences 4 in C-H and C-D bonds in the transition state may a l l contribute to the stability and reactivity of isotopically substituted molecules. Mason and Elliot (20) have investigated the reaction between benzoyl chloride and aniline, the N,N-dideuterio, 2 , 4 , 6 -trideuterio and N,N-2,4,6-pentadeuterio anilines. They have ascribed the faster rate of deuteriated anilines to the greater electron donating power of deuterium than protium and to a probable increase in the zero point energy of the N-H or N-D bonds in the transition state with zwitter-ion structure as indicated below: o Ph C NH2 CI Ph A -Llewellyn, Robertson and Scott (21) who have determined the rates of hydrolysis for the methyl iodide, bromide, p-toluene-sulphonate and the corresponding deuterium analogues, attributed the faster hydrolysis of the deuterio analogues to the possibility of a water molecule being partially bonded to the carbon which would produce much stiffer bending vibrations in the transition state, in which case the change in zero point energy would be in the right direction. A very interesting explanation of the secondary deuterium isotope effect is offered by Weston (22) who has come to the 5 conclusion that the difference in dipole moments, nuclear quadrupole coupling constants, chemical shift in NMR, kinetic isotope effect and isotope exchange equilibria can be explained almost completely on the basis of differences in vibrational energies and anharmonicity of the isotopically substituted species. A deuterium isotope effect which may be included in the category of secondary effects only because i t does not involve breaking of a C-H bond in the rate determining step was observed by Shiner (12,13,14) and Lewis, Boozer and Coppinger (15,16,17,18) in the solvoly-sis of alkyl halides and sulfonates. Deuterium substitution on the carbon adjacent to the reaction centre produces small but significant decrease in the rate of solvolysis for the deuterio compounds. The explanation of this effect is sought in the requirement of the developing carbonium ion for an electron supply at the transition state. Lewis et al postulate hyperconjugation of the empty p orbital on the carbonium ion with the orbitals of hydrogen on the adjacent carbon atom, whereas Shiner in addition to hyperconjugation also postulates solvation of the hydrogens adjacent to the reaction centre. Generally, two types of deuterium isotope effects may be distinguished: a) When the deuterium is attached to the reacting centre, such an effect is called an °( -deuterium isotope effect. Thus Streitwieser 1s investigation of the rates of acetolysis of cyclopentyl tosylates- o< -d and Stewart's study of ionisation of benzhydrols belongs to the classification of oC-effects. b) However when deuterium is attached to carbon immediately next to the reaction centre, then such effect is called -deuterium isotope effect. 6 Here the reaction of solvolysis of alkyl halides studied by Shiner, the ionisation of o( -deuterio phenylacetic acid investigated by Halevi belong to the category of -effects. The above references to the secondary deuterium isotope effect are by no means complete but have been collected with the view to show various explanations accounting for this effect. 7 OBJECT OF RESEARCH It was decided to study the secondary o( -deuterium isotope effect on the ionisation equilibria of the 4,4' identically di-substituted benzhydrols. These compounds are extremely well adaptable for this purpose for several reasons. (a) The carbinols and their deuterium analogues are easily obtainable by reduction of the corresponding ketones with LialH^ or LiAlD^ respectively. (b) The carbonium ions formed from the benzhydrols are fairly stable and therefore their concentration can be very conveniently measured by a spectrophotometric method. (c) The electron releasing character of the substituent groups can be varied to a great extent and the magnitude of the isotope effect may be studied as a function of the substituents. The following 4,4' identically di-substituted benzhydrols and their deuterium analogues were studied: dichloro, dimethoxy, bis(dimethylamino), dimethyl and unsubstituted benzhydrol. 8 METHODS OF APPROACH 1. The HQ and C o Acidity Functions. In the determinations of the magnitude of the deuterium isotope effect, an extensive use has been made of the H and C o o acidity functions, whose definitions and significance will be briefly discussed below. The HQ acidity function was originally derived by Hammett and Deyrup (23) from the investigation of ionisation equilibria of a particular class of indicators behaving in accordance with the Broensted - Lowry theory of acids and bases, namely: B + H+ BH+ 1. HQ, which measures the tendency to confer a proton on an uncharged base, has been defined by the equation: H 0 r PKJJH* -log(C B R + /CB) 2. where Kgy* is the dissociation constant of the conjugate base and Cg and Cgg-t- are the concentrations of the uncharged and conjugate bases respectively. The ratio of the concentration terms is an easily measurable quantity; either the colourless uncharged base becomes coloured upon protonation or a measurable shift occurs in the ultra-violet spectrum. By analogy with the above, the acidity functions H + and H + + may be derived (24). The H q function has been investigated very extensively especially for the water - sulfuric acid and anhydrous sulfuric acid - sulfur trioxide systems and its values are known for the dilute aqueous solutions in the pH range (23) up to 30$ fuming sulfuric acid (25). The usefulness of the HQ function has been amply demonstrated. Paul and Long (26), however, have indicated that discrepancies in the HQ will become apparent i f studies are made with indicators differing widely in structure. The equilibrium isotope effect is given by the equation: KH/ KD = K A / K B = CAH + °B / GBH + CA 3 where A and B refer to the uncharged protium and deuterium bases respectively and C has a meaning of concentration. The C 0 acidity function which has been more widely applied in this investigation, has been derived (27) on a similar basis to the Hammett HQ function, for an ionisation of the following type: ROH + H+ — R+ + H20 4 and has been defined by the following equation: 0 o = P V - l o g ( V /C R ( ) H) 5 The C Q function becomes particularly useful in the investigations of ionisation equilibria of carbonium ions which dissociate according to equation 4. The values of C Q have been determined from the pH range up to 92$ sulfuric acid (27) and estimated with the help of the J Q acidity function (28) up to 99$ sulfuric acid (29). Paul and Long favoured the use of the symbol J Q instead of C Q, and more recently Deno suggested the use of the symbol H R which was originally used by Williams and coworkers. For the purpose of this investigation i t was necessary to define a function RQ by the following equation: R - C - H 6. 0 - 0 o which has been applied to the ionisation of the type: RHOH++ ^ R+i" + H20 7. 1 0 . Rigorously, according to the definition, H 4 and C + should have been used; however, such acidity functions have not been evaluated as yet. Moreover, the experimental results herein as well as studies of Williams and coworkers (30) indicate the use of the HQ and GQ functions was, at least in this case, partly justified. 2. IR. UV and NMR spectra. Infrared spectra have been used mainly for the purpose of confirming the presence of the deuterium in the molecule and the NMR evidence has been applied to establish the isotopic purity of the sample. Ultraviolet spectra of the identically di-substituted benzhydrols and their change with increasing acidity have been studied firstly in the range 700 - 200 mp on the Cary Recording Spectrophoto-meter in order to establish the nature of the species present and secondly, at a definite wavelength, on Beckman DU Spectrophotometer in order to evaluate the ionisation constants and the magnitude of the isotope effect. More complete discussion of the ultraviolet spectra will be given later. 3 . Evidence for the existence of carbonium ions. It might be appropriate at this point to discuss briefly the principal evidence which conclusively established the existence of the carbonium ions. (a) Conductivity measurements. In the early investigations of carbonium ions i t was established by Walden (31) and Gomberg (32) that triphenylmethyl halides ionise in liquid sulphur dioxide to give conducting solutions comparable with those of methylammonium chloride and potassium iodide which are also good electrolytes in sulphur dioxide. This as well as more recent evidence (33) led to recognition of the existence of carbonium ions. (b) Spectrophotometry. Appearance of a spectrum in conducting solution but not in non-conducting solution of a given compound and variation of the intensity of the absorption with the conductivity, leads to the conclusion that the spectrum is associated with the ion formed in the ionisation of the compound (34,35). Comparison with a spectrum of related substances will decide which ion i s responsible for the absorption. (c) Cryoscopic measurements. The freezing point depression of a solvent is related to the concentration of the solute particles. This fact may be used to establish the number of solute particles responsible for the depression, once these have been qualitatively identified as ions. The cryoscopic evidence has been used successfully by Hantzsch (36) to identify the triphenyLnethyl carbonium ion and has led to recognition by Newman and Deno (37) of the ionisation of some triphenylmethyl carbinols as well as mesitoic acid (38). Much of the pioneering work in this field has been done by Hammett (39) and more recently by Gillespie (40,41,42,43*44). 12 EXPERIMENTAL 1. Synthesis of A. A1 identically di-substituted benzhydrols. (a) 4.4'-Dimethoxv benzhydrol. 