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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  i n 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 the  this thesis i n partial  r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y  of B r i t i s h Columbia, I agree that it  fulfilment of  freely  agree t h a t for  the Library  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 . permission f o r extensive  make  I further  copying of t h i s  thesis  s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e Head o f my  D e p a r t m e n t 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 or p u b l i c a t i o n of t h i s  gain  s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n  Department o f  Chemistry  The U n i v e r s i t y o f B r i t i s h Vancouver 8 , Canada. D a t e  shall  ftrf.  Columbia,  thesis  for financial permission.  ii ABSTRACT. The effect of c< -deuterium substitution upon the ionisation of five identically di-substituted benzhydrols was studied.  I t 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 i n character  but was greater i n 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 i s 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 i s almost completely ionised i n some of these solvents.  iii  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 8  1. The H and C Acidity Functions Q  Q  2.  IR, UV and NMR Spectra  10  3.  Evidence for Existence of Carbonium Ions  10 12  EXPERIMENTAL  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 i n A  37  of V and VI  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  4.  Simultaneous Appearance of Michler's Hydrol Yellow  .  42  and Green  43  5.  Spectrum of Michler's Hydrol Green  44  6. 7.  Spectrum of Michler's Hydrol i n 60$ Fuming Sulfuric Acid. 45 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  vi TABLES Page I  Values of the Ratio of Equilibrium Constants of Michler's Hydrol Blue i n Various Media  II  Ionisation and Isotope Effect Data for Benzhydrols .... 27  III  Temperature Variation of Optical Density of Michler's Hydrol Yellow  IV  Apparent pK++ Values for Michler's Hydrol Yellow i n  26  28  R  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  pKt+*. Values for Michler's Hydrol Green i n Fuming Sulfuric Acid  34  R  VIII  Ionisation and Isotope Effect Data for Michler's Hydrol 35  IX  Absorption Maxima of Michler's Hydrol  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  XIV  Absorption Maxima of Michler's Hydrol Yellow i n 81$ H S0  42  Absorption Maxima of Michler's Hydrol Yellow i n 96$ H S0  43  Absorption Maxima of Michler's Hydrol Yellow i n 98$ H S0  43  Absorption Maxima of Michler's Hydrol Green i n 10$ Fuming Sulfuric Acid  44  XVIII Absorption Maxima of Michler's Hydrol Green i n 30$ Fuming Sulfuric Acid  45  2  XV  2  XVI  2  XVII  XIX  4  4  4  The WFAB of Carbonium Ions and Their Parent Hydrocarbons  38  41  47  vii Pa£§ XX  Infrared Data for Benzhydrols  56  XXI  Variation of Extinction Coefficients and Isotope Effect of Michler's Hydrol Blue with Solvents  60  viii 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 i n % % HgSO^  53  10  Spectrum of Michler's Hydrol Green i n 30% Fuming H S0^ . . .  54  11  Simultaneous Appearance of Species V and VI  55  2  1  1  INTRODUCTION  Shortly after the discovery of deuterium i n 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 i s 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 i n 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 /k a 6.9 at 25° may be obtained (4). H  D  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 i n the rate determining step which led to the postulation of the following reaction mechanism:  2  CH.-CH-CH. + 3 |  3  2H + HGrO,"  3  OH  3 | /  +  CH -C-CH  3  3  +  3  HA  2  3  OCrO H  H <^A" u CH..-C) -CH  + H0  CH -CH-CH  4  2  + H CrO 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 i n 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. I t i s this secondary isotope effect \*hich was the object of this study and which shall be described i n more detail below. Halevi and Nussim (9) have shown that the presence of the deuterium on the o< -carbon atom i n phenyl acetic acid has a small but a significant effect upon the dissociation of the protium and deuterium compounds respectively: C/H CD 6 5 C  2  fiOOH  C,H_CD COO" + 6 5 2  H  +  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 i n terms of the difference of the inductive electron release which i s supposedlygreater from the deuterium than from hydrogen (10). Streitwieser, Jagow, Fahey and Suzuki ( l l ) 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 i n 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 i s similar i n  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 i n benzhydrol and 4,4'-  dimethoxybenzhydrol have pointed out that greater inductive electron release from deuterium, a difference i n the electronic wave function for the isotopically substituted species and vibrational differences  4 i n C-H and C-D bonds i n the transition state may a l l contribute to the stability and reactivity of isotopically substituted molecules. Mason and E l l i o t (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 i n the zero point energy of the N-H or N-D bonds i n the transition state with zwitter-ion structure as indicated below:  o Ph C CI A -  NH  2  Ph  Llewellyn, Robertson and Scott (21) who have determined the rates of hydrolysis for the methyl iodide, bromide, p-toluenesulphonate 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 s t i f f e r bending vibrations i n the transition state, i n which case the change i n zero point energy would be i n the right direction.  A very interesting explanation of the secondary deuterium isotope effect i s offered by Weston (22) who has come to the  5 conclusion that the difference i n dipole moments, nuclear quadrupole coupling constants, chemical shift i n NMR,  kinetic isotope effect and  isotope exchange equilibria can be explained almost completely on the basis of differences i n vibrational energies and anharmonicity of the isotopically substituted species. A deuterium isotope effect which may be included i n 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) i n the solvolysis 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 i s sought i n 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 i s attached to the reacting centre, such an  effect i s called an °( -deuterium isotope effect. Thus Streitwieser s 1  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 i s attached to carbon immediately next to the  reaction centre, then such effect i s 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 f a i r l y 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 H  Q  and C Acidity Functions. o  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 w i l l 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 i n accordance with the Broensted - Lowry theory of acids and bases, namely: B  +  H  BH  +  1.  +  H , which measures the tendency to confer a proton on an uncharged Q  base, has been defined by the equation: H  0  r PKJJH*  -log(C  B R +  2.  /C ) B  where Kgy* i s 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 i s an easily  measurable quantity; either the colourless uncharged base becomes coloured upon protonation or a measurable shift occurs i n the ultraviolet spectrum. and H  ++  By analogy with the above, the acidity functions 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 i t s values are known for the dilute aqueous solutions i n the pH range (23) up to 30$ fuming sulfuric acid (25).  The usefulness of the H  Q  function has been  amply demonstrated. Paul and Long (26), however, have indicated that discrepancies i n the H  Q  w i l l become apparent i f studies are made with  indicators differing widely i n structure. The equilibrium isotope effect i s given by the equation: K  H/ D  =  K  K  A  / K  B  =  C  AH ° B B H A +  / G  3  + C  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 H  Q  function, for an ionisation of the following type: ROH  +  H  +  —  R  +  +  4  H0 2  and has been defined by the following equation: 0 The C  Q  o  =  P V  -  l o g ( V /C  R()H  )  5  function becomes particularly useful i n 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 ( 2 9 ) .  favoured the use of the symbol J  Q  Paul and Long  instead of C , and more recently Q  Deno suggested the use of the symbol H  which was originally used by  R  Williams and coworkers. For the purpose of this investigation i t was necessary to define a function R  Q  by the following equation: R  -  0 - 0  C  -  H  6.  o  which has been applied to the ionisation of the type: RHOH  ++  ^  R" + +i  H0 2  7.  10.  Rigorously, according to the definition, H been used; as yet.  4  and C  +  should have  however, such acidity functions have not been evaluated  Moreover, the experimental results herein as well as studies  of Williams and coworkers (30) indicate the use of the H  Q  and G  Q  functions was, at least i n 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 i n 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 f i r s t l y i n the range 700 - 200 mp on the Cary Recording Spectrophotometer 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  w i l l 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 i n liquid sulphur dioxide to give conducting solutions comparable with those of methylammonium chloride and potassium iodide which are also good electrolytes i n 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 i n conducting solution but not i n 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 i s associated with the ion formed i n the ionisation of the compound (34,35).  Comparison with a spectrum of  related substances w i l l decide which ion i s responsible for the absorption. (c)  Cryoscopic measurements. The freezing point depression of a solvent i s 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 i n this f i e l d has been done by Hammett (39) and more recently by Gillespie  (40,41,42,43*44).  12  EXPERIMENTAL  1. (a)  Synthesis of A. A  1  identically di-substituted benzhydrols.  4.4'-Dimethoxv benzhydrol. 4,4'-Dimethoxy benzophenone (l2.1g) was dissolved i n  anhydrous tetrahydrofuran (100 ml) and LiAlH^ (0.5g) previously dissolved i n 25 mis of tetrahydrofuran was added to i t over a period of five minutes.  The mixture was refluxed for l g hour, then cooled  and poured over ice containing one ml of sulfuric acid, i n 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-6O ) mixture. 0  75$, m.p. 69-70°,  Yield  was about  ( l i t . : 69-70° (45)).  The deuterium analogue was prepared i n the identical manner with the exception that LiAlD^ was used i n place of the LiAlH^. Yield was about 50$> m.p. 69-69.5°.  The isotopic purity of this compound  was established by NMR to be 99$.  The absence of the proton absorption  in the deuteriated compound was considered a sufficient evidence of the purity of the deuterio analogue. (b)  4.4'-Dimethvl  benzhydrol.  4,4'-Dimethyl benzophenone (2.5g) was dissolved i n anhydrous tetrahydrofuran (25 mis) and LiAlH^ (0.2g) previously dissolved i n 10 mis tetrahydrofuran was added to i t over a period of five minutes.  * Note:  I t should be pointed out that a l l the yields reported herein are based on the f i r s t recrystallisation only, and that no attempt was made to recover any compound from the mother liquor.  13 The mixture was refluxed for 45 minutes. the  same as above.  Otherwise the procedure was  Recrystallisation was afforded from 1:1  ether-  petroleum ether mixture. Yield was about 40$, m.p. 69-70.5°,  (lit.:  69-700 ( 4 6 ) ) . The deuterium analogue was prepared as above by reduction of the 4,4'-dimethyl benzophenone with LiAlD^. m.p.  69-70°.  (c)  4.4*-Dichloro benzhydrol«  Yield was about 40$,  This compound was prepared i n a manner identical to that of 4,4'-dimethyl benzhydrol, by reduction of 4,4'-dichloro benzophenone with IdAlH^. Recrystallisation was made from 95$ ethanol. Yield was about 45$, m.p. 94-"95°, ( l i t . : m.p. 91.5°  (47)).  The deuterio analogue was prepared i n a similar manner. Yield was about 45$, 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 ( 4 8 ) . 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 u n t i l alkaline and the ether layer was separated without washing and l e f t to crystallise out. The Michler's Hydrol was repeatedly recrystallised f i r s t from ether and then from benzene, about ten times i n a l l . m.p. 101.5-103° ( l i t . : 102-103°  Yield was about 35$,  (49)).  The deuterio analogue was prepared i n an identical way by reduction of the Michler's ketone with LiAlD^.  Yield was about  14 35$, 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 i n 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 concentration of the sulfuric acid.  