Application of Linear Free Energy Relationships

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June 20, 1962 LINEARFREEENERGYIN TRIARYLMETHANEREACTIONS 2349

[CONTRIBUTIONFROM THE DEPARTMENTSOF CHEMISTRY,TEEC,EORGEWASHINGTONUNIVERSITY,WASHINGTON,D. c., A N D

THEWILLIAMM. RICEUNIVERSITY, HOUSTON,TEX.]

Application of Linear Free Energy Relationships to Some Reactions ofTriarylmethane Derivatives

B Y CALVIND. RITCHIE,~W.F. SAGERAND E. S. LEWIS

RECEIVEDDECEMBER16,1961

Several acid-base and oxidation-reduction reactions of a series of 3"- an d 4" - substituted derivatives of malachite green(4,4'-bis-(dimethylamino)-triphenylmethyl chloride) have been studied. The various ra te and equilibrium constants ar ecorrelated by the Ham met t equation , and the p-values obtained are compared with those calculated by the methods proposedin several recent theoretical studies. The constant s obtained for this series are combined

with da ta for a broader series of triarylm ethane derivatives in order to investigat e the effects of multiple variations in struc-ture. It is found th at Miller's equat ion for multiple variation in structure-reactivity correlations gives excellent agreementwith the data.

Reasonable agreement is found.

Introduction

A series of 3"- and 4"-substituted derivatives ofmalachite green (4,4'-bis (dimethylamino)-tri-phenylmethyl chloride) provide the opportunityto study a number of add-base and oxidation-reduction reactions in which the basic skeletalstructure of the substrate is unchanged from onereaction series to another. Thus, these reaction

series furnish an interesting set in which to investi-gate the applicability of structure-reactivity cor-relatias.

Th e base streng ths of a wide series of substitutedtriarylmethanols have been previously investi-gated,2 and the dat a can be combined to providea test of multiple variations in structure-reactivitycorrelations.'

In this paper, we shall be concerned with thevarious transformations indicated in Scheme I,with R = H or CH3.

Methods and Results

A series of 3"- and 4"-substituted leuco bases ofmalacbite green ( I) were prepared by the condensa-tion of the appropriately substituted benzaldehyde

with dimethylaniline in aqueous sulfuric acid.The carbinol bases I11 were prepared by oxidationof the leuco bases with excess chloranil (tetra-chloro-$-benzoquinone). Fluoborate salts of thedyes I1 were prepared by dissolving the carbinolbases in dilute aqueous acid and adding sodiumfluoborate to precipitate the salts.

The carbinol bases proved to be exceedingly dif-ficult to purify. Th e final compounds used in thisstudy showed extinction coefficients which weregenerally higher than any previously reported.Th e melting points of t he carbinol bases were alsogenerally higher than those which have beenreported in the literature.

Physical data and elemental analyses of thesecompounds are shown in Table I. The elemental

analyses are poorest for those compounds whichwe have found hardest to purify. The extinctioncoefficients obtained by dissolving tlhe carbinolbases in aqueous solution buffered at pH 4.0 agree

(1) To whom correspondence regarding this paper should be ad-

dressed at Department of Chemistry, The University of Buffalo,

Buffalo 14 , N. Y.

(2 ) (a) N. C. Deno, ct ai., J . Org. Chcm., 19, 155 (1954); J . Am.Chcm. S O L , 77, 3044 (1955) ; 7'7, 3051 (1955); 79 , 6805 (1957). (b)

0 . F. Ginsberg and N. S. Mel'nkova, J . Gcn.Chem., U.S. S. R.,I6,1109 (1955); C. A . , 61, 16368 (1957). (e) M. Gillois and P.Rumpf,

Compf. r en d. , 238, 591 (1954).(3) S. I. Miller, J . Am . Chem. SOL,81, 101 (1959).

with those obtained by oxidizing a known amountof the leuco base with an excess of chloranil andadding several lambda of the resulting solution toan aqueous solution buffered at pH 4.0.

1T

I t

I t

1T

SCHEME1.-R is H or CH,.

A rather disturbing feature of the carbinolbases is that we have been unable to observe anO H stretching band in the infrared spectra. Th atthe absence of the OH band in the infrared is notdue to hydrogen bonding is indicated by the factthat a band fails to appear when a dilute solutionof the carbinol base in carbon tetrachloride is

examined in a 1-cm. path length cell. An exami-nation of the n.m.r. s ectrum of the carbinol baseof crystal violet, 811, X = p-dimethylamino),also failed to show a band corresponding to aproton of an O H group. The presence of oxygenis shown, however, by the results of the elementalanalyses, and by a qualitative test for oxygen de-scribed in the Experimental section. Molecularweight determinations, bo th by depression of themelting point of camphor and by the isopiesticmethod for benzene solution, show that the carbinolbases are monomeric. There can be little doubt,



2350 CALVIND. RITCHIE,ll‘. F.SAGER A N D E. s. LEWIS 1701. 84

TABLE 1

PHYSICALCONSTANTSOF MALACHITEGREENDERIVATIVES

r-- --- --- --l euc o bases---- -Carbinol bases--Hydrogen, %M p. , a c . A m u , A m u , €mar.” f m a . , ~ M . P . , OC. Carbon, %

Subst. Obsd Lit. r n w emaxa mp H20 MeOH Obsd. Lit. Calcd. Found Calcd. Found

H 99 94 263 3.24 623

p ” - a 97 98 264 8.29 627

m”-CH3 86 85 264 3.24 622

m”-NO, I52 152 266 4.12 639

m”-OCH3 124 123 263 3.27 626

p”-DMA& 175 173 265 4.86 592

nt”-Cl 110 112 265 3.24 632

p”-CH3 100 94 262 3.30 620

P”-N02 179 177 267 4.10 648

p”-OCH, 105 105 268 3. 36 615

l-cm.-l-mole-l X lo-*. Dimethylamino.