4,4'-Dimethoxy benzophenone (l2.1g) was dissolved in anhydrous tetrahydrofuran (100 ml) and LiAlH^ (0.5g) previously dissolved in 25 mis of tetrahydrofuran was added to i t over a period of five minutes. The mixture was refluxed for lg hour, then cooled and poured over ice containing one ml of sulfuric acid, in order to decompose the complex. The solution was extracted with ether, washed and the ethereal extracts evaporated and the solid recrystallised from 1:1 ether-petroleum ether (3O-6O0) mixture. Yield was about 75$, m.p. 69-70°, ( l i t . : 69-70° (45)). The deuterium analogue was prepared in the identical manner with the exception that LiAlD^ was used in place of the LiAlH^. Yield was about 50$> m.p. 69-69.5°. The isotopic purity of this compound was established by NMR to be 99$. The absence of the proton absorption in the deuteriated compound was considered a sufficient evidence of the purity of the deuterio analogue. (b) 4.4'-Dimethvl benzhydrol. 4,4'-Dimethyl benzophenone (2.5g) was dissolved in anhydrous tetrahydrofuran (25 mis) and LiAlH^ (0.2g) previously dissolved in 10 mis tetrahydrofuran was added to i t over a period of five minutes. * Note: It should be pointed out that a l l the yields reported herein are based on the first recrystallisation only, and that no attempt was made to recover any compound from the mother liquor. 13 The mixture was refluxed for 45 minutes. Otherwise the procedure was the same as above. Recrystallisation was afforded from 1:1 ether-petroleum ether mixture. Yield was about 40$, m.p. 69-70.5°, ( l i t . : 69-700 (46)) . The deuterium analogue was prepared as above by reduction of the 4,4'-dimethyl benzophenone with LiAlD^. Yield was about 40$, m.p. 69-70°. (c) 4.4*-Dichloro benzhydrol« This compound was prepared in a manner identical to that of 4,4'-dimethyl benzhydrol, by reduction of 4,4'-dichloro benzophenone with IdAlH^. Recrystallisation was made from 95$ ethanol. Yield was about 45$, m.p. 94-"95°, ( l i t . : m.p. 91.5° (47)) . The deuterio analogue was prepared in a similar manner. Yield was about 45$, m.p. 94.5-95.5°. (d) 4.4'-Bis(dimethylamino) benzhydrol. Michler's Hydrol. This compound was prepared by reduction of the corresponding ketone by a method similar to that as described by Arient and Dvorak (48) . Ether was used as the solvent and the mixture was refluxed for over one hour and then poured over crushed ice containing one ml. of sulfuric acid. After the decomposition of the complex took place, the sulfuric acid was neutralised with sodium hydroxide until alkaline and the ether layer was separated without washing and left to crystallise out. The Michler's Hydrol was repeatedly recrystallised first from ether and then from benzene, about ten times in a l l . Yield was about 35$, m.p. 101.5-103° ( l i t . : 102-103° (49)) . The deuterio analogue was prepared in an identical way by reduction of the Michler's ketone with LiAlD^. Yield was about 14 35$, m.p. 101.5-103°. 2. Determination of the ionisation constants. The ionisation constants of the indicators were determined essentially by the method of Deno, Jaruzelski and Schiersheim (27) as follows: Method A. A stock solution of the appropriately substituted benzhydrols (with the exception of the Michler's Hydrol), and the deuterio analogues were made in 99»7$ glacial acetic acid (Baker and Adamson) and 0.1 ml aliquots were taken and diluted to 10 ml with appropriate concentration of sulfuric acid. The acetic acid introduced together with the sample was not evaporated and therefore a correction was made for the concen-tration of the sulfuric acid. The protium and deuterium compounds were weighed in the same molar concentration so that a direct comparison of the optical density was possible. The optical densities at approximately 25,50, 75 and 100$ ionisations were determined on the Beckman DU Spectrophotometer using silica cells of one cm path length. The measurements were done at the same wavelength for the protium and deuterium species. At least two measurements were made at 25 and 75$ ionisations and five readings at 50 and 100$ ionisations and the average of the results was taken. Due to the instability of some of the carbonium ions (fading of colour with time) i t was necessary to extrapolate to zero time. The disappearance of the carbonium ion with time seemed to follow second order kinetics in the case of 4,4'-dimethyl benzhydrol which were then used to obtain the i n i t i a l concentration of the coloured species. However the disappearance of colour was very slow in the case of the more stable cations and i t was not possible to 15 distinguish very well between the first and second order kinetics; in fact even the direct plot of concentration versus time gave good results. Some of the measurements of optical densities showed erratic behaviour during the first five to ten minutes; this was corrected by equilibrating the cells as well as the sulfuric acid used, at 25°. Even though the sulfuric acid does not absorb in the region studied, a blank cell containing the same concentration of acid as the sample was used to compensate for any possible absorption of light. A l l measurements were done at 25.0 + 0.1° (unless otherwise stated) using thermospacers. Method B. This method was used only for the Michler's Hydrol. The stock solution was made up in acetone and similarly 0.2 ml or 0.5 ml aliquots were taken and diluted with 0.02 M sodium acetate - acetic acid buffer to 10 ml. In this case the acetone was not evaporated and the buffer was added directly to the aliquot and the optical density measurements were taken. In a different series of measurements 1.0 ml aliquots were taken, the acetone was evaporated, the buffer was then added and the solution was warmed in order to ensure that a l l of the sample dissolved and the temperature was then brought to 25° in a thermostated bath and optical density readings taken. Since Michler's Hydrol is not completely ionised even at the optimum pH in certain solvents, i t was only possible to determine the "apparent" ionisation constant at 25° in these cases (49). For the other species of Michler's Hydrol which appear at much higher concentrations of sulfuric acid and which \*ri.ll be further discussed in the text, method A was used, with the exception that an acetone stock solution was prepared and 1.0 ml or 0.1 aliquots respectively were taken and the acetone was evaporated before 16 addition of the acid. Isotope Effects. (a) 4.4,-Dimethoxy benzhydrol. The ionisation constants of the protium and deuterium compounds were determined by method A, as described above. Due to the great stability of these carbonium ions i t was not necessary to apply extra-polation to zero time and the values of the pKg^+obtained are therefore somewhat more precise than for the other disubstituted benzhydrols. The optical densities were determined at 504 mu, and i t was found that the optical density of the deuterio compound was in a l l cases lower than that of the protium analogue. The ratio % / % was found to be 1.18. Other pertinent data appear in Table II. (b) 4i41-Dimethyl benzhydrol. The variation of optical density with concentration of sulfuric acid was determined at 468 mp for both, the protium and deuterium compounds, and the ionisation constants were evaluated by method A. Due to the instability of the cations i t was necessary to follow the disappearance of the carbonium ions with time and extrapolate to zero time. A first order kinetic plot gave a curved line, a second order kinetic plot, however showed a good straight line, with the exception of the first few points, which were irregular (Fig. l ) j this irregularity being due to the turbulence of sulfuric acid, which cleared after the system properly equilibrated itself. The appearance of turbu-lence was later avoided by equilibrating the cells and the acids before measurements. The decay of colour was always slower for the deuterio compound. The second order kinetic approximation is supported by Balfe and coworkers (50) who established that 4,4'-dimethoxy benzhydrol DISAPPEARANCE OF CARBONIUM ION WITH TIME (4,4*-DIMETHYL BENZHYDROL) 5 10 1. Protium Compound, Plot ot log(O.D.*IO) vs T i m e 2. Deuterium Compound, Plot of I / O.D. vs T i m e 3. Protium C o m p o u n d , Plot ot I / O.D. vs T i m e O . D . = Optical D e n s i t y Time - 0 . 7 5 - 0.73 0.71 - 0 . 6 9 20 min 0.67 i—• 18 disproportionates into the corresponding ketone and hydrocarbon. Both the protium and deuterium species were stable at fu l l ionisation for a considerable time which shows that the decay of colour usually observable at lower ionisations is not taking place; e.g. the amount of the unionised benzhydrol, which is the other reacting species, is negligibly small in this case. Similarly as for the 4,4'-dimethoxy benzhydrol, the optical densities were lower for the deuteriated species. The isotope effect for the 4,4'-dimethyl benzhydrol at 50$ ionisation was evaluated as follows. The optical densities at fu l l ionisation were 1.017 for the protium and 0.970 for the deuterium analogues respectively. The i n i t i a l optical densities at 50$ ionisation were evaluated by extrapolation to zero time from a second order kinetic plot (Fig. l) and were equal to 0.590 and 0.520 for the protium and deuterium compounds respectively. At this point i t is convenient to define a quantity Q by equation 8.: Q = concn. of coloured species concn. of colourless species concn. of R* concn. of R0H The quantity QH where subscript H (D) stands for the protium (deuterium)compound, is known from following• log QH r log 0.590 = 0.14 ± 0.02 1.017 - 0.590 similarly• log QD = 0.06 + 0.02 The isotope effect is given by equation 3 or alternatively by: 19 %A D - antilog (log 0^ - log QD) = antilog (0.14 - 0.06) = 1.20 + 0.05 The isotope effects were evaluated in a similar manner for the other benzhydrols. Other data appear in Table II. (c) 4.4'-Dichloro benzhydrol. The absorptions were determined at A M 483 mu for both, protium and deuterium compounds and the ionisation constants were evaluated by method A. Even though the carbonium ions formed were fairly stable, extrapolation to zero time was necessary, A second order kinetic approximation was used to obtain the i n i t i a l concentration of the species although first order kinetic plot as well as direct extrapolation from the graph of optical density versus time gave equally good results. In this case i t was not possible to distinguish between first and second order kinetics since fading of the colour was slow and the experimental error did not justify making such distinction. Again as before, the disappearance of the carbonium ion was slower for the deuteriated species. In addition its optical density was lower than that of the protium analogue. The ratio % / % was determined to be I .35. Other data appear in Table II. (d) Benzhydrol (51)? The protium compound was prepared by recrystallisation of the commercial product from carbon tetrachloride and petroleum ether (b.p. 85-95°), m.p. 66-67°. Benzhydrol -<K -d was synthesised by the method of Stewart (52) by the reduction of benzophenone with * The author is indebted to Dr. R. Stewart from the preparation of the benzhydrol- o< -d and Mr. A.L. Gatzke for the determination of the ionisation constants and evaluation of the deuterium isotope effect. 