3712
Vol. 79 [ C O N T R l U U T I O N FROM THE
DEPARTMENT OF
CHEMISTRY,
THEUSIVERSITY
O F ROCHESTER]
Mechanisms of Elimination Reactions. 11. Rates of Elimination from Some Substituted 2-Phenylethyl Bromides and 2-Phenylethyldimethylsulfonium Bromides' BY WILLIAMH. SAUNDERS, JR.,
AKD
RICHARD A. WILLIAMS^
RECEIVED DECEMBER 10, 1033
A series of $-substituted 2-phenylethyl bromides and 2-plienyletliyldinieth).lsuiioniuin broiriides has been prepared. The rates of their reactions with sodium ethoxide in ethanol were determined over a range of temperatures and the enthalpics and entropies of activation calculated. All of the compounds studied gave quantitative yields of the corresponding styrenes. T h e data fit the Hammett equation satisfactorily, giving p = 4-2.15 for the bromides and $2.64 for the sulfonium salts when the substituents are p-methoxyl, p-methyl, hydrogen and p-ehloro. The substituents p-acetyl and p-nitro require the use of U-values derived from the reactions of phenols aud anilines. Factors affecting the balance a n d timing of the bondmaking and bond-breaking processes in E2 eliminations are discussed, and some applications of these ideas to previous rcsults are made.
Recent studies of base-promoted elimination reactions have suggested the need for revision of some commonly accepted ideas. For instance, the usual marked preference for trans elimination may be greatly reduced when electron-withdrawing substituents are present on the P-carbon atom3 or when the four centers involved (H-C-C-X) cannot achieve ~ o p l a n a r i t y . ~Reactivities of diastereomeric I ,2-diphenyl-l-propyl derivatives show that the degree of eclipsing in the transition state may be greatly modified by changes in the leaving group, the solvent and the base.6 The absence of an appreciable sulfur isotope effect in eliminations from 2-phenylethyldimethylsulfoniuni bromide has been taken as indicating a transition state possessing considerable carbanion character. These and other results provide evirlencc that many E2 eliminations may depart rather widely from the conventional picture of a synchronous trans process. Recent evidence that steric6 as well as electronic factors may be important in elimination reactions also calls for a reconsideration of former concepts. We felt that a systematic study of electronic influences on rates of eliininations might shed further light on these problems. Consequently, the preparation of a series of p-substituted 2-phenylethyl bromides and 2-phenylethyldimethylsulfonium bromides was undertaken and the rates of their reactions with sodium ethoxide in absolute ethanol were determined. It was anticipated that the direction and magnitude of substituent effects would lead to information on the clectron distribution in the transition state. Syntheses of the desired compounds were accomplished by conventional procedures. The reactions employed in most cases are sumniarized in Fig. 1. A considerable number of the intermediates and products were unknown prior to this investigation. Details on these and other aspects of the synthetic !1) Paper I in this series: W. 11. Saunders, Jr.. a n d 79, 1612 (1057). (2) American Cyanamid Fellow, 1955-1956; Dealinit Slills I'elluw,
T H I S JOURNAL,
Summer, 1956. (3) J. Weinstock, R. G. I'earsrin and 17. G.Bordwell, T H I SJ < , r . ~ s n l . , 7 8 , 3468 (1SR6). 14) S. J. Cristol a n d N. I,. Hause, ibid.. 74, 2193 (1952); S 1 Cristol and R. P. Argatlbriglit, Abstracts of Papers, 130th N e e t i n i of tlie American Chemical Society, Atlantic City, N. J., September I + 21, 1D5R. ( 5 ) I). J. CI-.irn. F, D . Greene a n d C . €1. DePlly, T H I S J O U R N A I . , 78, 7!20 (14.ifi) iiii !! c I i i < , n f i XI:^ I. . ' ~ r c ~ n t , ~ lnhi /, , l , , 78, 2Xl3 [I!):~i~>, , t i i c i lire-