8 . 36

8.13

8.17

9.397.51

5.59

8 . 65

9.21

. . .

11 0

10.7

11.1

11.5

. . .

. . .9.17

8.20

11.5

11.5

163

14 0

160

151165

190

151

154

190

. .

117 79.7 78.1

, . 72.5 70.9

. . 72.5 72.7

. .80.0 79.7. . 70.7 69.9

. . i 0 . 7 71 . 3

140 76.7 77.0

. . 76.7 76.6

180 i i 2 7 7 . 2

. . . . . .

Subst.

TABLE11

EQUILIBRIUMA N D R ATECONSTASTSFOR MALACHITEGREENDERIVATIVES’

7.52

6.58

6.58

7.786.40

6.40

7.45

7.45

7.98

. .

7.85

6.21

6.62

7.44

6.10

6.61

7.37

7.52

7.92

H 0 0 6.06 2.40 0.761 2.94 0.67 6.84

m ’’-C1 0.373 0.399 5.80 1.35 1.44 2.58 0.34 6.23

WZ”-CH~ - ,069 - ,066 , . . . . . . . . . 7 .21

p”-CH, - ,170 - ,311 . . . . . . . . . . 7.27

m”-NOz ,710 ,710 5.73 0.442 1.8 0 2.17 0.10 5.84p’ ‘-NO* .778 ,778 . . 0.392 1.77 2.28 0.03 5.50

WZ”- OCH~ ,115 ,047 . . . . . . . . . . 6.95

p”-OCH3 - ,268 - ,778 . . . . . . . . . . 7.18

p”-DMAy - .83 -2.00 . . . . . . . . , . 9.36

p’’-Br ,232 0. 15 0 . . . . . . . . . . 6.38

H. C. Brown and D. McDaniel, J.Oug. Chem., 23 , 420 (1958). b H. C. Brown and Y .Okamoto, J.A n z . C h e m . SOC. ,80,4978 (1958). Ref. 2 ; aqueous solution, d Relative values, see text, methanol solution. e k , set.-'. k , l.-mole-’-min. -I. 0 Dimethylamino. Temperature 25’.

then, that we have indeed obtained the carbinolbases (111,R = H).

Because of the solubility limitations of the car-binol bases and leuco bases in water, we haveexamined all of the reactions in methanol solution.Buffer solutions were prepared by partial neutrali-

zation of various tert iary amines with p-toluene-sulfonic acid. The inertness of the amine towardthe dyes was established in each case by the useof different buffer concentrations.

Spectrophotometric measurements have beenemployed throughout, and the procedures aredescribed in detail in the Experimental section.

Conversion of Dyes to Methyl Ethers (I1 F! 111,R = CH,) .-The relative equilibrium constantsfor the transformation of the methyl ethers to thedyes were measured in anhydrous methanol usingtribenzylamine-tribenzylammonium p-toluenesulfo-nate buffers. The results are reported in Table I1

The relative rates of reaction of the dyes withmethoxide were measured in triethylamine-tri-ethylammonium p-toluenesulfonate buffers. Therates, in each case, were measured at two differentbuffer ratios The ratio of pseudo-first-order rateconstants obtained agreed with those calculatedassuming th at the rate of reaction of the dye withmethanol was negligible. The results are reportedin Table I1 as the log of the pseudo-first-order rateconstants, log k l = log k z + log K b (triethylamine)

Conversion of Dyes to Protonated Methyl Ethers(IX (R=CHQ) + 11)-&’hen acid is added to asolution of one of the dyes, there is an immediate

as PK a -pK t r ibe nz yla m ine .

change in the color of the solution, followed by aslow fading of the color. When equilibrium isreached, the visible spectrum of the solution isidentical to the original spectrum except for dimin-ished intensity. No absorbance corresponding toth at expected of the monoprotonated dye is ob-

servable except in the case of crystal violet.From the changes observed it may be deduced

that the monoprotonated dye reacts with solventto form the diprotonated methyl ether. The samebehavior is noted in water, and Hantzsch4was ableto isolate the triprotonated carbinol base in the caseof crystal violet.

The equilibrium constants for the conversion(I1 & IX, R=CH3) were measured by addingknown amounts of p-toluenesulfonic acid to ca.

molar solutions of the dyes in methanol. Theresults are shown in Table 11.

Basicity of Leuco Bases (IV Ft I).-The basicitiesof the leuco bases were measured in dichloroaceticacid-dichloroacetate buffers in which the extent ofprotonation of the leuco bases is small. The secondionization of t he leuco bases has, therefore, beenneglected. The values of the first ionization con-stant are reported in Table 11, and are calculatedusing the value of pKa = 6.4 for the ionization ofdichloroacetic acid.5

The Reaction of the Leuco Base of MalachiteGreen (I, X = H) with Chloranil (I + II).-Thestoichiometry of the reaction of th e leuco base of

(4 ) A . Hantzsch, B c r . , 33 , 7 52 (1900).