20 LiAlD^. Spectra of both compounds were identical in shape and position of the wavelength, and the ionisation constants were determined at 442 mu which corresponds to the -\ Stock solution of the benzhydrols was prepared in water and 0 ,1 ml aliquots were pipeted into 5.0 ml volumetric flasks. Appropriate correction was made to account for the dilution of the sulfuric acid with the introduced sample. Fading of the colour with time was observed and extrapolation to zero time was necessary. The decay of the deuteriated species was slower and also its optical density was lower than that of the protium analogue. No fading of the colour was noticed in 96$ sulfuric acid, which corresponded to the f u l l ionisation. The isotope effect K^/KQ was found to be 1 . 2 9 . Also a primary kinetic isotope effect which was assumed to be due to the reaction of the carbonium ion with the undissociated benzhydrol, in which a hydride ion transfer occurs, was found to be: kn/ko = 1.7« (e) 4.4'-Bis(dimethvlamino) Benzhydrol. Michlerf3 Hydrol. Species at 605 mu. The ionisation constants of the protium and deuterium compounds were determined by method B, e.g. in sodium acetate - acetic acid buffers. The concentrations of the Michler's Hydrol were 2.5 x 10~5, 6.25 x 10-5 , 1.25 x 10-5 M/1 and 2$, 5$ and 99$ acetone solutions respectively were used in the determinations. The absorptions were measured at A J J ^ = 605, 607 and 610 mp respectively for both the protium and deuterium compounds. The acetone was introduced with the sample, and without evaporation of acetone, the buffer or acid were added directly to i t . This method gave excellent reproducibility of measurements. An attempt was also made to determine the ionisation constants in the buffer alone (e.g. acetone was evaporated), however 21 i t was found that i t was necessary to use much higher concentration of the dye to obtain suitable readings, and the reproducibility of measure-ments was not very good. This was due to the fact that the buffer solutions had to be heated in order to get the dye into solution and then equilibrated again which could have caused undesirable side reactions. For this reason, the determinations of the isotope effect are reported only for the acetone solutions. Since the molecule of Michler's Hydrol contains two tertiary nitrogens, the situation is somewhat more com-plicated than with other benzhydrols and, as might be expected, there is competition between oxygen protonation and the subsequent formation of the carbonium ion, and nitrogen protonation which results in formation of the colourless species. Thus the following equilibria result as shown in Fig. 2. The carbonium ion, II, blue in colour, starts to appear at a pH 7.5, reaches a maximum intensity of absorption at pH 4«72 and then disappears again at pH 2.0. The colourless species, whose spectra start.to appear around pH 4.7, persist up to 60% sulfuric acid. The evidence for the presence of the species I, II and IV is apparent from the investigations of UV spectra of Michler's Hydrol which were taken at various acidities, in sulfuric acid - water system (Figs. 4, 5, 6, 7, 8 ) . The complete discussion of the spectral evidence will be given later. There might be some doubt as to the existence of the species III, where only one nitrogen is protonated, but i t is fairly obvious that such an entity exists at lower acidities. However i t occurs in a range too short for i t to be properly identified. Some evidence for its existence might be inferred from the shape of the ionisation curve of Michler's Hydrol Blue cation; one way to account for the almost flat part of the curve is to postulate the existence of another species (III) which 22 IONISATION EQUILIBRIA OF MICHLER'S HYDROL FIG. 2 vi. v. 23 reaches only a small concentration. This is illustrated in Fig. 3 . To account for the shape of the observed optical density the following explanation is offered: A hypothetical curve is assumed which represents the f u l l ionisation of II, measured in terms of the amount of the coloured species present. If another species exists, such that i t removes the colour, then a decrease in intensity of absorption of the coloured species should be observed. Moreover, to account for the almost flat part of the curve, the existence of two such colour removing species must be assumed. The fi r s t one ( i l l ) reaches only a small concentration and then starts to be removed by the other (IV), so that the concentration of II at any instant should be given by: M = M at f u l l l o c a t i o n " < tm] 4 fiv] ) 10. where square brackets stand for optical density. It can be seen from Fig, 3 that the above assumptions are reasonable. Considering the structure of III, i t is not unreasonable to assume that the protonation of one nitrogen will influence the basicity of the other nitrogen to only a small extent, and thus the protonation of the other nitrogen should occur at only slightly higher acidities. Goldacre and Phillips (49) reported in their findings that the optical density of Michler's Hydrol Blue as well as other basic dyes reached a maximum which persisted for several pH units. This may be true for other basic dyes but as can be verified from Fig, 3 i t does not apply to Michler's Hydrol Blue. The fact that they have observed a flat part of the ionisation curve instead of a peak may well be due 2k IONISATION OF MICHLER'S HYDROL 6.0 pH 1. Hypothetical Ionisation Curve of Michler's Hydrol Blue 2. Actual Ionisation Curve of Michler's Hydrol Blue in HOAc-NaOAc Buffer 3 . and 4. Hypothetical Ionisation Curves of Species HI and IV respectively Data are tor the Protium Analogue to the low optical densities (0.150) employed in their investigations and perhaps also to temperature fluctuations. The optical densities at the top of the ionisation curve vary actually very l i t t l e but the peak is unmistakable when high concentrations of the dye are used. It has been shown before (49) that almost a l l of the basic dyes containing nitrogen, in which category Michler's Hydrol belongs, are not completely ionised even though maximum optical density has been reached. For this reason i t was possible to determine only the "apparent" ionisation constant at 25°. Due to the extreme stability of Michler's Hydrol Blue cation i t was not necessary to extrapolate to zero time, in fact, the readings taken after 24 hours were identical with the i n i t i a l ones. Reproducibility was excellent for the 2% acetone solution, however only fairly good when straight buffer (no acetone) was used. At this point i t is necessary to mention and discuss the low extinction coefficient obtained in this investigation compared with that obtained by Mason and Grinter (53) who reported a value of 80,000, in sulfuric acid-water system. The value of €. m a x obtained by Mason is about eight times as high as that obtained in this investigation. This might perhaps imply that the Michler's Hydrol prepared in this laboratory was impure or that some serious mistake occurred during measurements. However i t was surprising to note that when a system of 99$ acetone and 1% sulfuric acid-water, was used for investigation of the ionisation constants, i t was found that the extinction coefficients were about twice as high as Mason's values. Due to the presence of acetone i t was necessary to go to higher acidities of sulfuric acid (0.01 N) to obtain maximum ionisation. It may be that Mason's work was done in a similar system. 26 The equilibrium isotope effect following systems: (a) 2$ by volume acetone solution of the acetic acid buffer. The acetone was not (b) 5$ by volume acetone solution of the acetic acid buffer. The acetone was not (c) 99$ by volume of acetone solution of of sulfuric acid. The results are summarised in Table I. Table I. Values of the Ratio of Equilibrium Constants of Michler's Hydrol Blue in Various Media. Ionising medium 2% acetone - buffer 1.08 5$ acetone - buffer 1.04 99$ acetone - 1$ (sulfuric 1.03 acid-water) The high value of the isotope effect for the 2$ acetone solution is attributed to the error arising from low optical density used. Other pertinent data are shown in Table II. was determined for the dye in 0.02N sodium acetate -evaporated. dye in 0.02 sodium acetate -evaporated. the dye with added 1$ by volume 27 Table II Ionisation and. Isotope Effect Data for Benzhydrols 4,4 '-Identically A max li-substituted benzhydrols •A max mu mu l i t . max 6 max l i t . -P% + l i t . KH / KD 0Me OMe- <X -d 504 504 507 116,200 106,700 109,700 5.85 5.92 1.18 Me Me- <* -d 468 468 472 86,800 72,000 74,200 10.31 10.39 10.4 1.20 CI CI- *-d 483 483 485 110,000 108,100 138,000 13.97 14.10 13.96 1.35 H H- «.