approach are given under Experimental. Some departures from the general scheme were necessary. Direct acetylation' of I I I a and IVa yielded ITIe and IVe, respectively. Nitration7 of I I I a gave IIIf. Attempts t o prepare Vf were unsuccessful and were abandoned when it became apparent that the reaction of Vf with base would be too fast to measure. The compounds examined in the kinetic studies were IIIa-f in the bromide series and Va-d in the sulfonium bromide series. The corresponding styrenes (VI) were, for the most part, obtained by treatment of the sulfonium salts with aqueous sodium hydroxide, followed by isolation and careful distillation of the styrenes. The ultraviolet absorption spectra of the styrenes were determined in absolute ethanol. Extinction coefficients at the maxima used for analysis are recorded in Table I. These values were used in the calculatiori of olefin yields in the kinetic experiments. Tan1,c I U L T R A V I O L E T r ~ B S O R P i ' i O NOF
p-SURbTITUTED STTREShS
IN
95% E.rrrasor, Cpd.U
X m ~ x ,nw
hldar extinction X 10-4
YIC4 VIb \.'IC VItl \;Ie YIf
218 352 258 253 '780 300
1.3@ 1.69 1.92 1.07 2.07' 1 ,37"
* Lit.' 1.35 'i Sce Fig. 1 for nuinhering of coinpouritls. X lo4. G. Baddeley, E. \\'rcnch anti R. Williamson, J . Cheiii. Soc., 2110 (1953), give 2.15 X lo4. h f . J . Kamlet and D. J. Glover, THISJocRNAr,, 7 7 , 5696 (1955), give 1.39 X l o 4 . The rates of the reactions of I11 and V with sodium ethoxide in absolute ethanol were followed acidirnetrically. Olefin yields were calculated from the ultraviolet absorption of the reaction mixtures a t the appr0priat.e maxima. Kate constants and olefin yields are collected in Table IT. The olefin yields are obviously within experimental error of loo%;;,and thus the rate constants are attributable entirely to elimination.* The reactions were all ( i ) E: I,. Firremati and S. h f . I \ I r L S l \ a i n t b i d . , 64, 113.5 ( 1 0 1 0 ) . IS) T h e s t y r e n e yield iron1 llTa is reporled t o IIC Q4.6%> by E. r). Iliiqhes. C. K . I n g o l d S. Ai.isternian a n d 13. J. M c N i i l t y . J . CIZLIIZSo(., Y!l!> (1940). 'l'lie s i i l f ~ m i ~ iiodide m corresponding to Va is repurted t o S I 1 ' : olt,lin I,? ii D I T : i ~ l i ~ C s , K . r n w d d :and C A . h l a w , I h i J , 207:) (l!i I S ) . 'I'1ii.i~ ,~iinl\s1-s ~ ~ t r ~ > l u ytvl i, rl L ~ r ~ ~ i t i. ~~i ~~ cl~ ~ l ~ l i u ~ ~
-
ELIMINATION RATESOF 2 - P H E N Y L E T H Y L BROMIDES
July 20, 1957 X - O M g B r
liaOEt
___)
EtOH
I
PBra
3713
X-O-CH=CHz VI
X ~ - - F ~ ~ C H Z S ___) CCH3Br H ~ Sa-CH2CHzS(CH3)zBr
CHaKOz V a, X = H ; b, X = CHI; C, X = CHBO; d, X = C1; e, X = CHsCO; f, X = NO2 Fig. 1.-Synthetic scheme.
Activation enthalpies for the sulfonium salts run 3-5 kcal. higher than for the bromides, but the reactions of the sulfonium salts are actually faster because of a compensating difference in entropies of activation. The much larger entropies of activation for the sulfonium salts probably can be ascribed mainly to entropies of solvation. Since TABLE I1 the reaction is between oppositely charged ions, RATE CONSTASTS AND OLEFINYIELDSFOR ELIMINATIONS solvent molecules should be released on going to the FROM 2-ARYLETHYL BROMIDES A S D 2-.ARYLETHYLDIMETHYLtransition state. The variation in entropy of acSULFONIUM BROMIDES WITH SODIUM ETHOXIDE IS . ~ B S O L U T E tivation among the bromides (except IIIf) appears ETHANOL to be within experimental error. The difference Olefin kz X 105, yield, Cpd.a T, OC. 1. mole-' sec.-lC for IIIf may have mechanistic significance, but we IIIa 814 f 9 4 59.40 9 9 . 7 3 ~0 3 cannot exclude the possibility of systematic errors 50,20 IIIa 342 f 4 . 3 9 9 . 5 f .3 in the rate constants aggravated by the high speed 133 5 2 . 1 9 9 . 6 1 . 4 of the reaction. Entropies of activation for the 40.75 IIIa IIIa 41.7 =!= 0 . 6 6 9 9 . 6 1 .I sulfonium salts are spread over a wider range (ca. 5 30.05 100.1 f . 2 e.u.), and there is a tendency for high A S + to be IIIb 494 f 6 . 0 59.40 IIIb 202 13 . 3 9 9 . 6 1 .4 associated with high AH+.g 50.20
cleanly second order and the constants were steady t o a precision of 1-2% in practically all cases. The data were fitted to log k vs. 1/T plots by the method of least squares and the enthalpies and entropies of activation calculated. These values are recorded in Table 111.