( 3 ) I< P Bell, “The Proton in Chemistry,” Cornell University

Press, l thacs . 6 .Y..1959. p . 4 4,




malachite green with chloraniP to produce the cor-responding dye in methanol solution degassed withnitrogen was found to be accurately represented

by eq. 1 to about 50% completion. After this

Ar3CH -I- Q + H + Ar3C++ H2Q (1)

point, th e amount of dihydrochloranil presentbecame progressively greater than the amount ofdye. Since the concentration of dye plus unre-acted leuco base was always equal to the initialconcentration of leuco base, it appears that thecomplication is caused by a slow reaction of chlor-ani1 with solvent.

The rate of oxidation of the leuco base followedsecond-order kinetics accurately to about 50%completion. If air was not removed from thesolutions, the kinetics were second order to onlyabout 25y0 completion, and the rate constant de-termined was about 20% higher than that de-termined in degassed solutions.

A tenfold change in the hydrogen ion concentra-tion of the reaction solution caused no change inthe ra te constant.

If the reaction was carried ou t in solution buffereda t a pH where th e carbinol was the stable species,the concentration of dye initially rose, and thendeclined as the reaction proceeded. Although noquantitative measurements were made, this ob-servation indicates that the dye is the initiallyproduced species in the reaction.

The second-order rate constants, calculated frominitial rates of reaction in 0.1 M acetic acid inmethanol, are reported in Table 11.

Evidence for the Existence of MonoprotonatedCarbinol Base in Aqueous Solution.-It was notedth at the extinction coefficient of malachite green isconsiderably greater in methanol than in water(cu. 25Y 0) . The integrated intensities of the visiblebands differ by only 7 % . When a solution of the

dye in methanol is rapidly diluted with water,buffered at FH 4.0, the absorbance of t he dye iscu. 7% greater than that normally measured.The absorbance of the solution slowly decreasesto the normal value.

These observations are simply explained if oneassumes that the dye is in equilibrium with themonoprotonated carbinol base (I1a V I) in aqueoussolution.’ The position of this equilibrium isindependent of $€Iand would be exceedingly dif-ficult to determine in studies with aqueous solutionsalone.

The integrated intensities of solutions of the dyesin acetonitrile are the same as those in methanol,and we take this as evidence that practically nomonoprotonated methyl ether exists in the meth-anol solutions.

Discussion

Application of the Hammett equations to thedata of Table 11, using only m”- and “+R” 9”-substituted compounds in the correlationg gives theresults shown in Table 111.

(6) J . H. Jones, et a l . , J . A s s o c . Of. A g r i c . Chcm. , 88 , 977 (1955) .

(7) The possibility of monoprotonated carbinol base in certain

cases has been suggested by: C. C . Barker, G. Hallas and A. Stamp,

J. C h e m . SOL. ,3791 (1960).

( 8 ) L. P. Hammett , “Physical Organic Chemistry,” McGraw-Hill

Book Ca.,Inc.,S e w Ycrk, N . Y.,1840, Chap. V I I .

TABLEI11

HAMMETTEQUATIONCORRELATIONS

IV I equil. MeOH, 25’ 0.466

11 is 111( R = CHI) equil. MeOH,

25’ 2.64

I1 -t 111 ( R = CHI) rate, MeOH,

25’ 0.940IX F? I1 ( R = CHa) equil. MeOH,

25 ” -1.33

-0 .81

1.83

-559

Reaction P

I -P 11, rate MeOH, 25”

vrnax, of dyes H20,2 5 “ , crn.-l

I1 + I11 (R = H)“ equil. H20, 20”

0 Da ta taken from ref. 2b.

-log KO

6 .05

2.37

2.93

0.824

i . 0 1

16,040

-0 .66

Also shown in Table I11 are the results of theapplication of the Hammett equation to the cor-relation of the wave lengths of maximum absorb-ance of the dyes, and to t he da ta for the basicityof the carbinol bases in water. Plots of the dataare shown in Figs. 1-7.

0:

Fig. 1.-Hammett plot of basicities of leuco bases.

0 5 t

0 02 0 4 06 o e

0

Fig. 2.-Hammett plot of basicities of methyl ethers

One other p-value may be determined from thepresent data by noting that p for the reaction ofthe dyes to form the diprotonated methyl ethers(I1F? IX, R=CH,) must be the sum of p for thediprotonation of the methyl ethers (I11 F? IX,

(9) Procedure recommended by: R. W. Taft , Jr . , an d I. C. Lewis,

J . Am . C h c m . Soc . , 81, 5343 (1959).



2352 CALVIND. RITCHIE,w.F.SAGER A N D E. s.LEWIS

55

Vol. 84

'

r

2. 5 -0-I

I

2.3-

I . . . . . . . . . .o \

0 2 0 4 06 0 8

G

Fig. 3.-Hammett plot of ra te constan ts for reaction of dyes

with methoxide.

0 2 0.4 0 6 0 8

a:

Fig. 4.-Hammett plot of equilibrium constant s for conver-

sion of dyes to dipro tonated methyl ethers.

I . 040 2 0 6 0 8

cr:

Fig. 5.-Hammett plot of rat e cons tant s for reaction of

leucos with chloranil.