-d 442 442 442 43,600 13.25 13.36 13.3 1.29 NMe NMe2- « -d 605 605 6 1 0 2 1 0 , 6 4 0 1 9,040 80 ,000 2 5.61 NMe2 NMe2- K -d 605 605 14,200^ 12,500 1.08 NMe 2 NMe2- v, -d 605 605 17,000^ 14,500 1.04 NMe2 NMe2- o(-d 608 608 1 5 6 , 5 0 0 5 156,500 I . 0 3 1 In buffer, not completely ionised at 25°• 2 Value obtained by Mason ( 5 3 ) . 3 2% acetone - buffer, not completely ionised at 2 5 ° . 4 5% acetone - buffer, not completely ionised at 25°. 5 99$ acetone - 1% (sulfuric acid-water). 28 FURTHER INVESTIGATIONS ON MICHLER'S HYDROL. 1. Species at 401 mu. At much higher concentration of sulfuric acid (60$) a new species appears which has an absorption band in the visible region with a peak at 401 mu. The complete spectrum of this species is shown in Fig. 9. This species is yellow in colour and starts to appear in 60$ sulfuric acid reaching its maximum absorption at 100$ sulfuric acid. For the sake of simplicity, the species at 401 mu will be called Michler's Hydrol Yellow. The optical density is again less for the deuterio compound as in the case of other benzhydrols. A time dependence study has shown that the species is very stable, though perhaps slightly less stable than the Michler's Hydrol Blue cation. The spectrum taken after 24 hours gave optical densities only 0.020 units less than the original readings, which is almost within experimental error. Visually i t was possible to observe the colour even after one week of standing. A temperature study has shown that the species is not completely ionised even in 100$ sulfuric acid. The data are shown in Table III,. For this reason only the "apparent" ionisation constant could be determined. Table III Temperature Variation of Optical Density of Michler's Hydrol Yellow. Temperature Optical Density 25.0 + 0.1° 0.865 1.0 + 0.5° 0.495 40.0 + 0.5° 1.168 Data are for the protium. compound in 100$ sulfuric acid. The fact that the species is not completely ionised even at optimum 29 conditions suggests, by analogy with Michler's Hydrol Blue, that there is some competition between the oxygen and nitrogen protonation reactions. Assuming that the colourless species which appears in the range from pH 1 , 0 to 60$ sulfuric acid has both of the nitrogens protonated, then an obvious conclusion presents itself that the species at 401 mu is a carbonium ion in which both of the nitrogens are protonated. This type of reasoning however, is not correct, since as will be shown in the subsequent section the carbonium ion in which both of the nitrogens are protonated in addition appears at a concentration of sulfuric acid over 100$ , e.g. in fuming sulfuric acid. Thus the species which appears in the range of 60 - 100$ sulfuric acid must be a carbonium ion in which only one of the nitrogens i s protonated. The equilibrium involved in the formation of this species is the following: H H IV V The ionisation constant should be given by: pKR*+ = RQ+ log(C H*+ /CBHOH** ) N « where RQ is defined as above (p.^ ). This may be derived as follows: RHOH** + H + ^ RH*** -r HgO 12. RH+++ — R*+ + H* 1 3 . Adding 10 and 11 one obtains: RH0H+* ^ R*+ + H20 1 4 . 30 Since the ionisation equilibrium, given by 10 , is governed by C^and that given by 11 , by H + functions i t follows that: R + - C * - H +. 1 5 . The function RQ was used here for reasons outlined above (p. S). Data in Table IV support in part the above assumptions, since the values of p%++ are sufficiently constant between 85 - 96$ sulfuric acid and only slight drift of values occurs. Such drifts were already observed by Deno et al (27) in his investigations of ionisations of benzhydrols. However, there is a larger decrease of p%++ values below 85$ sulfuric acid. The same effect was observed by Deno et al (27) who ascribe i t to the high concentration of the indicator used. Since high concentration of the dye (12.5 x 10~5 M/l) had to be used in this investigation, i t i s assumed that the inconstancy of the values below 85$ sulfuric acid was due to the same effect as found by Deno. It is also possible that the ionisation constant p % 4 + should follow the R + values. However such a function has not been evaluated as yet. The only study on the Hammett H+ function has been attempted by Brand and Horning (54) who estimated that the H + values are parallel to the HQ values, being more negative by 0 . 3 units, in fuming sulfuric acid. The actual value of RQ however, i s immaterial for the determination of the isotope effect, since this depends only on the ratio of the concentrations of the R + + and RHOH-*"*" species. The isotope effect %/Kn was evaluated to be 1 .12. Only the first four values were used in computing the isotope effect, since these were thought to be more accurate ( 2 7 ) . Other data are found in TableVIII. 31 Table IV. Apparent pKR++ Values for Michler's Hydrol Yellow in 73 - 96$ $ Sulfuric Acid 96.00 93-96 90.44 85.18 81.43 77.58 72.62 2. Species at 435 mu. At a concentration of sulfuric acid over 100$ a new species appears with an absorption band in the visible region, with a peak at 435 mu and a shoulder at 400 mu. The complete spectrum is shown in Fig. 10. The shape of the spectrum resembles that of the Michler's Hydrol Blue cation at 605 mu, however i t is obvious that i t is not due to the same species. This entity which is yellow in colour (slightly darker than the species at 401 imi) starts to appear at concentrations above 100$ sulfuric acid and reaches its maximum absorption at 30$ fuming sulfuric acid. The species at 435 mu will be called Michler's Hydrol Green. The absorption of the deuterio compound is again lower as in a l l previous cases. The species is reasonably stable as might be inferred from the approximate constancy of measurements for the first twenty minutes. On the other hand, a decrease in the absorption was observed after twenty hours, the Sulfuric Acid. P%5* • P KR^ A P KR + + -8.11 -8.18 0.07 -8.00 -8.05 0.05 -7.90 -7.95 0.05 -7.51 -7.54 0.03 -7.45 -7.47 -7.15 -7.18 -6.92 -6.94 32 optical density dropped from the original value of 1.200 to 0 . 8 0 0 (cone. 2.5 x 10-5 M/l). The temperature dependence investigation at the point of maximum ionisation (which occurs at 30$ fuming sulfuric acid) revealed that no change of optical density occurred other than that due to the decay of the species itself. If an assumption is made that the species is an ion, then i t is fully ionised at that point. The results are summarized in Table V. Table V. Temperature Data for Michler's Hydrol Green. Temperature Optical Density 25.0 ± 0.1° 0.755 16.0 ± 0.5° 0.739 36.O ± 1.0° 0.721 Measurements are for the protium compound in 3 0 $ fuming sulfuric acid (cone. 1.25 x 10"5 M/l). The same sample was used in a l l three determinations and the measurements at 25° were made firs t , followed by a reading at 16° and then at 36°. It is obvious from Table V that the optical density decreases for the readings at 25° .> 16° > 36°• .Considering that a minimum of lg hour elapsed between the first reading at 25° and last reading at 36° due to the fact that i t was necessary to change the temperature of the thermostated bath. From previous measurements i t appears that there is a decrease in optical density of 0.020 units in 40 minutes (as shown in Table VI); i t might be reasonable to assume that there will be a decrease of optical density of at least O.O3O units in l | - hour. This has actually been observed. This then immediately implies, that the temperature has l i t t l e effect upon the ionisation equilibrium and 33 that the species i s i n fact f u l l y ionised i n 30$ fuming sulfuric acid. Table VI. Decay of Michler's Hydrol Green with Time. Time, min. Optical Density 8 .5 0.760 22 .5 0.750 31.0 0.747 4 0 . 3 0.742 The data are for the protium compound in 30$ fuming sulfuric acid at 25° • The question now arises as to the identity of the species responsible for the absorption at 435 mu. At f i r s t i t i s necessary to establish whether a new species absorbing at 435 mu (VI) really exists and that i t i s not merely a solvent effect causing a shift of V from 401 mu to 435 mu. Thus i f VI i s to be a distinct entity, then i t might be expected that at some intermediate concentration of sulfuric acid (between 100 - 105$) , both species V and VI should appear at the same time. That this i n fact happens i s obvious from the spectrum shown in Fig. 11 where i t can be seen that both species appear at the same time i n about 2% fuming sulfuric acid. Moreover, the extinction coefficient of VI i s about 10 times that of V, which would be very hard to explain on the basis of sudden increase of ionisation of V, but which can be easily understood i f presence of species VI i s assumed. The spectra of V and VI are quite distinct which constitutes further evidence of existence of VI. 3. Evidence for the identity of VI. If one assumes that the species at 401 mu i s V then the nature of VI follows from i t . Realising the fact that VI appears at higher acidity, that V s t i l l contains one unprotonated nitrogen, then i t must follow that VI i s a carbonium ion i n which both of the nitrogens are protonated: H . _ + M M * = T Me,MH V VI Moreover, i f this i s true, then the dissociation of the conjugate base VI should obey the Hammett H + f function. Since such a function has not been evaluated as yet, the H 0 function was used to compute the values of the pK^ +++ and as may be seen from the Table VII the values of pKj£+++ are reasonably constant. Table VII. p K g V a l u e s for Michler's Hydrol Green i n Fuming Sulfuric Acid. % Sulfuric Acid *H ' KD 105.65 H . 