The protium and deuterium compounds were  weighed i n 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 s i l i c a 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 i n 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 f i r s t and second order kinetics; i n fact even the direct plot of concentration versus time gave good results. Some of the measurements of optical densities showed erratic behaviour during the f i r s t 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 i n the region studied, a blank c e l l 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 i n 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 i n order to ensure that a l l of the sample dissolved and the temperature was then brought to 25° i n a thermostated bath and optical density readings taken.  Since Michler's  Hydrol i s not completely ionised even at the optimum pH i n certain solvents, i t was only possible to determine the "apparent" ionisation constant at 25° i n 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 i n 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 extrapolation 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 i n a l l cases lower than that of the protium analogue.  The ratio % / % was found to be 1.18. Other  pertinent data appear i n Table I I . (b)  4i4 -Dimethyl benzhydrol. 1  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 f i r s t order kinetic plot gave a curved line, a second order  kinetic plot, however showed a good straight line, with the exception of the f i r s t 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 i t s e l f .  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 i s supported by Balfe  and coworkers (50) who established that 4,4'-dimethoxy benzhydrol  DISAPPEARANCE  OF CARBONIUM ION WITH  TIME  (4,4*-DIMETHYL  BENZHYDROL)  -0.75  -  0.73  0.71  -0.69  5 1.  Protium  Compound, Plot ot  2.  Deuterium  3.  Protium  Compound, Compound,  O.D. = Optical  Time  10 Plot Plot  log(O.D.*IO) of ot  vs  I / O.D. I / O.D.  0.67  Time vs  vs  2 0 min  Time  Time  Density  i—•  18 disproportionates into the corresponding ketone and hydrocarbon. Both the protium and deuterium species were stable at f u l l ionisation for a considerable time which shows that the decay of colour usually observable at lower ionisations i s not taking place; e.g. the amount of the unionised benzhydrol, which i s the other reacting species, i s negligibly small i n Similarly as for the 4,4'-dimethoxy benzhydrol, the optical  this case.  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 f u 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 i s convenient to define a quantity Q by equation 8.: Q =  concn. of R* concn. of R0H  concn. of coloured species concn. of colourless species  The quantity Q where subscript H (D) stands for the protium H  (deuterium)compound, i s known from following• log Q  H  r  log  0.590 1.017 - 0.590  =  0.14 ± 0.02  similarly• log Q  D  = 0.06 + 0.02  The isotope effect i s given by equation 3 or alternatively by:  19  %A  D  - antilog (log 0^ - log Q ) D  = antilog (0.14 - 0.06) =  1.20  +  0.05  The isotope effects were evaluated i n a similar manner for the other benzhydrols. Other data appear i n Table I I . (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 f a i r l y 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 f i r s t 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  f i r s t 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 i t s optical density was lower than  that of the protium analogue.  The ratio % / % was determined to be I . 3 5 .  Other data appear i n Table I I . (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 i s 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 i n 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 i n 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 i t s  optical density was lower than that of the protium analogue.  No fading  of the colour was noticed i n 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, i n which a hydride ion transfer occurs, was found to be: kn/ko (e)  =  1.7«  4.4'-Bis(dimethvlamino) Benzhydrol. Michler 3 Hydrol. f  Species at 605 mu. The ionisation constants of the protium and deuterium compounds were determined by method B, e.g. i n sodium acetate - acetic acid buffers.  The concentrations of the Michler's Hydrol were 2 . 5 x 10~5,  6.25 x 1 0 - 5 , 1.25 x 10-5 M/1 and 2 $ , 5$ and 99$ acetone solutions respectively were used i n 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 i n 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 measurements was not very good. This was due to the fact that the buffer solutions had to be heated i n 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 i s somewhat more complicated than with other benzhydrols and, as might be expected, there i s competition between oxygen protonation and the subsequent formation of the carbonium ion, and nitrogen protonation which results i n formation of the colourless species. Thus the following equilibria result as shown i n Fig. 2.  The carbonium ion, II, blue i n 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 i s 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 w i l l be given later. There might be some doubt as to the existence of the species III, where only one nitrogen i s protonated, but i t i s f a i r l y obvious that such an entity exists at lower acidities.  However i t occurs i n a range too  short for i t to be properly identified.  Some evidence for i t s 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 i s 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 i s illustrated in Fig. 3 .  To account for the shape of the observed optical density the following explanation i s offered: A hypothetical curve i s assumed which represents the f u l l ionisation of II, measured i n terms of the amount of the coloured species present. If another species exists, such that i t removes the colour, then a decrease i n 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 f i 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]  where square brackets stand for optical density.  4 fiv] )  10.  I t can be seen from  Fig, 3 that the above assumptions are reasonable. Considering the structure of III, i t i s not unreasonable to assume that the protonation of one nitrogen w i l l 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 i n 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 p H 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 i n 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 i s 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, i n 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, i n 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 f a i r l y good when straight buffer (no acetone) was used. At this point i t i s necessary to mention and discuss the low extinction coefficient obtained i n 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  i s about eight times as high as that obtained i n this investigation. This might perhaps imply that the Michler's Hydrol prepared i n 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. Mason's work was done i n a similar system.  