R=CHI) and tha t for the formation of the methylether from the dyes (I1a 111, R=CHa). Thus,p for the diprotonation of t he methyl ethers is

The Reaction of Chloranil with the Leuco Bases.-Any mechnnism proposed for the oxidation mustbe in accord with the observed second-orderkinetics, and the observation th at t he dye is theinitially produced species. The simplest mech-anism that is in accord with these observations,

-1.31.

\\\I ,

0 0 2 0.4 0.6 0.8

4Fig. 6.-Hammett plot of basicities of carbinol bases in

aqueous solution.

160-

um.x10-4

158-

1.56 -0

.O 0.2 0.4 0.6 0 8

0:

Fig. 7.-Hammett plot of frequencies of absorption of dyes.

and which appears reasonable from the mechanismof other triphenylmethane oxidations,"' is a hy-dride transfer

ArpCH + Q ---f Ar&+ + HQ -

r.d.

fast (2 )HQ - + H + --+- HzQ

Obviously, however, our data do not rule outother mechanisms in which a slow second-orderreaction is followed by a fast step producing thedye. Nor can we rule ou t a mechanism similar toth at proposed by Swain and Hedberg" for the oxi-dation of leuco malachite green by ceric sulfate.Assuming mechanism 2, if the charge on th ecarbonium ion were fully developed in the transi-tion s ta te of t he rate-determining step, we shouldexpect a p of nearly the same magnitude as th atfor the equilibrium conversion of the methyl e therto the dye (-2.6). Since the observed p is only-0.81, very little charge has developed in thetransition state .

Basicity of the Amino Groups in the Leucos an dMethyl Ethers.-The p for the basicity of the aminogroup in the leuco bases (-0.47) is very close tothat which one would predict from the basicityof anilines in methanol (-3.0212), and the fall-offfactor for a benzene ring (0.30318)and a saturated

(10) R.Stewart, C o n . J . Chem. , 35, 76 6 (1957).

(11) C. G. Swain and R. Hedberg, J . A m . Chem. Soc., 7 2 , 3375

(12) H. H. Jaffe,Chcm. Reus., 58, 191 (1953).

(13) €I.H . Jaff.5, J . C h r m . P h y s . , It , 415 (1953).

(1950).




/ 2 0 - 2 -4 -6

IC?

Fig. 8.-Hammett plot of basicities of triarylmethanols in

water.

carbon atom (0.35814). Thus one predicts p =

-0.33 ( i .e . , -3.02 X 0.303 X 0.358).Rho for the reaction I11 $ VI, R = CH3,

may be estimated as one-half th at for reaction I11IX, R = CHa, if we assume th at the protonated

amino group of VI does not appreciably change theeffect of the X group on the second amino group.

The value estimated in this manner (-0.655)is higher than expected by a significant amountwhen compared with p for the reaction I $ IVof the leuco bases.

Since the only change from the leuco base situa-tion is that a hydrogen has been replaced by amethoxyl on the central carbon, we must ascribethe differences in p values to an increased transmis-sion by the central carbon when H is replaced by

Qualitatively, this observation is in agreementwith the prediction tha t the transmission propertiesof groups are changed by substi tution.l6

It should be noted t ha t this change is in the op-posite direction to that found in the aliphaticseries,16 however. In the aliphatic system, anelectron-withdrawing substituent attached tocarbon diminishes the transmission properties ofthe carbon, while in the present system, the substi-tution of OCHs, an electron - withdrawing substit-uent, for hydrogen has increased the transmissionproperties of the carbon.

From the arguments presented in the earlierpaper,16 this result requires a value of q16 which isof the same sign as p , rather than of opposite sign

as in the aliphatic series. We shall proceed toshow that this result is consistent with other proper-ties of the system.

Basicity of the Methyl Ethers.-From thevalue of p for the basicity of the methyl ethersfound in this work, we predict a p of -1.95 inaqueous solution, using the equation

(1.8+ 67/D)Aur = p"

OCH3.

(3 )

(14) R. W. Taft, Jr., and I. C. Lewis, J . A m . C h c m. Soc., 80 , 2436

(1958).

(15) C. D. Ritchie, J . P h y s . C h r m . , 66 , 2091 (1961).

(16) Q is the constant in the equation: log K - Z u i p o + z'uiu,g +i 1.i

log K O,which was derived by Miller (cy. ref. 3).

0

-8 0 8 16

f(4.

Fig. 9.-Plot of basicities of triarylmethanols according to

eq. 4.

where D is the dielectric constant of the solvent,and Au r is the difference in the u constan t of thereaction site in the reactants and products.

If one assumes tha t the effect of an OH groupis very nearly tha t of an OCH3 group, this valuemay be compared with that found for the basicityof the carbinol bases in water (-1.83). Theagreement is excellent.

Multiple Variations in Structure-Reactivity Cor-relation.-Deno2a has determined the basicity of alarge number of mono-, di- and trisubstitutedtriphenylmethanols in aqueous solution, and hascalculated a p for this series of -4.0. A plot ofDeno's pK values and the values for the malachitegreen series versus the sum of the u values of the

substituents is shown in Fig. 8. The plot is cer-tainly not linear.