6 9 11.74 104.50 11.94 12.03 IO3.4O 12.17 12.25 102.25 12.37 12.47 101.15 12.45 12.56 Best value 12.12 12.21 Concentration of the dye = 1.25 x TO - 5 M/l The drift of the pKR++* values may well be due to the fact that the values of HQ between 100 - 107$ sulfuric acid were determined (55) using p-nitrotoluene, nitrobenzene and p-chloronitrobenzene which are structurally different from Michler's Hydrol. That such difference may arise has been pointed out by Long and Paul (26). It is also possible that the H++, function would f i t the data better. However, as has been pointed out before, the actual value of HQ is immaterial in the evaluation of the isotope effect. This was found to be: KH/KD = 1.24. Other data are shown in Table VIII. Table VIII Ionisation, and Isotope Effect Data for Michler's Hydrol. Compound m^ax emax pKR4 VKD mu Michler's H 605 17,000 Hydrol 1.04 Blue D 605 14,500 Michler* s H 401 6,930 -7.63 Hydrol 6,350 -7.69 1.12 Yellow D 401 Michler's H 435 59,000 -12.12 Hydrol 58,600 1.24 Green D 435 -12.21 H 442 -13.30 Benzhydrol 1.29 D 442 -13.41 Symbols H and D refer to protium and deuterium analogues respectively. The data in Table VIII indicate that the isotope effect increases in the series Michler's Hydrol Blue, Yellow and Green and the increase may be associated with the number of protonated nitrogens. Moreover, the values of the wave length of absorption, extinction coefficient and the isotope effect for Michler's Hydrol Green are very 36 similar to that of benzhydrol. The significance of this will be discussed later in conjunction with the interpretation of the ultra-violet spectra. 4. Evidence for true equilibria. The quantitative aspect of the equilibria: VI — V ^ IV ^ III ^  II has been investigated in order to establish whether the postulated equilibria are truly reversible and that no sulfonation or any other s i d e r e a c t i o n s o c c u r r e d . In the discussion below the concentrations . of individual species have been adjusted so that a direct comparison may be possible. In actual fact a higher concentration of VI ( l 2 . 5 x 10"*5 M/l) was used and the spectrum was determined in a one cm cell with 9 mm spacer, then after appropriate dilution the spectrum was taken in a one cm cell without spacer in order to compensate for the dilution and then after further dilution a cell of 10 cm path length was used. Starting with the species VI, appearing in 30$ fuming sulfuric acid solution i t was possible to observe a spectrum where both species V and VI appear at the same time (Fig. 11). Diluting further, so that the final concentration of sulfuric acid was 96$, a quantitative spectrum of the species V alone was obtained. To obtain the spectrum of Species II, a slightly different technique was used. The 96$ sulfuric acid solution of the dye was diluted with water, cooled in ice, the sulfuric acid was :treated with sodium hydroxide until alkaline, the alkaline solution was extracted with ether, the ethereal extracts evaporated to dryness and the solid dissolved in the appropriate volume of sodium acetate -acetic acid buffer (pH 4.7) and the spectrum taken. It was found that the spectrum was identical with that of II in a l l respects. Moreover, the optical density was identical with that of a fresh sample of Michler's Hydrol Blue of the same concentration. This proves conclusively that the equilibria in the ionisation of Michler's Hydrol are truly reversible and that no sulfonation or side reactions occur. 5. Difference in A m a Y of V and VI As the values for the A j ^ ^ in Table VIII show, the species V (carbonium ion with one nitrogen protonated) absorbs at 401 mu whereas the species VI (carbonium ion with both nitrogens protonated) absorbs at 435 mu. The bathochromic shift which occurs by going from V to VI is somewhat baffling, since i t would be expected that species V would absorb at a higher wavelength than VI, because i t has one dimethylamino group available for resonance. When the structure of the species V and VI is investigated, i t is observed that V i s unsymmetrical whereas VI is symmetrical along its axis. Now, Brooker and his colleagues (56,57), who have investigated the symmetrical and unsymmetrical cyanines, found that the unsymmetrical cyanines absorb at a much lower wavelength, than the symmetrical ones. Considering again the species V, i t can be seen that the difference in basicities of the dimethylamino and the protonated dimethylamino groups i s extremely great, which in turn implies that the difference in energies of the principal contributing structures will be great and that more energy must be supplied before the transition occurs. For these reasons the absorption of the unsymmetrical V occurs at a lower wavelength than that of the symmetrical VI. The theoretical reasons for the above phenomena have been quantitatively worked out by Dewar (58,59). 38 INVESTIGATION OF ULTRAVIOLET SPECTRA. These were determined in the range of 200 - 700 mu on the Cary Model 14, Recording Spectrophotometer using cells of 1 cm path length, unless otherwise specified. Although concentrations of the samples were varied throughout the investigation, the interpretation of the spectra is made in terms of the extinction coefficient, so that the results are directly comparable. Occasionally a blank determination was also carried out in order to ensure that the solvent did not absorb in the range studied. Since no temperature control was attempted the quantitative data are only approximate,• this however, should not affect the interpretation of the spectra to a great extent. A l l the data apply to the protium compound only. 1. Spectrum of Michler's Hydrol (Fig. 4) . The spectrum of this compound was investigated using distilled water as solvent with one drop of 0.01 N sodium hydroxide added in order to keep the pH at the value of about 7.0. The spectrum showed the following absorption maxima, which are given in Table IX. Table IX. Absorption Maxima of Michler's Hydrol. Band mu € max 258 25,300 203 46,000 Since Michler's Hydrol has two dimethylamino groups in the molecule i t would be expected that also absorption in the 3OO mu region should occur (60,61,62). The failure to observe a distinct peak in the latter region is probably due to the low concentration 39 and to the limited solubility of the dye used. The peak at 3OO mp can be observed when spectrum i s taken in ethyl alcohol solution. The only observable absorption in the 3OO mu region in water solution is the t a i l of the absorption band at 258 mu, which extends into the region of 35O mu. The spectrum of Michler's Hydrol closely resembles that of aniline (60,61,62) or substituted anilines ( 6 1 , 6 3 ) . 2. Spectra of Michler's Hydrol Blue. Investigations of the spectra of this species were done in the sulfuric acid - water system in the range of 6.0 to 0,0 pH units, (a) Spectrum at pH 6 . 0 (Fig. 5 ) . Table X shows the following observed maxima: Table X. Absorption Maxima of Michler's Hydrol Blue at pH 6.0 Band mu e max 605 4,000 36O 1,000 300 shoulder 258 19,600 203 36,000 The band at 605 mu has been identified as the absorption due to the polarisation along the major axis of the carbonium ion (58,64); and the peak at 36O mu being due to polarisation along the minor axis of the carbonium ion (58,64). It may be seen that the band at 258 mu decreased in proportion to the formed carbonium ion. The bands at 300, 258 and 203 mu are associated with the unionised Michler's Hydrol. This confirms Goldacre's (49) assumption that the undissociated carbinol I is in 0 40 equilibrium with the carbonium ion II, at least at the beginning of the ionisation curve of Michler's Hydrol Blue. As will become obvious from the following spectral data the case becomes more complicated as the ionisation increases and at the peak of the ionisation curve at least two equilibria are involved, (b) Spectrum at pH 5 .0 (Fig. 6) . The absorption maxima are found in Table XI. Table XI. Absorption Mayjir.a o f Michler's Hydrol Blue at pH 5 .0 . Band mu € max 605 9,000 360 1,900 300 shoulder 253 11,500 203 32,000 again the intensity of the absorption at 605 mu rises as the intensity of the band at 253 ^ decreases. Also the position of the peak at 258 mu shifted to shorter wavelength by 5 mji and i t s shape has changed considerably. This i s probably due to the formation of the colourless species III. The absorption bands at 203, 300 are due to the undissociated Michler's Hydrol. (c) Spectrum at pH L.O (Fig. 7). Table XII gives the observed peaks. The carbonium ion (605,360 mp.) has already started to disappear. The band at 253 ^ has decreased only slightly in intensity, which would be in accordance with the assumptions made previously to account for the almost flat part of the ionisation curve of Michler's Hydrol Blue (Fig. 3 ) . A totally new peak at 218 mu appeared which may be ascribed with some certainty to the di-protonated species (IV), on the basis of analogy with protonated anilines and substituted anilines (65). The band at 203 np now appears as shoulder. Table XII. Absorption Maxima of Michler's Hydrol Blue at P H 4.0. Band mu € ^ 605 8,600 36O 1,000 300 shoulder 253 8,800 218 11,600 203 24,000 shoulder (d) Spectrum at pH 0.0 (Fig. 8). The spectrum at this acidity which is strikingly simple is shown in Table XIII. Both bands are most likely due to the di-protonated species IV. The extinction coefficient of the more intense band is in the range observed for the protonated anilines (61,65). The spectrum remains essentially unchanged from pH 0.0 to 60$ sulfuric acid. Table XIII. Absorption, MqTrijnft of Michler's Hydrol at pH 0.0. Band mu € ^ 253 2,000 217 17,200 42 3» Spectram of Michler's Hydrol Yellow. The spectra of this species were determined in the range of 60 to 100$ sulfuric acid. Only the spectrum in 96$ sulfuric acid is shown in Fig. 9. The conclusions were made on the basis of other spectra taken in the above range of sulfuric acid. (a) Spectrum in 81$ sulfuric acid. The