I t may be that  26 The equilibrium isotope effect  was determined for the  following systems: (a) 2$ by volume acetone solution of thedye i n 0.02N sodium acetate acetic acid buffer.  The acetone was not evaporated.  (b) 5$ by volume acetone solution of thedye i n 0.02 sodium acetate acetic acid buffer.  The acetone was not evaporated.  (c) 99$ by volume of acetone solution of the dye with added 1$ by volume of sulfuric acid. The results are summarised i n 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 99$ acetone - 1$ (sulfuric acid-water)  1.04 1.03  The high value of the isotope effect for the 2$ acetone solution i s attributed to the error arising from low optical density used.  Other pertinent data are shown i n Table I I .  27 Table II Ionisation and. Isotope Effect Data for Benzhydrols 4,4'-Identically li-substituted benzhydrols  A max •A max mu lit. mu  6  max  max lit.  -P% lit.  + K  H D /K  0M OMe- <X -d  504 504  507  116,200 106,700  109,700  5.85 5.92  Me Me- <* -d  468 468  472  86,800 72,000  74,200  10.31 10.39  10.4  CI CI- * - d  483 483  485  110,000 108,100  138,000  13.97 14.10  13.96  H H- «.-d  442 442  442  43,600  13.25 13.36  13.3  NMe NMe - « -d  605 605  610  NMe NMe - K -d  605 605  14,200^ 12,500  1.08  NMe 2 NMe - v, -d  605 605  17,000^ 14,500  1.04  NMe NMe - o(-d  608 608  e  2  2  2  2  2  2  2  10,640 9,040  156,500 156,500  1  80,000  1.18  2  1.20 1.35 1.29  5.61  5  1  In buffer, not completely ionised at 2 5 ° •  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).  I.03  28 FURTHER INVESTIGATIONS ON MICHLER'S HYDROL. 1.  Species at 401  mu.  At much higher concentration of sulfuric acid (60$) a new species appears which has an absorption band i n the visible region with a peak at 401 mu. Fig. 9.  The complete spectrum of this species i s shown i n  This species i s yellow in colour and starts to appear i n  60$ sulfuric acid reaching i t s maximum absorption at 100$ sulfuric acid.  For the sake of simplicity, the species at 401 mu w i l l be called  Michler's Hydrol Yellow. The optical density i s again less for the deuterio compound as in the case of other benzhydrols.  A time dependence  study has shown that the species i s 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 i s 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 i s not completely ionised even i n 100$ sulfuric acid. The data are shown i n 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 i n 100$ sulfuric acid. The fact that the species i s not completely ionised even at optimum  29 conditions suggests, by analogy with Michler's Hydrol Blue, that there i s 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 i t s e l f that the species at 401 mu i s a carbonium ion i n which both of the nitrogens are protonated.  This type  of reasoning however, i s not correct, since as w i l l be shown i n the subsequent section the carbonium ion i n which both of the nitrogens are protonated i n addition appears at a concentration of sulfuric acid over 100$, e.g. in fuming sulfuric acid. Thus the species which appears i n the range of 60 - 1 0 0 $ sulfuric acid must be a carbonium ion i n which only one of the nitrogens i s protonated.  The equilibrium involved i n  the formation of this species i s the following:  H  H  IV  V  The ionisation constant should be given by: pK *+ R  =  R+ Q  log(C *+ /CBHOH** ) H  N  «  where RQ i s defined as above (p.^ ). This may be derived as follows: RHOH** + RH  H ^  RH*** -r HgO  +  —  +++  12.  + H*  13.  R* + H 0  14.  R*  +  Adding 10 and 11 one obtains: RH0H * ^ +  +  2  30  Since the ionisation equilibrium, given by 1 0 , i s governed by C^and that given by 11, by H  functions i t follows that:  +  R  +  The function R  Q  -  C*  -  15.  H +.  was used here for reasons outlined above (p. S ) .  Data i n Table IV support i n part the above assumptions, since the are sufficiently constant between 85 - 96$ sulfuric  values of p%++  acid and only slight d r i f t of values occurs.  Such drifts were already  observed by Deno et a l (27) i n his investigations of ionisations of benzhydrols.  However, there i s a larger decrease of p%++  below 85$ sulfuric acid.  values  The same effect was observed by Deno et a l  (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 i s also possible that the ionisation constant p % the R + values.  4 +  should follow  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, i n fuming sulfuric acid. The actual value of R  Q  however, i s immaterial for the  determination of the isotope effect, since this depends only on the ratio of the concentrations of the R isotope effect %/Kn  ++  and RHOH*"*" species.  was evaluated to be 1.12.  -  The  Only the f i r s t four  values were used i n computing the isotope effect, since these were thought to be more accurate ( 2 7 ) .  Other data are found i n TableVIII.  31 Table IV. Apparent pK++ R  Values for Michler's Hydrol Yellow in 73 - 96$ Sulfuric Acid.  $ Sulfuric Acid  2.  P%5*  • P R^  96.00  -8.11  -8.18  0.07  93-96  -8.00  -8.05  0.05  90.44  -7.90  -7.95  0.05  85.18  -7.51  -7.54  0.03  81.43  -7.45  -7.47  77.58  -7.15  -7.18  72.62  -6.92  -6.94  K  A  P R K  ++  Species at 435 mu. At a concentration of sulfuric acid over 100$ a new species  appears with an absorption band i n the visible region, with a peak at 435 mu and a shoulder at 400 mu. in Fig. 10.  The complete spectrum i s shown  The shape of the spectrum resembles that of the Michler's  Hydrol Blue cation at 605 mu, however i t i s obvious that i t i s not due to the same species.  This entity which i s yellow i n colour  (slightly darker than the species at 401 imi) starts to appear at concentrations above 100$ sulfuric acid and reaches i t s 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 i s again lower as in a l l previous cases.  The species i s  reasonably stable as might be inferred from the approximate constancy of measurements for the f i r s t twenty minutes. On the other hand, a decrease i n the absorption was observed after twenty hours, the  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 i t s e l f .  