Plotting only the monosubstituted derivativesversus u gives a p of -4.0. If one then examinesthe eq. 4 for multiple variations in structure,'jit is seen that the difference between this value

log K = ( V I + uz + C I ) , ~ O + (UIUZ + 0 1 4 3 4UZ'J3)q + log KO (4)

and that for the malachite green series is dueto the term q (h.,p - pa = + (az+ a&). Assign-ing a u value of -2.00 to the dimethylaminogroup,18we thenf indq = -0.55.l9

The sign of q is the same as tha t of p as requiredfrom the previous section.

Figure 9 shows a plot of the first two terms on the

right of eq. 4 versus the experimental pK valuesfor all of the experimental da ta available. Theline drawn is the line of uni t slope predicted by theequation.

We feel tha t this correlation, involving a range inK of over twenty powers of ten, is a striking con-

(17) W . F. Sager end C. D. Ritchie, J . A m . Chem. SOC. ,83 , 3498

(1961).

118) H. C. Brown and Y.Okamoto, i b id . , 80, 4979 (1958); the

extreme value of u was chosen because of the nature of the present

system.

(19) This treatment implicitly assumes the propeller model for the

triarylcarbonium ions. Cf.N. N. Lichtin and M. J. Vignoli, J , A m .C h r m . Soc.,19, 679 (1957).



2354 CALVIND. RITCHIE,W. F. SAGERA N D E. S.LEWIS Vol. 84

firmation of the utility of multiple variations instructure-reactivity correlations.

ExperimentalMaterials.-Eastman Kodak Co. p-toluenesulfonic acid

was recrystallized from methanol a nd dried a t 100" forseveral hours.

Dichloroacetic acid was distilled a t atmospheric pressurethrough a 50-cm. Vigreux column. The fraction boilingbetween 193.5 and 194.5' was collected and used for thepreparation of buffers.

Practical grade chloranil was recrystallized from ben-zene to constant melting point. Dihyorochloranil was re-crystallized from ethanol.

Reagent grade anhydrous methanol was used withoutfurther purification.

Tribenzylamine and triethylamine were Eas tman KodakCo. white label reagents.

Temperatu re Control.-The Cary model 14 spectropho-tometer was equipped with a thermostated cell compartme ntwhich was kept a t 25 & 0.5' by means of water circula tedfrom a constan t temperature bath . All solutions werethermostated in the bath prior to mixing or measuring.

Preparation of Leuco Bases.-The leuco bases used in thisstu dy were prepared by condensation of t he appropr iatelysubsti tuted benzaldehyde with dimethylaniline using st and-ard procedures.'j The melting point s of these compoundsagree well with previously reported values as shown in TableI .

Preparation of Carbinol Bases.-The carbinol baseswere prepared by th e oxidation of the corresponding leucobases with excess chloranil.

The leuco base (2 g.) was dissolved in 150 ml. of ethanoland 4 g. of chloranil was added in one portion. The result-ing mixture was refluxed for 1 hour. At this time, 200 ml.of hot 10% sodium hydroxicle solution was added. Thesolution was immediately extracted with several 100 ml.portions of hot n-heptane. The n-heptane extract waswashed with hot 10% sodium hydroxide, dried with sodiumsulfate , and set aside to cool slowly. The sides of the flaskwere scratched vigorously to init iate crystallization.

The precipitate thus obtained was repeatedly recrystal-lized from n-heptane until constant melting point and molarabsorbance in the visible spectrum of the derived dyes wereobtained.

The absorbances of the compounds in the visible rangewere obtained by add ing several lambda of an acetonesolution of the carbinol bases to an aqueous solution buf-fered at pH 4.0. Spectra were obtained with a Cary model14 spectrophotometer.

Physical constants for the compounds are shown in TableI .

Fluoroborate salts of the dyes were prepared by addingsodium fluorobordte to aqueous solutions of the dyes buffered;i t PH 4.0. The fluoroborates were recrystallized from water.

Basicity of Methyl Ethers in Methanol.-Buffers pre-pared by adding p-toluenesulfonic acid to methanol solu-tions of t ribenzylamine were found to be in the proper rangefor measurement of the equilibria between the dyes and themethyl ethers.

A typical experiment was performed by adding an aliquotportion of a solution of the dye, or carbinol, in methanol toa buffered portion of methanol, waiting for establishmentof the equilibrium, and measuring the absorbance of theresulting solution in the visible range with a Cary model14 spectrophotometer.

A second aliquot of the dye solution was added to nietli-ariol buffered a t p H = P K t r i b e n ~ y l a m i n e- 1.00 in order to

determine the absorbance of the dye. I t was found that:tt this PH the dissociation of the methyl ethers was com-plete.

The various buffers were prepared by adding knowiiamounts of tribenzylamine to a methanolic solution 1 .08 XlO-.a M in p-toluenesulfonic acid, which had been stand-ardized against s tandard sodium hydroxide.

The inertness of t he amine toward th e dye was estab-lished by repeating the measurement using a solution havinga different buffer concentration but the same ratio as pre-viously used.

The final concentration of dye plus ether was in each caseahout l O - ' M .

equationThe cquilit)riulll constant. \\ere calcrllatatl f r t ) l n (h e

PKt. - f'Ktribensylsmina = lOg[A/(Amax - A ) ] +log[(Rali)/(RaNH')] (5)

where A is the absorbance of a solution with buffer ratio(R+)/(RaNH+), A,,, is th e absorbance of a solution con-taining the same amount of added dye, but buffered atpH = pKamine- 1.00, and K, is the equilibrium constantfor the reaction

Ar3C++ CH30H Ar30CH3 + H f

Table 11. shows a series of typical experimental data .