%C
IIIb 75.0 f 1 . 1 4 l 0 0 . 0 f .1 40.75 22. 8' ........ IIIb 30.05 IIIC 378 4.7 99.3 f0.4 59.40 IIIC 151 f 1 . 5 100.5f .3 50.20 40.75 IIIC 52.8 5 0.71 99.8f .3 16.2' IIIC . .... . . . 30.05 IIId 9 9 . 1 3Z 0.5 1410 =!= 22 50.20 IIId 585 f 1 1 . 9 40.75 9 9 . 4 5 .4 30.05 IIId 9 9 . 0 5 .1 191 f 3 . 3 IIIe 3720b 30.05 . . . . .. . . . 20.10 IIIe 100.4 f 0 . 3 1340 f 27 IIIe 464 f 5 . 1 10.35 100.Of .2 0.0 135 f 2 . 4 IIIe 1 0 0 . 2 f .1 30.0.5 f4200b IIIf ...... ,. IIIf 10.35 9140 f 280 99.0 f0.7 0.0 IIIf 2680 f 24 9 9 . 4 1 .5 1970 f 26 40.75 Va 99.4f .2 500 f 7 . 5 30.05 Va 9 9 . 4 5 .3 20.10 133 f 1.1 Va 9 9 . 4 f .2 40.75 1000 f 12 Vh 9 9 . 5 f .4 232 f 2 . 3 30.05 Vb 9 9 . 9 f .3 55.1 f 0.69 20.10 Vb 1 0 0 . 2 f .2 40.75 484 f 3 . 6 VC 99.5f .4 30,05 111 f 1 . 4 VC 99.3f .2 30.05 2440 f 38 Vd 9 9 . 0 5 .2 Vd 680 f 12 20.10 9 9 . 2 3 ~.3 Vd 177 3Z 3 . 8 10.35 99.03Z . 1 a See Fig. 1 for numbering of compounds. Extrapolated from the Arrhenius plot of data a t other temperatures. e Deviations listed are average deviations.
Inspection of the rate constants shows that the elimination reaction in both series is strongly accelerated by electron-withdrawing substituents. method. The reasons for the discrepancies are not entirely clear, but chances of loss of product should be greater in the chemical than in the spectrophotometric analyses.
TABLEI11 ASD ESTHALPIES OF ACTIVATIONFOR ELIMINAENTROPIES TIONS FROX z-r\RYLETHTL BROMIDES AND 2-ARYLETHYLDIMETHYLSULFOSIUM BROMIDES AH,*^ AS*,. Cpd.
T,O C . 0
kcal.-mole-l
ca1.-mole-1, OC.--I
IIIC 50 20.8 -7.3 IIIb 50 20.3 -8.2 IIIa 45 19.6 -9.2 IIId 40 18.7 -9.2 IIIe 20 17.6 -7.0 IIIf 5 17.7 -0.8 vc 45 25.4 11.6 Tb 30 25.0 12.2 Va 30 23.3" 7.7 Vd 20 22.2 7.2 Mid-point of the temperature range of the data. b Calculated from the slope (method of least squares) of a log k PIS. l / T plot and the equation AH* = E. - R T . cCalculated from the intercept (method of least squares) of a lit.8 reports E, = 23.9 kcal., which is log k vs. l / T plot. equivalent to AH* = 23.3 kcal.
Rate constants a t 30" (some extrapolated from data a t other temperatures) for the $-substituents methoxyl, methyl, hydrogen and chlorine in each series were fitted t o the Hammett equationlo by the method of least squares, using the U-values tabulated by J a i X 1 O b Table I V records the results of these calculations. Reaction constants for the entire series of bromides I I I a to IIIf are also tabulated. It is evident that the calculations employ(9) J. E. Leffler, J. Org. Chcm., 20, 1202 (19.551, discusses such relationships and quotes many examples. (IO) (a) L. P. Hammett, "Physical Organic Chemistry," McGrawHill Book, Co.,Inc., New York, X. Y . , 1940, Chap: V I I ; (b) H. H. JaffC, Chem. Revs.. 63, 191 (1953).
I\-ILLIABI H. SAUNDERS, JR.,
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TABLE IV RATES O F ELIXINATIOY
HAMMETT CORRELATIOS O F Cpd.
P
1.1
Se
-log koQ
IIIa-d 2.154 i 0.242d 0.091 0.987 3.269 IIIa-fb 2.342 i ,115 ,154 ,996 3.270 IIIa-f" 3 . 5 0 5 i ,338 ,308 ,982 3.148 Va-d 2.639 i ,157 ,059 ,996 2.365 Calculations by the least squares procedure as outlined it1 ref. lob. Using u*-values for p-nitro arid p-acetyl. Using ordinary U-values for p-nitro and p-acetyl. -411 uvalues are from ref. 9b. Standard deviation of p . e Standard deviation of experimental points from the least squares line. f Correlation coefficient. 0 Calculated value of the intercept.
ing u*-values'Ob (those derived from the reactions of phenols and anilines) give a far better fit with the p-acetyl and p-nitro groups. Figure 2 shows that the use of ordinary U-values for these two substituents produces marked curvature."
AND
KICHARD A. WILLIAMS
Vol. T ! )
with the substituent is p0ssib1e.l~ Since the two systems under discussion have leaving groups of different charge types, the objection might be raised that this factor in itself influences the value of p . What p measures, however, is a diference in electron density a t the P-carbon between the ground state and the transition state. The effect of charge type should therefore cancel, particularly in view of independent evidence' that the C-S bond is little disturbed on going to the transition state. The nature of the transition state in E2 eliminations can be discussed best in terms of the structures VI-V1II.j In this representation VI1 is the s s s
>c-c< B
I
H
H VI
>C-C