I f an assumption i s  made that the species i s an ion, then i t i s 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 i n 30$ 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 f i r s t , followed by a reading at 16° and then at 36°. It i s obvious from Table V that the optical density decreases for the readings at 25° .> 16° > 36°• .Considering that a minimum of l g hour elapsed between the f i r s t 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  i s a decrease i n optical density of 0.020 units i n 40 minutes (as shown in Table VI);  i t might be reasonable to assume that there w i l l  be a decrease of optical density of at least O.O3O units i n 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 f a c t f u l l y ionised i n 30$ fuming s u l f u r i c a c i d .  Table VI. Decay of Michler's Hydrol Green with Time. Time, min.  O p t i c a l Density  8.5  0.760  22.5  0.750  31.0  0.747  40.3  0.742  The data are f o r the protium compound i n 30$ fuming s u l f u r i c acid at 25° •  The question now arises as to the i d e n t i t y o f the species responsible f o r 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) r e a l l y exists and that i t i s not merely a solvent e f f e c t causing a s h i f t of V from 401 mu to 435 mu.  Thus i f VI i s to be a d i s t i n c t entity,  then i t might be expected that a t some intermediate concentration of s u l f u r i c acid (between 100 - 1 0 5 $ ) , both species V and VI should appear at the same time.  That t h i s i n fact happens i s obvious from the spectrum  shown i n F i g . 11 where i t can be seen that both species appear at the same time i n about 2% fuming s u l f u r i c a c i d .  Moreover, the e x t i n c t i o n  c o e f f i c i e n t o f VI i s about 10 times that of V, which would be very hard to explain on the basis of sudden increase of i o n i s a t i o n of V, but which can be e a s i l y understood  i f presence o f species VI i s assumed.  The spectra of V and VI are quite d i s t i n c t which constitutes further evidence of existence of VI.  3.  Evidence for the i d e n t i t y o f VI. I f 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 a c i d i t y , 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 . _ + = T Me,MH  MM*  V  VI  Moreover, i f t h i s i s true, then the d i s s o c i a t i o n of the conjugate base VI should obey the Hammett H  + f  function. Since such a function has  not been evaluated as yet, the H values of the pK^+++ of pKj£+++  0  function was used to compute the  and as may be seen from the Table VII the values  are reasonably constant. Table VII.  p K g V a l u e s f o r Michler's Hydrol Green i n Fuming S u l f u r i c A c i d . % S u l f u r i c Acid  *H  ' D K  105.65  H.69  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  12.12  12.21  Best value  Concentration o f the dye  =  1.25 x T O  - 5  M/l  The d r i f t of the pK ++* values may well be due to the fact that the R  values of H between 100 - 107$ sulfuric acid were determined (55) Q  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 i s also  possible that the H , function would f i t the data better. However, ++  as has been pointed out before, the actual value of H i s immaterial Q  in the evaluation of the isotope effect. K /K H  D  This was found to be:  1.24. Other data are shown i n Table VIII.  =  Table VIII Ionisation, and Isotope Effect Data for Michler's Hydrol. Compound  ^max mu  e  max  pK  R4  Michler's Hydrol Blue  H  605  17,000  D  605  14,500  Michler* s Hydrol Yellow  H  401  6,930  -7.63  D  401  6,350  -7.69  Michler's Hydrol Green  H  435  59,000  -12.12  435  58,600  -12.21  Benzhydrol  D  VD K  1.04  1.12  1.24  H  442  -13.30  D  442  -13.41  1.29  Symbols H and D refer to protium and deuterium analogues respectively. The data i n Table VIII indicate that the isotope effect increases i n 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 w i l l be  discussed later i n conjunction with the interpretation of the ultraviolet spectra. 4.  Evidence for true equilibria. The quantitative aspect of the equilibria: VI —  V ^  IV ^  III ^  II  has been investigated i n order to establish whether the postulated equilibria are truly reversible and that no sulfonation or any other side reactions occurred.  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 i n a one cm c e l l with 9 mm spacer, then after appropriate dilution the spectrum was taken i n a one cm c e l l without spacer i n order to compensate for the dilution and then after further dilution a c e l l 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$, a quantitative spectrum of the species V alone was obtained. To obtain the spectrum of Species I I , a slightly different technique was used.  The 96$ sulfuric acid solution of the dye was  diluted with water, cooled i n ice, the sulfuric acid was :treated with sodium hydroxide u n t i l alkaline, the alkaline solution was extracted with ether, the ethereal extracts evaporated to dryness and the solid dissolved i n the appropriate volume of sodium acetate acetic acid buffer (pH 4.7) and the spectrum taken.  I t was found that  the spectrum was identical with that of II i n 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 i n the ionisation of Michler's Hydrol are truly reversible and that no sulfonation or side reactions occur. 5. Difference i n A m  a  Y  of V and VI  As the values for the A j ^ ^ i n 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 i s 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 i s investigated, i t i s observed that V i s unsymmetrical whereas VI i s symmetrical along i t s 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 i n basicities of the dimethylamino and the protonated dimethylamino groups i s extremely great, which i n turn implies that the difference i n energies of the principal contributing structures w i l l 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 i n 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 i s made i n terms of the extinction coefficient, so that the results are directly comparable.  Occasionally a blank determination was also  carried out i n order to ensure that the solvent did not absorb i n 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 d i s t i l l e d  water as solvent with one drop of 0.01 N sodium hydroxide added i n order to keep the pH at the value of about 7.0. The spectrum showed the following absorption maxima, which are given i n 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 i n the molecule i t would be expected that also absorption i n the 3OO mu region should occur (60,61,62).  