TABLEI V

DETERMINATIONO F EQUILIBRIUM COXSTANTS

ArC+ 4- CH30H ArJCOCH3+ H+;

p"-NOz derivative

Buger Ab - pK - Buffer Ab- pK -ratioa sorbance pK.,i,, ratioa sorbance pK.,i..

O . l O b 1.220 . . . 2 . 0 0 b 0.707 0.397

.30b 1.138 0.403 4.00 b ,486 ,392

, 280 .405

l . O O b 0.891 .368 9.00c ,281 ,405

.50b 1.057 ,388 9.00b

a Buffer ratio = (R3N)/[R3NH+). * (R3?jH+) = 1.08

Basicity of Leuco Bases.-The basicities of the leuco

bases were measured by an analogous method using theabsorbance at about 260 mp .The buffer solutions were prepared by adding known

concentrations of dichloroacetic acid to a methanolic solu-tion 3.78 X 10-8 M in sodium methoxide which had beenstandardized against standard hydrochloric acid.

The buffers were such tha t only a small extent of proto-nation occurred, so th at it was possible to neglect the secondionization of t he leuco bases .

In order to calculate equilibrium constants from the ob-served absorbances, it was necessary to know the absorb-ance of th e monoprotonated leuco bases. It was foundthat for those leuco bases which did not have chromophoricgroups on the third ring, the diprotonated leuco base hada negligible absorbance a t 260 rnp . It was also noted tha tthe absorbance of leuco crystal violet was three-halves th atfor leuco malachite green. From these observations itappeared reasonable to assume tha t the absorbance of theleuco bases was the sum of the absorbances of t he individualchromophore groups present . Thu s, the molar absorbances

may be expressed

where EL is the molar absorbance of the monoprotonatedleuco base, Eo that for the free base and E2 that for thediprotonated leuco base.

E2 was determined by measuring the absorbance of theleuco base in a methanolic solution 10-1 M in p-toluenesul-fonic acid, and Eoby measuring the absorbance of the leucobase in pure methanol.

The equilibrium constants were then calculated using anequation analogous to 5. Typical experimental dat a areshown inTable V .

x 10-3 A I . c (K~P\;H+)= 2.02 x 1 0 - 4 M .

El = '/z(Eo+ E z )

TABLEV

DETERMINATIONOF EQUILIBRIUMCONSTANTS

IV I + H+; nz"-C1 Substituent

ratio Absorbance f i K - P K u c hHuBer'L

. . . 1 .874b . . . . .0.291 1.799 -0,525

0.962 1.670 - ,538

1.625 1.580 - .542

2.23 1.519 - ,564

"Buffer ratio = ( H A ) / ( A - ) . b Absorbance in puremethanol.

Conversion of Dyes to Diprotonated Methyl Ethers .-Carefully measured volumes of standardized -toluenesul-fonic acid in methanol were added to ds ol ut io ns ofthe dyes in methanol (prepared bl- adding the fluorobordtesalt to methanol ). After equilibrium had been established,



June 20, 1962 LINEARFREEENERGYI N TRIARYLMETHANEREACTIONS 2355

r

-0.61

I-0.8

‘g‘9

100 300 70 0

TIME (SECS.).

Fig. 10.-Pseudo-first-order ra te plot for reaction of niala-

cliite green with mctlioxide: I, buffer ratio = 1.64; 11,

buffer ratio = 5 . 5 7 ; 0,(R3A7HC) = 5.41 X 10-3M; 0 ,

( R ~ x H + )= 1.08x 10-3.1’1.

the absorbances of t he solutions in the visible region \vcreineasured . Calculations were analogous to those describedabove.

Some typical dat a are shown in Table V I .

TABLEVI

DETERMINATIONOF EQUILIBRIUMCONSTANTS

IX, ( R = CH3) I1 f CH8OHp’; X = H(A,,,,

= 1.502)

sulfonic acid Absorbance 8K

1 .67 X 1.370 0.761

3 .34 x 10-2 1.314 ,631

6 . 6 8 X 1.131 ,691

1.34 X 10-1 0.860 .747

1.67 X 10-1 ,760 77 2

3 .34 x 10-1 ,491 .i90

Reaction of Dyes with Methoxide.-Buffer solutions pre-pared by adding p-toluenesulfonic acid to solutions of tri -cthylainine in methanol were found to give a range ofPH in which the dyes are completely converted to the m ethy lethers.

The reactions were followed by adding an aliquot portionof a solutiori of the dye fluoroborate in methanol to a quantityo f the buffer, placing the resulting solution in a 1-cni. pathleiigth cell and measuring th e absorbance 51s.time by placiiig(tie cell in the therrnostated conipartmciit o f a Cary inodcl1.1 spectrophotometer set to record absorbarilength oi maximum absorbance of the dye, a t a ktinwii

(.hart speed.First-order plots of the da ta gave the pseudo-first-order

rate constants, kl. Runs at two different buffer ratioscstablished that the reactions are first order with respectto triethoxide, and th at t he rate of reaction with methanolis iiegligible. Run s at different buffer concentrations estab-lished the inertness of triethylamine toward the dy e.