observed bands are shown in Table XIV. Table XIV. Absorption Maxima of Michler's Hydrol Yellow in 81$ HgSO^  Band mu € ^ 401 2,850 286 1,440 256 2,000 217 13,500 As mentioned above, the bands at 217 and 256 mu are most likely due to the di-protonated species IV. The peak at 256 mu has shifted slightly towards a longer wavelength by 3 mu; probably a solvent effect. The peak at 401 mu is associated with the carbonium ion V, being probably due to the polarisation along its major axis. The peak at 286 mu is ascribed to the carbonium ion V, polarisation along minor axis. It can be seen from the next spectrum that the intensity of the band at 286 mu increases at the same time as the band at 401 mu, which supports the above assumption. (b) Spectrum in 96$ sulfuric acid (Fig. 9). The absorption maxima are shown in Table XV. The intensity of the carbonium ion (401,286 mu) has increased as would be expected. The intensity of the absorption of the 256 mu band has also increased; there is no explanation for this phenomena at present. Table XV. Absorption Maxima of Michler's Hydrol Yellow in 96$ H2S0^ Band mu 6 max 401 6,700 286 1,660 256 2,560 217 13,000 (c) Spectrum in 98$ sulfuric acid. The absorption bands observed at this acidity are given in Table XVI. Table XVI Absorption Maxima of Michler's Hydrol Yellow in 98$ HgSO^  B a n d 'r1 * max 401 6,800 286 2,160 256 2,240 217 12,600 Again the intensity of the bands due to the carbonium ion rise and the peaks at 256 and 217 mu decrease as would be expected. 4. Simultaneous appearance of Michler's Hydrol Yellow and Green. At a concentration of about 2$ fuming sulfuric acid, both species, Michler's Hydrol Yellow and Green appear at the same time and the resulting spectrum is a superposition of both (Fig. 11). The appearance of both species was effected by diluting the sample in 30$ 44 fuming sulfuric acid with a few drops of 60$ sulfuric acid. The presence of both species can be clearly seen from the spectrum. The concentration of the dye is in this case not specified. 5. Spectrum of Michler's Hydrol Green. The spectra of this species were determined in the range of 5 to 30$ fuming sulfuric acid. The spectrum below 250 mp i s not reliable due to the absorption of the solvent. Only one spectrum taken in 30$ fuming sulfuric acid i s shown in Fig. 10, however data are also reported for 10$ fuming sulfuric acid. (a) Spectrum in 10$ fuming sulfuric acid. Table XVII shows the observed maxima. Table XVII. Absorption Maxima of Michler's Hydrol Green in 10$  fuming sulfuric acid. B a n d € max 435 22,400 300 2,400 The band at 435 mp is attributed to the carbonium ion VI, being due to the polarisation along the major axis.. The absorption peak at 300 mu is probably due to polarisation along the minor axis. This is supported by the fact that the intensity of the peak at 300 mp increases at the same time as the band at 435 mu. (b) Spectrum in "30$ fuming sulfuric acid (Fig. 10). Absorption bands are shown in Table XVIII, Intensity of both bands increases as would be expected i f i t is assumed that the species ionises. There is also indication that the band which was present at 256 mp in lower concentration of sulfuric acid has dis-45 appeared. Since this peak is associated with the di-protonated species IV its disappearance would indicate f u l l ionisation of VI in 30$ fuming sulfuric acid. However this can not be concluded with certainty-due to the absorption of solvent in the 250 mu region. Table XVIII. Absorption Maxima of Michler's Hydrol Green in 30$  Fuming Sulfuric Acid. Band mu c r fc max 435 68,000 300 4,800 6. Spectrum of Michler's Hydrol in 60$ fuming sulfuric acid. In the concentration of sulfuric acid above 30$ fuming a new species VII appears with an absorption band in the 615 mu region. From visual observations i t appears that the fading of colour i s very fast; the new species seems to disappear completely in about half an hour. Since no quantitative measurements were made, the nature of this new species VII remains unknown at present. It is probably a decomposition product of Michler's Hydrol. 7 . Conclusions derived from ultraviolet spectra. In the following discussion,, the influence of nitrogen protonation upon the position of the wavelength of the first absorption band is summarized. The wavelength of the first absorption band (designated further by WFAB) is defined as the absorption band of the carbonium ion appearing farthest towards red in the region 200 - 700 mu. It was noted earlier (61,65,66) that protonated anilines or substituted anilines absorb essentially at the same wavelength as the parent hydrocarbons. Resemblance of the spectrum of Malachite Green ((p_Me5 MHCAH.)9CrHjC+ ) in concentrated sulfuric acid to that of 46 triphenylmethyl carbonium ion was also noted by Branch and Tolbert (67). A similar observation was made in this study where i t was found that the spectrum of Michler's Hydrol Green resembles the spectrum of benzhydryl carbonium ion not only in WFAB but also in the region of 3OO mp. Table XIX shows that the conclusion may be extended to triaryl + carbonium ions, where a substituent p-MegNH has approximately the same WFAB as the unsubstituted compound. From the above as well as other evidence (35,60-63,65-69). i t may be inferred that the protonated p-dimethylamino group in carbonium ions behaves as i f i t would be replaced by hydrogen alone, and that the WFAB of the nitrogen protonated species will be within + 15 mp of the WFAB of the unsubstituted compound. This assumption may be valid, providing that the p-dimethylamino group is not sterically hindered. The above postulate is further supported by the size of the isotope effect of the Michler's Hydrol Green which is comparable with that of benzhydrol itself. Similar evidence to the effect that the protonated amino group is completely out of resonance with the benzene ring was presented by Schneider (70) on the basis of his NMR studies. 9 47 Table XIX The WFAB of Carbonium Ions and Their Parent Hydrocarbons. Carbonium ion Crystal Violet* (a) Crystal Violet 4* (b) Malachite Green + + (d) Crystal Violet + + + > (f) Malachite Green+  + (g) Michler* s Hydrol Green WFAB mu 589 635 450 436 434 435 Ref. 53,64 35 67 69 67 Parent Hydrocarbon Malachite Green + (c) VIII (e) Malachite Green •+ Triphenyl methyl carbonium ion Benzhydryl carbonium ion WFAB mu 620 446 450 428 442 (a) (p-Me NC H ) C* 2 6 4 3 (b) (p-Me NHC H )(p-Me NC H ) C + 2 6 4 2 6 4 2 (c) (p-Me NC H ) CH C + 2 6 4 2 6 5 (d) (p-Me NHC H )(p-Me NC H )C H C + 2 6 4 2 6 4 6 5 (e) (p-Me NHC H )(C H ) C + ^ 2 6 4 6 5 2 (f) (p-Me NHC H ) (p-Me NC H )C + 2 6 4 2 2 6 4 (g) ( p - M e ^ H ^ C ^ C * Ref. 53,64 68 67 67 Note: The parent hydrocarbon refers here to a carbonium ion where p-Me2NH+group has been replaced by hydrogen. 48 SPECTRUM OF MICHLER'S HYDROL FIG. 4 200 300 400 nyj Wavelength SPECTRUM OF MICHLER'S HYDROL BLUE AT pH6.0 SPECTRUM OF MICHLER'S HYDROL BLUE AT pH 5.0 FIG. 6 SPECTRUM OF MICHLER'S HYDROL BLUE AT pH 4.0 52 53 SPECTRUM OF MICHLER'S HYDROL YELLOW IN 96% H 2S0 4 300 400 500 m/j Wavelength 54 SPECTRUM OF MICHLERS HYDROL GREEN IN 30% FUMING H 2S0 4 300 400 Wavelength 500 mj 55 S I M U L T A N E O U S A P P E A R A N C E O F S P E C I E S V A N D V I 200 300 400 500nyj Wavelength INFRARED SPECTRA These were determined i n order to establish the presence of deuterium i n the isotopically substituted molecule. The spectra were made i n KBr discs with the exception of benzhydrol and 4 , 4 ' -dimethoxy benzhydrol which were determined i n carbon tetrachloride. Table XX summarizes the vibrational frequencies of the C-H and C-D stretching and bending vibrations respectively. The C-H stretching frequencies were obscured due to the presence of phenyl and methyl hydrogens. Table XX. Infrared Data for Benzhydrols. 4 , 4 ' i d e n t i c a l l y Stretching Bending di-substituted frequency frequency benzhydrols cm"1 c m - l 0ME 1 ,364 OMe- <* _d 2,140 1,004 Me ' 1,357 Me- % _d 2,150 1,002 CI 1,350 CI- c< -d 2,140 1,000 NMe2 1,400 ? NMe2- c< -d 2,120 995 H obscured H- * -d 2,130 998 57 SOLVENT EFFECT UPON IONISATION OF MICHLER'S HYDROL BLUE. Since Michler's Hydrol Blue i s not f u l l y ionised i n the acetic acid - sodium acetate buffer, an attempt was made to investigate some other solvents which would bring about complete ionisation, or at least increase the ionisation to a greater extent. Such use of solvent properties was made by Stewart (7l) i n the investigation of the basicity of Michler's ketone. At the same time this would permit the evaluation of the isotope effect at higher ionisations and enable comparison of results with those obtained i n the 2% and 5$ acetone solutions i n acetic acid - sodium acetate buffer. The solvents investigated wereJ acetone, acetonitrile, glacial acetic acid, nitromethane and ethyl alcohol. The molar concentrations of the dye were identical i n a l l solvents, so that direct comparison of the data was possible. In a l l cases, with the exception of nitromethane and acetic acid, the dye was dissolved i n 99$ by volume of the solvent and to i t was added 1$ by volume of sulfuric acid of desired concentration. The acidity of the sulfuric acid was usually within the range of 1.0 to 3.0 pH units. Nitromethane and acetic acid were excep-tional since the dye ionised i n them without the addition of the acid. A shift of about 2 - 5 mu towards a longer wavelength was observed i n the organic solvents compared to that i n water. The results are summarized i n Table XXI. As may be noted from data i n Table XXI the extinction coefficients for the acetone, nitromethane, acetic acid and acetonitrile solutions of the dye are a l l approximately of the same magnitude. Thus i t may be assumed that the species i s essentially f u l l y ionised i n these solvents. The comparison with extinction coefficients of the other benzhydrols reveals that they are