The failure to observe a distinct  peak i n the latter region i s 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 i n ethyl alcohol solution. The only observable absorption i n the 3OO mu region i n water solution i s 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 i n  the sulfuric acid - water system i n 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).  I t may be seen that the band at 258 mu decreased  i n 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 i s i n  0  40 equilibrium with the carbonium ion II, at least at the beginning of the ionisation curve of Michler's Hydrol Blue. As w i l l 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 i n 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. band at 253 ^  The  has decreased only slightly i n intensity, which would  be i n accordance with the assumptions made previously to account for the  almost f l a t 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  (d)  ^  €  605  8,600  36O  1,000  300  shoulder  253  8,800  218  11,600  203  24,000 shoulder  Spectrum at pH 0.0 (Fig. 8). The spectrum at this acidity which i s strikingly simple i s  shown i n Table XIII. Both bands are most l i k e l y due to the di-protonated species IV. The extinction coefficient of the more intense band i s i n 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 i n the range of  60 to 100$ sulfuric acid. shown i n Fig. 9.  Only the spectrum i n 96$ sulfuric acid i s  The conclusions were made on the basis of other  spectra taken in the above range of sulfuric acid. (a)  Spectrum i n 81$ sulfuric acid. The observed bands are shown i n Table XIV. Table XIV. Absorption Maxima of Michler's Hydrol Yellow i n 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 i s associated with the carbonium ion V, being probably due to the polarisation along i t s major axis.  The peak at 286  mu i s 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 i n 96$ sulfuric acid (Fig. 9). The absorption maxima are shown i n 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 i s no explanation for this phenomena at present. Table XV.  Absorption Maxima of Michler's Hydrol Yellow i n 96$ H S0^ 2  (c)  Band mu  6 max  401  6,700  286  1,660  256  2,560  217  13,000  Spectrum i n 98$ sulfuric acid. The absorption bands observed at this acidity are given i n  Table XVI. Table XVI Absorption Maxima of Michler's Hydrol Yellow i n 98$ HgSO^ B a n d  'r  1  * 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 i s a superposition of both (Fig. 11).  The  appearance of both species was effected by diluting the sample i n 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 i s i n this case not specified. 5.  Spectrum of Michler's Hydrol Green. The spectra of this species were determined i n 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 i n 30$ fuming sulfuric acid i s shown i n Fig. 10, however data are also reported for 10$ fuming sulfuric acid. (a)  Spectrum i n 10$ fuming sulfuric acid. Table XVII shows the observed maxima. Table XVII. Absorption Maxima of Michler's Hydrol Green i n 10$ fuming sulfuric acid. € max  B a n d  435  22,400  300  2,400  The band at 435 mp i s attributed to the carbonium ion VI, being due to the polarisation along the major axis.. The absorption peak at 300 mu i s probably due to polarisation along the minor axis. This i s 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 i n "30$ fuming sulfuric acid (Fig. 10). Absorption bands are shown i n Table XVIII, Intensity of both bands increases as would be expected i f i t i s assumed that the species ionises.  There i s also indication that the band which  was present at 256 mp i n lower concentration of sulfuric acid has dis-  45 appeared.  Since this peak i s associated with the di-protonated species  IV i t s disappearance would indicate f u l l ionisation of VI i n 3 0 $ fuming sulfuric acid.  However this can not be concluded with certainty-  due to the absorption of solvent i n the 250 mu region. Table XVIII. Absorption Maxima of Michler's Hydrol Green i n 30$ Fuming Sulfuric Acid. Band mu  r  6.  c  fc  max  435  68,000  300  4,800  Spectrum of Michler's Hydrol i n 60$ fuming sulfuric acid. In the concentration of sulfuric acid above 30$ fuming a  new species VII appears with an absorption band i n 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 i n about half an  hour. Since no quantitative measurements were made, the nature of this new species VII remains unknown at present.  I t i s 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 f i r s t absorption band i s summarized. The wavelength of the f i r s t absorption band (designated further by WFAB) i s defined as the absorption band of the carbonium ion appearing farthest towards red i n the region 200 - 700 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_Me MHC H.) CrHjC ) i n concentrated sulfuric acid to that of +  5  A  9  mu.  46 triphenylmethyl carbonium ion was also noted by Branch and Tolbert ( 6 7 ) . A similar observation was made i n this study where i t was found that the spectrum of Michler's Hydrol Green resembles the spectrum of benzhydryl carbonium ion not only i n WFAB but also i n the region of 3OO mp. Table XIX shows that the conclusion may be extended to t r i a r y l +  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 i n carbonium ions behaves as i f i t would be replaced by hydrogen alone, and that the WFAB of the nitrogen protonated species w i l l be within + 15 mp of the WFAB of the unsubstituted compound. This assumption may be valid, providing that the p-dimethylamino group i s not sterically hindered. The above postulate i s further supported by the size of the isotope effect of the Michler's Hydrol Green which i s comparable with that of benzhydrol i t s e l f .  Similar evidence to the effect that the protonated  amino group i s 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  Ref.  WFAB  mu Crystal Violet*  Parent Hydrocarbon  WFAB mu  Ref.  (a)  589  53,64  Crystal V i o l e t * (b)  635  35  Malachite Green (c)  620  53,64  Malachite Green (d)  450  67  VIII  446  68  Crystal Violet  436  69  Malachite Green •+  450  67  Triphenyl methyl carbonium ion  428  Benzhydryl carbonium ion  442  4  ++  + + + >  (f)  Malachite Green (g) +++  434  Michler* s Hydrol Green  435  (a) (b)  2 6 4 2 +  6 4 2 6 5 +  e  6 4  2 6 4 6 5  (p-Me NHC H )(C H ) C  +  2  6 4  6 5 2  (p-Me NHC H ) (p-Me NC H ) C 2  (g)  6 4  (p-Me NHC H )(p-M NC H )C H C  ^  (f)  6 4 3  (p-Me NC H ) C H C  2  (e)  67  +  2  (d)  67  (p-Me NHC H )(p-Me NC H ) C 2  (c)  (e)  (p-Me NC H ) C* 2  Note:  +  6 4 2  +  2 6 4  (p-Me^H^C^C*  The parent hydrocarbon refers here to a carbonium ion where p-Me NH group has been replaced by hydrogen. +  2  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 9 6 % H S0 2  300  4  400 Wavelength  500 m/j  54  SPECTRUM OF MICHLERS HYDROL GREEN IN 30% FUMING H S0 2  300  4  400 Wavelength  500 mj  55  S I M U L T A N E O U S  200  A P P E A R A N C E  300  OF  S P E C I E S  V  400 Wavelength  A N D  VI  500nyj  INFRARED SPECTRA  These were determined i n order to establish the presence of deuterium i n the i s o t o p i c a l l y substituted molecule.  The spectra  were made i n KBr discs with the exception o f benzhydrol and 4 , 4 ' dimethoxy benzhydrol which were determined i n carbon t e t r a c h l o r i d e . Table XX summarizes the v i b r a t i o n a l frequencies o f the C-H and C-D stretching and bending vibrations respectively.  The C-H stretching  frequencies were obscured due to the presence o f phenyl and methyl hydrogens.  Table XX. Infrared Data f o r Benzhydrols. 4,4'identically di-substituted benzhydrols 0M  Stretching frequency cm" 1  c  m  Bending frequency - l  1,364  E  2,140  OMe- <* _d Me  1,357  '  Me- % _d  1,004  2,150  1,002  1,350  CI CI- c< -d  2,140  1,400  NMe  2  NMe - c< -d 2  2,120  H H- * -d  1,000 ?  995 obscured  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 i o n i s a t i o n , or at l e a s t increase the i o n i s a t i o n to a greater extent.  Such use of solvent  properties  was made by Stewart ( 7 l ) i n the i n v e s t i g a t i o n o f the b a s i c i t y of Michler's ketone.  At the same time t h i s would permit the evaluation o f the isotope  e f f e c t at higher ionisations and enable comparison of r e s u l t s with those obtained i n the 2% and 5$ acetone solutions i n acetic acid - sodium acetate buffer.  The solvents investigated wereJ acetone, a c e t o n i t r i l e ,  g l a c i a l acetic acid, nitromethane and ethyl alcohol.  The molar  concentrations of the dye were i d e n t i c a l i n a l l solvents, so that d i r e c t comparison o f 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 s u l f u r i c acid o f desired concentration.  The a c i d i t y of the s u l f u r i c a c i d was usually within the  range of 1.0 to 3.0 pH u n i t s .  Nitromethane and acetic acid were excep-  t i o n a l since the dye ionised i n them without the addition o f the a c i d . A s h i f t of about 2 - 5 mu towards a longer wavelength was observed i n the organic solvents compared to that i n water.  The r e s u l t s are  summarized i n Table XXI. As may be noted from data i n Table XXI the extinction c o e f f i c i e n t s f o r the acetone, nitromethane, acetic acid and a c e t o n i t r i l e solutions o f the dye are a l l approximately o f the same magnitude.  Thus  i t may be assumed that the species i s e s s e n t i a l l y f u l l y ionised i n these solvents.  The comparison with extinction c o e f f i c i e n t s o f the other  benzhydrols reveals that they are o f the same order o f magnitude as those  58  for Michler's Hydrol Blue i n the above solvents. Since no temperature study of the ionisation equilibria was ma.de, the claim that the species i s 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 i n magnitude to that of the dye i n water (Table I I ) . This i s explained i n part by the fact that Michler's Hydrol forms an ethyl ether i n ethyl alcohol (72).  It appears  that the more important governing factor i s 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 i t s formation  would be greatly suppressed i n 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 i n 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.  I t seems that the effect of the solvents i n this case  i s highly specific. The exceptional behaviour of nitromethane i s not very well understood. Even though nitromethane i s a pseudo acid, i t s apparent pK  a  (73) would be too small to effect the ionisation of Michler's  Hydrol Blue.  Thus i t i s assumed that nitromethane contained probably  a trace of n i t r i c acid which would be sufficient to produce ionisation. The formation of the carbonium ion i n acetic acid i s of some interest since the ion appears i n 0 . 0 1 N acetic acid, disappears  at higher  acidities and re-appears again i n 90-100$ acetic acid. The appearance  59 of the species may be presumably associated with the change i n dielectric constant of the medium, e.g. decrease of dielectric constant with increasing concentration of acetic acid. The isotope effect has been investigated i n acetone, acetonitrile and ethyl alcohol and was found comparable to the values obtained i n 2% and 5$ acetone solutions of the dye i n acetic acid - sodium acetate buffer.  In addition the isotope effect for the equilibrium between II  and III was studied i n 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.  I t should be noted  that i n 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 i n 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 i n hybridisation i s from sp to sp3. 2  This i s i n fact found;  moreover, the isotope effect  1.11 for the equilibrium between I I and III i s well comparable with the value of % A n I and I I .  =  1» ^ obtained for the equilibrium between 0  60  Table XXI. Variation of Extinction Coefficients and Isotope Effect of Michler's Hydrol Blue with Solvents. Concentration of acid to produce maximum ionisation  Solvent  X max mu  €  ^ max  max lit.  Kg,  H 156,500 Acetone  0.01N H.SO. 2  4  610  D 161,500 H 151,500  Glacial Acetic Acid  148,000^  607.5 D 151,500  (\ h  J  0.01N H SO  H 145,000  607  0.10N H S0. 2  4  1.04 D 143,500  4  Ethyl Alcohol  1.03  D 156,500 H 161,500  Mitro methane  Acetonitrile  608  H  18,500  D  16,500  607  1.04  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 i s 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 i n the CH^ group of toluene (74) found that the protons i n the CEpD are more shielded than the protons i n the CH^ group;  this implied greater electron donating power from deuterium than  from protium.  Study of the chemical shift i n n-C^FyH and the corresponding  deuterio compound revealed (75) that the fluorine nuclei i n the CF^D group are 0.60  + 0.05 ppm more shielded than those in the CF H group* 2  This, presumably, implied again greater electron releasing power of deuterium. Both of these studies are strongly i n support of Halevi's assumptions.  