Some typical first-order plots are shown in F ig. 10.Reaction of Leuco Bases with Chloranil .-In order to

cstablisli the stoichioiiietry of the reaction of chloranil withlllc leuco bases, tlic complete ultraviolet and visible spectrao f chloratiil, dihydrochloranil, leuco malachite grcrn andirialachite green fluoroborate were measured on carefullypurified materials in methanol solution. Solutions con-taining mixtures of a ny two of the above components wereobserved to give spectra exactly corresponding to the su mof the spectra of th e individual componen ts.

The spectra obtained were used to analyze the spectraof reaction mixtures prepared from known amounts of

chlordnil and leuco base at various reaction times.X typical experiment was performed by preparing master

solutions uf the leuco base and of chlo ranil of equal concen-

Concn. of p-toluene-

TIME (mins.),

Fig. 11.-Second-order ra te plot for reaction of leuco

nialachi te green with chlo ranil ; (Ar3CH)o = ( Q ) o= 2.39 X

.lf; absorbance measured in 1-mm . path length cell.

trations ( c a . 5 X 3 1 ) in lY0acetic acid in methanol(the amou nt of acid determined to give complete conversionof the methyl ether to the dy e, and only a small extent ofprotonation of the leuco base) , and then mixing equal

volumes of th e solutions.Complete ultraviolet and visible spectra were obtained a t

18and 24 hours reaction time.The absorbance of t he solution a t 620 mp was used t o

calculate the amount of dye present. Assuming 1 : l

stoichiometry, and using the spect ra of t he pure com-ponents, a theoretical spectrum was calculated and com-pared with the experimental. Xearly perfect agreementwas obtained in both cases.

After 48 hours reaction time, the experimental andtheoretical spectra did not agree . Analysis of the experi-mental spectrum revealed that more dihydrochloranil thandye was present. The spectrum of the master solution ofchloranil also revealed the presence of dihydrochloranilafter 48 hours.

Second-order p lots of absorbance dat a for more concen-trated reaction mixtures were accurately linear to about5070 completion. The presence of air resulted in deviationsfrom linearity much sooner, and the calculated rate con-

stants were higher than those obtained from degassed solu-tions by about 20%.Because of the complicating factors, initial rates werc

used t o calculate rate consta nts for the reaction. Thecons tant s obtained in th is manner agreed reasonably wellwith those obtained from second-order plots of degassedsolutions, and were more reproducible.

-4typical second-order plot is shown in Fig . 11.Runs using various concent rations of ace tic acid wcrc

made to establish the independence of rate constant 01 1

acid concentration over a tenfold raiige of liydrogen io11concentration, (i .e. , 100-fold range of concentrations ofacetic acid ).

When the reactions were carried o u t in the absence o Iacetic acid, th e color of the solution increased in intensity.and then decreased until no color remained after severdlhours. Addition of acid to the solution resulted in the ini-mediate appearance of color.

Infra red Spectra.-Infrared spectra of the carbinolbases were measured in C S and CC1, solutions with a Per-

kin-Elmer model 21 spectrophotometer.Molecular weight determinations were performed oi i

crystal violet carbinol using st anda rd techniques.QualitativeTestfor OxygeninMalachiteGreen Carbinol.-

Anhydrous hydrochloric acid was passed through a solutioriof 7 5 mg. of malachite green carbinol i u 20 ml. nf carboiitetrachloride until precipitation was complete.

The lightly colored solvent was carefully decanted, andthe gumm y precipitate was dissolved in 0.10 ml. of absoluteethanol. The resulting ethanol solution was then analyzedfor water by gas chromatography o n an 8’ polyethyleneglycol colum n. Th e presence of water was clearly shon.riby the chromatogram.

A blank of the alcohol, treated in the same tiiaiiiier, didiiot show the presence of water.



2356 M A R KN. RERICKAN D ERNESTL. ELIEL VOl. 84

Acknowledgments.-We gratefully acknowledgethe use of the laboratory facilities of the CosmeticsDivision of the U. S. Food and Drug Administra-tion for the early phases of this work. C. D. R.also wishes to express his appreciation to the preparing the manuscript.

Robert A. Welch Foundation for a Post-doctoralfellowship under which much of the present workwas carried out, and to the Department of Chemis-try, University of Buffalo, for clerical assistance in

[CONTRIBUTIONFROM THE DEPARTMENTOF CHEMISTRYA N D RADIATIONLABORATORY’OF THE UNIVERSITYOF SOTREDAME,NOTREDAME,IND.,AN D THE DEPARTMENTOF CHEMISTRYOF PROVIDENCECOLLEGE,PROVIDENCE,R. I.]