of the same order of magnitude as those 58 for Michler's Hydrol Blue in the above solvents. Since no temperature study of the ionisation equilibria was ma.de, the claim that the species is fully ionised cannot be fully substantiated. Ethyl alcohol was an exception, and i t was found that the extinction coefficient of the Michler's Hydrol Blue was low, comparable in magnitude to that of the dye in water (Table II). This is explained in part by the fact that Michler's Hydrol forms an ethyl ether in ethyl alcohol (72). It appears that the more important governing factor is the formation of the nitrogen protonated species III, with a charge localised on the nitrogen, which would be more favoured in highly polar solvents such as water or ethyl alcohol. On the other hand i t might be expected that its formation would be greatly suppressed in organic solvents, whereas the formation of the carbonium ion, with a greater charge delocalisation, should not be affected to a great extent. That this assumption seems correct may be seen from the data in Table XXI. However, there appears to be no correlation between the physical properties of the solvents, such as the dielectric constant, and the extinction coefficient of Michler's Hydrol Blue. It seems that the effect of the solvents in this case is highly specific. The exceptional behaviour of nitromethane i s not very well understood. Even though nitromethane is a pseudo acid, its apparent pKa (73) would be too small to effect the ionisation of Michler's Hydrol Blue. Thus i t is assumed that nitromethane contained probably a trace of nitric acid which would be sufficient to produce ionisation. The formation of the carbonium ion in acetic acid is of some interest since the ion appears in 0 . 0 1 N acetic acid, disappears at higher acidities and re-appears again in 90-100$ acetic acid. The appearance 59 of the species may be presumably associated with the change in dielectric constant of the medium, e.g. decrease of dielectric constant with increas-ing concentration of acetic acid. and ethyl alcohol and was found comparable to the values obtained in 2% and 5$ acetone solutions of the dye in acetic acid - sodium acetate buffer. In addition the isotope effect for the equilibrium between II and III was studied in 99$ acetone and i t was found that now the isotope effect was reversed with the value of K H / K D was 0.90. It should be noted that in the formation of the carbonium ion II from the unionised Michler's Hydrol I, the hybridisation of the central carbon atom changes from sp3 to sp^. Thus in the reverse process, e.g. formation of the nitrogen protonated species III from the carbonium ion II, an inverse isotope effect would be expected since now the change in hybridisation is from sp 2 to sp3. This is in fact found; moreover, the isotope effect with the value of % A n = 1» 0 ^ obtained for the equilibrium between I and II. The isotope effect has been investigated in acetone, acetonitrile 1.11 for the equilibrium between I I and III is well comparable 60 Table XXI. Variation of Extinction Coefficients and Isotope  Effect of Michler's Hydrol Blue with Solvents. Concentration of acid to produce maxi- X max € max Solvent mum ionisation mu ^ max l i t . Kg, H 156,500 Acetone 0.01N H.SO. 608 1.03 2 4 D 156,500 Mitro methane 610 H 161,500 D 161,500 Glacial H 151,500 148 , 0 0 0 ^ Acetic 607.5 Acid D 151,500 (h\ H 145,000 Aceto- J 0.01N nitrile H SO 607 1.04 4 D 143,500 H 18,500 Ethyl 0.10N Alcohol H2S0. 607 1.04 4 D 16,500 Symbols H and D refer to the protium and deuterium analogues respectively. (a) See reference 64. (b) Values of r\max were rising slowly with time at lower ionisations. Limiting values were used for evaluation of the isotope effect. 61 DISCUSSION AMD CONCLUSIONS. (a) Deuterium Isotope Effect. The actual underlying causes of the secondary deuterium isotope effect are not fully understood at present. Two main trends are apparent when the explanation of the isotope effect is sought. The theory that deuterium has greater inductive electron releasing power than protium, was proposed as being the principal cause of this effect, by Halevi (9,10). Van Dyke Tiers who studied the effect of deuterium substitution upon the NMR chemical shift in the CH^  group of toluene (74) found that the protons in the CEpD are more shielded than the protons in the CH^  group; this implied greater electron donating power from deuterium than from protium. Study of the chemical shift in n-C^FyH and the corresponding deuterio compound revealed (75) that the fluorine nuclei in the CF^ D group are 0.60 + 0.05 ppm more shielded than those in the CF2H group* This, presumably, implied again greater electron releasing power of deuterium. Both of these studies are strongly in support of Halevi's assumptions. On the other hand, Halevi's explanation of the effect of o< -deuterium substitution in phenylacetic acid is based on the reported 0.009 1 difference in the C-H and C-D bond lengths (76), where the shorter C-D distance implies greater electron density about the carbon atom and thus weaker acid properties. However the acidity changes are explicable entirely on the basis of changes of the stretching and bending frequencies of the methyl group (ll) which are sufficiently lowered in the anion (77) to account for the acid weakening effect of deuterium. Comparison of the results obtained in this investigation with those of Halevi reveal that the isotope effects, even though comparable in magnitude, are actually in the reverse direction to that which would 62 be predicted on the basis of greater electron release from deuterium. This suggests that the explanation of the isotope effect can not be sought simply on the basis of the inductive effect of deuterium but presumably more than one factor is operative (l9). The other point of view, that the isotope effect should be sought principally in the difference of vibrational frequencies between the ground and transition states in the rate process (p.3) was put forward by Streitwieser et al (ll) who have put their assumptions on a semi-quantitative basis using an aldehyde C-H as a model for the incipient carbonium ion. Their approach i s briefly summarized below. The assignment of frequencies of the aldehydic C-H bond was taken as ~2800 cm~" (stretching), ~1350 cm""'* (in the plane bending) and ^ 800 cm'""'- (out-of-plane bending). The assignment of the tertiary C-H bond, which changes to the H-C"** in the transition state, was taken as • 2900 cm"-'- (stretching) and I34O cm"^- (bending). The changes may be represented as 2900 ~2800, 1340 -*• /VI350 and 1340 ^800. The decrease in stretching frequency of ~100 cm""-'- would result in k^/k^ = 1.06. One bending vibration remains essentially unchanged, the reduction in the second of ~550 cm-1 leads to an isotope effect 1^ /kp - I . 3 8 . Consequently i t appears that the most important factor involved in the c( -deuterium isotope effect is the isotopic inhibition of the out-of-plane bending vibration of the cation which is looser than the corresponding vibration in the ground state. Weston (22) who has examined some of the experimental data for the isotope effect, states that a l l the evidence presented in favour of an electronic isotope effect is explicable on the basis of vibrational effects. Moreover, i t would be difficult to conceive of a kinetic experiment which could measure an electronic isotope effect 63 separately from the usual vibrational effect. More recent investigation of Seltzer (78) of the cis-trans isomerisation of maleic acid and maleic acid-2,4-d2, and that of Bender (79) of the hydrolysis of ethyl acetate and ethyl acetate- o( -d^ are also in support of Streitwieser 1s explanation of the isotope effect. The results of the present investigation reveal isotope effects comparable in magnitude and in the same direction as would be predicted on the basis of Streitwieser's considerations. In addition, i t was observed that the isotope effects in the benzhydrol series vary with the nature of the substituent group. Thus for an intrinsically more basic substituent the positive charge resides mainly on this substituent and the isotope effect is consequently very small. However i f the sub-stituent is more electron withdrawing in character, then the positive charge resides more on the central carbon atom to which the deuterium is attached, and the isotope effect is greater. This is well demonstrated by the two extremes: Michler's Hydrol Blue, where the nitrogen can greatly stabilise the positive charge and the isotope effect is extremely small (Kjj/Kp = I .O3), and the 4,4'-dichlorobenzhydryl cation, where the positive charge density is probably much greater round the central carbon atom and consequently the isotope effect is much greater (K^ /Kp - I . 3 5 ) . The same trend was observed in the series Michler's Hydrol Blue, Yellow and Green which is discussed in the following section. The above observations of the variation of the isotope effect with the substituent are not explained very well on the basis of Streitwieser's assumptions who claims that the presence of a net positive charge on a trigonal carbon apparently does l i t t l e to the vibration frequencies of attached bonds ( l l ) . This suggests that the isotope effect might be sought, in addition to Streitwieser's explanation, in the difference in polarisability of the C-H and C-D bonds respectively. Ingold et al (BO) have shown in their study of benzene and hexadeuteriobenzene that the static polarisability of the deuteriated benzene was lower than for the protium compound. The same conclusions were reached by Dixon and Schiessler (8l) on the basis of their investigation of refractive indices of benzene, hexadeuteriobenzene,cyclohexane and cyclohexane-d^2. They interpreted the lowering of molecular refract-ivity for the deuterio compounds as a lowering of the static polarisability of the molecule. Thus in the case of the 4 , 4 ' -dichlorobenzhydryl cation due to the great electron demand of the central, partially positive carbon atom, the C-H bond should be more polarisable than the C-D bond, and consequently the electron release should be greater from hydrogen than from deuterium. On the other hand, in the case of Michler's Hydrol Blue, the electron demand of the central carbon atom i s very small which might in turn imply that the C-H bond is only slightly more polarised than the C-D bond. In support of both Streitwieser's explanation and the polarisability explanation is the inverse isotope effect (Kpj/KD = 0 . 