On the other hand, Halevi's explanation of the effect of  o< -deuterium substitution i n phenylacetic acid i s 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 ( l l ) which are sufficiently lowered i n the anion (77)  to account for the acid weakening effect of deuterium.  Comparison of the results obtained i n this investigation with those of Halevi reveal that the isotope effects, even though comparable in magnitude, are actually i n 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 i s operative ( l 9 ) . The other point of view, that the isotope effect should be sought principally i n the difference of vibrational frequencies between the ground and transition states i n the rate process (p.3) was put forward by Streitwieser et a l ( l l ) who have put their assumptions on a semiquantitative 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'""'- (outof-plane bending).  The assignment of the tertiary C-H bond, which changes  to the H-C"** i n 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 i n stretching  frequency of ~100 cm""-'- would result i n k^/k^  =  1.06. One bending  vibration remains essentially unchanged, the reduction i n 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 i n the c( -deuterium isotope effect i s the isotopic inhibition of the out-of-plane bending vibration of the cation which i s 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 i n favour of an electronic isotope effect i s explicable on the basis of vibrational effects.  Moreover, i t would be d i f f i c u l t 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-d , and that of 2  Bender (79) of the hydrolysis of ethyl acetate and ethyl acetate- o( -d^ are also i n support of Streitwieser s explanation of the isotope effect. 1  The results of the present investigation reveal isotope effects comparable i n magnitude and i n the same direction as would be predicted on the basis of Streitwieser's considerations.  In addition, i t was  observed that the isotope effects i n 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 i s consequently very small.  However i f the sub-  stituent i s more electron withdrawing i n character, then the positive charge resides more on the central carbon atom to which the deuterium i s attached, and the isotope effect i s greater. This i s well demonstrated by the two extremes: Michler's Hydrol Blue, where the nitrogen can greatly stabilise the positive charge and the isotope effect i s extremely small (Kjj/Kp  =  I.O3), and the 4,4'-dichlorobenzhydryl  cation, where the  positive charge density i s probably much greater round the central carbon atom and consequently the isotope effect i s much greater (K^/Kp - I . 3 5 ) .  The same trend was observed i n the series Michler's  Hydrol Blue, Yellow and Green which i s discussed i n 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, i n addition to Streitwieser's explanation, i n the difference in polarisability of the C-H and C-D bonds respectively.  Ingold et  a l (BO) have shown i n 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-  i v i t y for the deuterio compounds as a lowering of the static polarisability of the molecule.  Thus i n 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, i n the case of Michler's Hydrol Blue, the electron demand of the central carbon atom i s very small which might i n turn imply that the C-H bond i s only slightly more polarised than the C-D bond. In support of both Streitwieser's explanation and the polarisability explanation i s the inverse isotope effect (Kpj/K  D =  0.90)  observed i n the equilibrium of Michler's Hydrol Blue with the species III.  Here the change i n hybridisation i s from sp to sp^ and the 2  polarisability differences now operate i n the direction opposite to that when the carbonium ion i s formed from i t s uncharged base, (b) Behaviour of Michler's Hydrol i n acidic solutions. Investigations of Michler's Hydrol were made at acidities ranging from the pH 7.0 to 6 0 $ 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 i n the case of Michler's Hydrol Green (%AD  = 1.24). The latter value i s comparable with the isotope effect  obtained for benzhydrol, where ^ / j ) = 1.29. K  K  I t i s clear that the  isotope effect for Michler's Hydrol increases for the series Blue, Yellow, Green i n that order, and the increase i s ascribed to the decreasing number of the resonating groups. This seems to agree well with the assumptions made previously i n connection with the ultraviolet spectra, where i t i s 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 H and G functions. Q  Q  Even though the evidence i s not conclusive i t suggests that Michler's Hydrol Yellow i s a cation with one nitrogen protonated and Michler's Hydrol Green i s 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.  I t may be assumed with some certainty that the species  III exists and i s the unionised Michler's Hydrol with one nitrogen protonated and that the species IV i s 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 i n the delocalisation of the positive charge.  The isotope  effect was examined i n some solvents and was found to be comparable with that i n 2% and 5$ acetone solutions of the dye i n the buffer.  67 SUGGESTIONS FOR FURTHER RESEARCH.  The r e s u l t s of present i n v e s t i g a t i o n suggest that the isotope e f f e c t should be studied for the 4>4'-dinitro benzhydrol, where i t i s hoped that the great electron withdrawing  character  of the n i t r o group should show an isotope e f f e c t much greater than so f a r observed i n the benzhydrol s e r i e s . i n t e r e s t to study the e f f e c t of  I t might also be of some  o< -deuterium substitution upon the  e q u i l i b r i a of appropriately substituted benzhydryl carbanions where now probably the e f f e c t i v e amount of negative charge on the central carbon atom w i l l determine the d i r e c t i o n of the isotope e f f e c t .  As  another alternative, i t i s suggested to study the e f f e c t 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 s u f f i c i e n t l y r e s t r i c t e d to the out-of-plane bending v i b r a t i o n , and thus the scope of Streitwieser's predictions may be tested.  Determinations  of the isotope e f f e c t s i n some of the above systems w i l l be undertaken i n the near future i n t h i s 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. 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