Reduction with Metal Hydrides. IX. Reaction Paths in the Reduction of Epoxideswith Lithium Aluminum Hydride and Aluminum Chloride’

BY M A R KN. RERICKAND ERNESTL. ELIEL

RECEIVEDJANUARY 3, 1962

The nature of the reduction products of triphenylethylene oxide with “mixed hydride” (lithium aluminum hydride-aluminum chloride) has been found t o depend on the proportions of LiAlHl and AlCll in the reagent. With a 1:3 (AlCla:LiAlH4) reagent, presu mably containing AlHa as the acti ve reducing agent, t he product is the previously observed phenyl-benzhydrylcarbinol, PhzCHCHOHPh, probably formed by direct, electrophilically assisted reductive ring opening of th eepoxide. With a 4: 1 reagent, presumably AlHClz and excess AlCla, the main product is PhaCCH*OH, accompanied by hy -drogenolysis products of PhtC HCH OHP h. With this reagent, reduction seems to be preceded by rearrangement of t heep.oxide to the aldehyde PhsCCHO (phepyl shift) as the major product an d the ketone PhzCH COPh (hydride shift) as t heminor product. Treatm ent of t he epoxide with AlCh gave the aldehyde an d ketone in a 3 : l ratlo, as distinct from treat-ment with BFa which gives almost entirely the aldehyde. Th e reaction of the epoxide with AlCla seems to involve a com-

plex of t he chlorohydrin PhzCClCHOHPh as the first intermediate. This unstable chlorohydrin was prepared by treat-ment of the epoxide with hydrogen chloride. Upon treat men t with a limi ted amoun t of LiAIHI (C0.25 mole) it also gavea mixture of Phs CCHO and PhzCHC OPh with the aldehyde predominating, a nd upon reduction with mixed ( 4 : 1) hydrideit gave PhsCCHrOH, whereas with excess LiAlH, alone it gave mainly PhZCHCHOHPh. In this lat ter reduction, the keton ePhzCHCOPh is not an intermediate, since deuteride reduction gave PhZCDCHOHPh, as shown by n.m.r. spectrum, ratherth an PhzCH CDOHPh. Similar results are obtained with p-diisobutylene oxide which gives mainly MeaCCMeZCHzOHwith the 4:l reagent but mainly MeaCCHOHCHMet with the 1:3 reagent.

In previous p ub l i~ at io ns ~~~from this Laboratory,the reduction of epoxides with lithium aluminumhydride-aluminum halide mixtures (“mixed hy-drides”6) has been described. Of particular in-terest in connection with the present investigationis the reduction of triphenylethylene oxide (I) andof @-diisobutylene oxide (11). The former gaverise to phenylbenzhydrylcarbinol (111) and the

BFa /O\ “m.h .”(CeHa)tCCHOt (C~H~)~C-CHCSH~

-V I

(CsW6)zCHCHOHCsH

111

AlClr /o\(CHa)dWCHa)zCHO-(CH,)2CH-C~( C H,) ~

VI1 I1

“m.h.”-(CHt )lCC( CHa)2CH20HIV

(C O H ~ Z C H C O C J L (C&)~CCH~OH

(CH&CCHOHCH( CH&I X

VI11VI

( 1 ) The Radiation Laboratory is operated under contract with the

Atomic Energy Commission. Enquiries regarding this paper should

be directed to E. L. E., University of Notre Dame. Presented before

the Division of Organic Chemistry at the meeting of the American

Chemical Soaet y at New York,N. Y.,September 12, 1960.

(2) Paper VIII,E. L. Eliel and M. N. Rerick. J . A m . C h e m . Soc.,

(3) E. L.Elid and D. W . Delmonte, ibid., 80, 1744 (1958).

(4) E.L.Eliel and M. N. Rerick, ibjd., 81, 1362 (1960).

(5) For a review of mixed hydride reductions cf. M. N. Rerick,

“Selecti ve Reduction of Organic Compounds with Complex Metal

Hydrides ,” Metal Hydrides, Inc., 33 Congress Street, Bever ly, Mass.,

1959, snd E. L.Eliel, Roc. Chem. Progr., I S , 129 (1961).

83, 1367 (1960).

latter to 2,2,3,3-tetramethylbutanol-1( I V ) . I twas postulated that the reduction of I to I11 in-volved a hydride shift in the acidic reaction mediumproducing phenyl benzhydryl ketone (VI) as anintermediate. This postula te was based on thedemonstrationa tha t reduction of styrene oxide withlithium aluminum deuteride-aluminum chloridegave 2-phenylethanol-1-d, evidently v i a phenyl-acetaldehyde, and that similar reduction of iso-butylene oxide gave isobutyl-1-d alcohol v i a iso-butyraldehyde. Th e mechanism of reduction ofI to I11 was inferred by analogy, passing over thefact that in this reduction lithium aluminum hy-dride pre-treated with allyl bromide was usedrather than lithium aluminum chloride-aluminumhalide. At t he time, this did not seem to matter,since allyl bromide (believed to be a generator ofinorganic bromide in situ) had been employed inter-changeably with aluminum halides in other reduc-tions.: Th e reduction of I1 to IV was similarlypostulated t o involve shift of a t-butyl group togive tetramethylbutyraldehyde (VII) as an inter-mediate.4 In this case the reality of the inter-mediate was demonstrated by isolating it from the

reaction of IT with aluminum chloride in the ab-sence of hydride.‘

The postulated rearrangement of I to VI becamea matter of concern in the light of a recent surveyswhich suggests that phenyl usually migrates in pref-erence to hydrogen and especially in view of thedemonstration’ th at treatment of triphenylethyleneoxide (I) with boron trifluoride etherate gives tri-phenylacetaldehyde (V) by a phenyl shift rather

(6) R. E. Parker and N. 5. Isaacs, Ckm. Revs., 69, 737 (1959).

(7) A. C.Cope, P. A. Trumbull and E.R. Trumbull. J , A m . Chcm.Soc., 80, 2844 (1968).

Application of Linear Free Energy Relationships

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