9 0 ) observed in the equilibrium of Michler's Hydrol Blue with the species III. Here the change in hybridisation is from sp 2 to sp^ and the polarisability differences now operate in the direction opposite to that when the carbonium ion is formed from its uncharged base, (b) Behaviour of Michler's Hydrol in acidic solutions. Investigations of Michler's Hydrol were made at acidities ranging from the pH 7.0 to 60$ fuming sulfuric acid. The region of 40 to 60$ fuming sulfuric acid was investigated only qualitatively. 65 The primary interest was to determine the magnitude of the isotope effect for the series Michler's Hydrol Blue, Yellow and Green. The isotope effect was found to be very small (K^/K^ = I.03) for Michler's Hydrol Blue, but increased for Michler's Hydrol Yellow (K^/K^ = 1.12) and reached i t s maximum value in the case of Michler's Hydrol Green (%AD = 1.24). The latter value is comparable with the isotope effect obtained for benzhydrol, where K^/ Kj) = 1.29. It is clear that the isotope effect for Michler's Hydrol increases for the series Blue, Yellow, Green in that order, and the increase is ascribed to the decreasing number of the resonating groups. This seems to agree well with the assumptions made previously in connection with the ultraviolet spectra, where i t is postulated that the protonated p-dimethylamino group has essentially no effect upon the rest of the molecule. Attention has also been devoted to the identification of the species Michler's Hydrol Yellow and Green. A certain amount of information has been derived from the ultraviolet spectra, magnitude of the isotope effect and the behaviour of the species towards the HQ and GQ functions. Even though the evidence is not conclusive i t suggests that Michler's Hydrol Yellow is a cation with one nitrogen protonated and Michler's Hydrol Green is a carbonium ion with both of the nitrogens protonated. The identities of the colourless species III and IV have also been investigated and the conclusions were arrived at from the spectral evidence and the shape of the ionisation curve of the Michler's Hydrol Blue. It may be assumed with some certainty that the species III exists and is the unionised Michler's Hydrol with one nitrogen protonated and that the species IV is the unionised Michler's Hydrol 66 with both nitrogens protonated. (c) Solvent effect upon ionisation of Michler's Hydrol Blue. The results indicated that the species II i s almost fully ionised in acetone, acetonitrile, glacial acetic acid and nitromethane. The almost f u l l ionisation of Michler's Hydrol to the blue carbonium ion was explained on the basis of the relative stabilities of II and III which differ in the delocalisation of the positive charge. The isotope effect was examined in some solvents and was found to be comparable with that in 2% and 5$ acetone solutions of the dye in the buffer. 67 SUGGESTIONS FOR FURTHER RESEARCH. The results of present investigation suggest that the isotope effect should be studied for the 4>4'-dinitro benzhydrol, where i t i s hoped that the great electron withdrawing character of the nitro group should show an isotope effect much greater than so far observed i n the benzhydrol series. I t might also be of some interest to study the effect of o< -deuterium substitution upon the equilibria of appropriately substituted benzhydryl carbanions where now probably the effective amount of negative charge on the central carbon atom w i l l determine the direction of the isotope effect. As another alternative, i t i s suggested to study the effect of o( -deuterium substitution i n appropriately substituted fluorene derivatives where i t i s hoped that the bending C-H motion w i l l be sufficiently restricted to the out-of-plane bending vibration, and thus the scope of Streitwieser's predictions may be tested. Determinations of the isotope effects i n some of the above systems w i l l be undertaken i n the near future in this laboratory. 68 BIBLIOGRAPHY 1. Urey, H.C., Brickwedde, F.G., and Murphy, G.M.: Phys. Rev., 32., 164 (1932). 2. Cremer, E., and Polanyi, M . : Z. physik. Chem., BJ2, 443 (1932). 3. Eyring, H., and Sherman, A.: J. Chem. Phys., 1, 435 (1933). 4. Wisberg, K.B.: Chem. Revs., £5., 713 (1955). 5. Streitwieser, A., Jr.: ibid, j>6, 571 (1956). 6. Bigeleisen, J.: J. Chem. Phys., 1 2 , 675 (1949). 7. Westheimer, F.H., and Nicolaides, N.: J. Am. Chem. Soc, 21, 25 (1949). 8. Westheimer, F.H.: Chem. Revs., 4J>, 419 (1949). 9. Halevi, E.A., and Nussim, N.: Bull. Res. Counc. of Israel, k, 263 (1956). 10. Halevi, E.A.: Tetrahedron, 1, 174 (1957). 11. Streitwieser, A., Jr., Jagow, R.H., Fahey, R.C., and Suzuki, S.: J. Am. Chem. Soc, 80, 2326 (1958). 12. Shiner, V.J., Jr.: ibid, 2k, 5285 (1952). 13. Shiner, V.J., Jr.: ibid, 21, 2925 (1953)* 14. Shiner, V.J., Jr.: ibid, 76, I6O3 (1954). 15. Boozer, C.E., and Lewis, E.S.: ibid, 2k, 63O6 (1952). 16. Boozer, C.E., and Lewis, E.S.: ibid, 2k, 794 (1954). 17. Lewis, E.S., and Boozer, C.E.: ibid, 2k, 791 (1954). 18. Lewis, E.S., and Coppinger, G.M.: ibid, 76, 4495 (1954). 19. Stewart, R., Gatzke, A.L., Mocek, M., and Yates, K.: Chem. and Ind. 331 (1959). 20. Elliott, J.J., and Mason, S.F.: ibid, 488 (1959). 21. Llewellyn, A., Robertson, R.E., and Scott, J.M.W.: ibid, 723 (1959). 22. Weston, R.E., Jr.: Tetrahedron, 6, 31 (1959). 23. Hammett, L.P., and Deyrup, A.J.: J. Am. Chem. Soc, 2721 (1932). 69 2 4 . Hammett, L.P.: Physical Organic Chemistry. Chapt. IX. McGraw H i l l Book Co., Inc., New York (1940). 25. Brand, J.C.D.,Horning, W.C., and Thoraley, M.B.: J. Chem. Soc, 1374 (1952). 2 6 . Paul, M.A., and Long, F.A.: Chem. Revs., 21, 1 (1957). 27. Deno, N.C., Jaruzelski, J.J., Schiersheim, A.: J. Am. Chem. Soc, XL, 3044 (1955). 28. Gold V., and Hawes, B.W.V.: J. Chem. Soc, 2102 (l95l). 29. °eno, N.C., and Taft, R.W., Jr.: J. Am. Chem. Soc, 2k> 244 (1954). 3 0 . Lowen, A.H., Murray, M.A., and Williams, G.: J. Chem. Soc, 33I8 (1950). 3 1 . Walden, P.: Ber., 2018 (1902). 3 2 . Gomberg, M.: ibid, 2ly 2403 (1902). 33. Lichtin, N.N., a n d Bartlett, P.D.: J. Am. Chem. Soc, 23, 5503 ( l 9 5 l ) . 34. Flexser, L.A., Hammett, L.A., and Dingwall, A.: ibid, JJ2, 2103 (1935). 3 5 . Adams, E.Q., and Rosenstein, L.: ibid, 3&, 1452 (1914). 36. Hantzsch, A.: Z. physik. Chem., 61, 257 (1908). 37. Newman, M.S., and Deno, N.C.: J. Am. Chem. Soc, 22, 3644 (l95l). 38. Newman, M.S., and Deno, N.C.: ibid, 22, 365I (l95l). 39. Hammett, L.P., and Treffers, H.P.: ibid, j>9_, 1708 (1937). 40. Gillespie, R.J., Hughes, E.D., and Ingold, C.K.: J. Chem. Soc, 2473 (1950). 41. Gillespie, R.J.:ibid, 2493 (1950). 42. Gillespie, R.J.: ibid, 2517 (1950). 43. Gillespie, R.J.: ibid, 2537 (1950). 44. Gillespie, R.J.: ibid, 2542 (1950). 45. Bethel, D., and Gold, V.: ibid, 1905 (1958). 46. Weiler, J.: Ber., 2, 1184 (1874). 47. Montagne, P.J.: Rec. Trav. Chim., 24,, 114 (1905). 48. Arient, J., and Dvorak, J.: Chem. Listy, i±8, 1581 (1954). 70 49. Goldacre, R.J., and Phillips, J.N.: J. Chem. Soc, 1724 (1949). 5 0 . Balfe, M.P., Kenyon, J., and Thain, E.M.: ibid, 790 (1952). 51. Gatzke, A.L.: M.SC. Thesis, University of British Columbia, Vancouver, British Columbia. (1959). 52. Stewart, R.: J. Am. Chem. Soc, 22, 3057 (1957). 53* Mason, S.F. and Grinter, R.: Steric Effects in Conjugated Systems, edited by G.W. Gray, Butterworths Scientific Publications, London, 1958. 54. Brand, J.C.D., and Horning, W.C.: J. Chem. Soc, 1374 (1952). 55. Brand, J.C.D.: ibid, 997 (1950). 56. Brooker, L.G.S.: Rev. of Modern Physics, 14, 275 (1942). 57. Brooker, L.G.S., White, F.L., Sprague, R.H., Dent, S.G., Jr., and Van Zandt, G.: Chem. Rev., 41, 325 (1947). 58. Dewar, M.J.S.: J. Chem. Soc, 2329 (1950). 59. Dewar, M.J.S.: Recent Advances in the Chemistry of Colouring Matter. The Chemical Society, London, 1956, p.64. 60. Bowden, K., and Braude, B.A.: J. Chem. Soc, 1068 (1952). 61. Wohl, A.: Bull. Soc Chim., 6, I3I2 (1939). 62. Remington, W.R.: J. Am. Chem. Soc, 62, I838 (1945). 63. Klevens, H.B., and Piatt, J.R.: ibid, 71, 1714 (1949). 64. Barker, C.C.: Steric Effects in Conjugated Systems, edited by B.G. Gray, Butterworths Scientific Publications, London, 1958. 65. Doub, L. and Vandenbelt, J.M.: J. Am. Chem. Soc, §2, 2715 (1947). 66. Moede, J.A., and Curran, C.: ibid, 71, 852 (1949). 67. Branch, G., and Tolbert, B.: ibid, 21, 781 (1949). 68. Branch, G., and Walba, H.: ibid, 26, 1564 (1954). 69. Perekalin, V.V., Savostjanova, M.V., and Morozova, R.I.: Zhur. Obshchei Khim., 22., 821 (1952). 7 0 . Schneider, W.G.: Private Communication. 7 1 . Stewart, R.: J. Am. Chem. Soc.: In the Press. 71 72. Fischer, 0.: Ber., 3356 (1900). 73. Pearson, M., and Dillon, R.: J. Am. Chem. Soc, 25., 2439 (1953). 74. Van Dyke Thiers, G.: J. Chem. Phys., 22, 963 (1958). 75. Van Dyke Thiers, G.: J. Am. Chem. Soc, 22, 5585 (1957). 76. Miller, S.L., Aamodt, L.C., Dousmanis, G., Townes, C.H., and Kraitchman, J.: J. Chem. Phys., 20, 1112 (1952). 77. Edsall, E.T.: ibid, A, 1 (1936). 78. Seltzer, S.: Chem. and Ind., I3I3 (1959). 79. Bender, M.L., Feng, M.S., and Jones, J.M.: ibid, 1350 (1959). 80. Ingold, C.K., Raisin, C.G., and Wilson, C.L.: J. Chem. Soc, 915 (1936). 81. Dixon, J.A., and Schiessler, R.W.: J. Am. Chem. Soc, 26, 2197 (1954). 

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