D. S. Noyce, D. Hartter, R. Pollack
Jul 1, 1968
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0
Influential Citations
14
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Journal
Journal of the American Chemical Society
Abstract
The acid-catalyzed dehydration of 1,2-diphenylethanol(l) proceeds readily in 50-60 affords trans-stilbene. Racemization of active 1,2-diphenylethanol is 58 times more rapid than dehydration. dehydration is more rapid. k H / k D being 1.83. of the 1,2-diphenylethyl cation followed by rate-limiting proton loss t o give trans-stilbene. sulfuric acid, a n d T h e reaction is very sharply acid catalyzed, a plot of log k cs. -Ho having a slope of 1.32. In deuteriosulfuric acid, The rate of dehydration of 2-deuterio-l,2-diphenylethanol is smaller than that of 1, All of these facts a re consistent with a reaction pathway involving the reversible formation study of the acid-catalyzed dehydration of alcohols, A as the reverse of the acid-catalyzed hydration of olefins, provides a way of gaining much pertinent information regarding the mechanism of proton transfer to unsaturated sys tems. In previous studies from these laboratories the acid-catalyzed dehydrations of P-hydroxy acids and of P-hydroxy ketones have been examined to compare with proton-initiated reactions of unsaturated acids and unsaturated ketones. The related pair, cinnamic acid and p-hydroxy-P-phenylpropionic acid, have been examined in detail4 and it was shown that these reactions are characterized by ratel i m i t i n g proton transfer to or from carbon. In order to gain more insight into these reaction mechanisms and their generality, we have undertaken a s t u d y of the kinetics and mechanisms of the reactions of stilbene and 1,2-diphenylethanol (1). It is the purpose of the present report to present the evidence which serves to establish the mechanism of the acid-catalyzed dehydration of 1,2-diphenylethanol in reasonable detail. Experimental Section5 Preparation of Materials. 1,2-Diphenylethanol (l), mp 67.067.5", was prepared by reduction of deoxybenzoin with sodium borohydride in methanol. Partial resolution of 1 was carried out following the procedure of Gerrard and Kenyon.6 Material with [O(lz5D +4.29" was used to determine the rate of racemization of 1. Following the procedure of Curtin and K e l l ~ m , ~ reduction of cisstilbene oxide with lithium aluminum deuteride gave t/zreo-2-deuterio-1,2-diphenylethanol (2), mp 65.8-66.4" ( M 7 mp 64.4-65.4"). Anal. Found: 6.87 atom excess D, corresponding to 9 6 . 2 z monodeuteration. Similarly, reduction of trans-stilbene oxide with lithium aluminum deuteride afforded eryf/iro-2-deuterio-1,2(1) Supported in part by grants from the National Science Foundation, GP 1572 and GP 6133X. Partial support was also provided by a grant from the Petroleum Research Fund administered by the American Chemical Society. Grateful acknowledgment is made to the donors of these funds. (2) A portion of this work has been reported in a preliminary communication: D. S. Noyce, D. R. Hartter, and F. B. Miles, J . Am. Chem. SOC., 86, 3584 (1964). (3) Shell Fellow in Chemistry, 1963-1964. (4) (a) D. S. Noyce and C. A. Lane, J . Am. Chem. SOC., 84, 1635 (1962); (b) D. S. Noyce, P. A. King, F. B. Kirby, and W. L. Reed, ibid., 84, 1632 (1962). (5) Analyses are by the Microanalytical Laboratory, University of California, Berkeley, Calif. Deuterium analyses were carried out by Mr. Josef Nemeth, Urbana, Ill. (6) W. Gerrard and J. Kenyon, J . Chem. SOC., 2564 (1928). (7) D. Y . Curtin and D. B. Kellom, J . Am. Chem. SOC., 75, 6011 (1 953). diphenylethanol (3), mp 66.0-66.2' (lit.' mp 64.4-65.4"). Anal. Found: 6.98 atom % excess D, corresponding to 97.8% monodeuteration. The infrared spectra were identical with those reported by Curtin and Kellom. The nmr spectra were distinctive and in accord with the structural assignments. l-Deuterio-1,2-diphenylethanol (4) was prepared by reduction of deoxybenzoin with lithium aluminum deuteride. Deoxybenzoin-a,a-d? was prepared by exchange in alkaline solution. A solution of 10 g of deoxybenzoin in 70 ml of purified dioxane was added to 70 ml of D 2 0 in which 2 g of sodium had been dissolved. The mixture was refluxed overnight. On cooling, two layers formed. The aqueous layer was extracted with three 100-ml portions of ether, and the combined organic fractions were concentrated under reduced pressure. The exchange was repeated. Deoxybenzoin-a,a-& was reduced with lithium aluminum hydride and 1,2-diphenylethanol-2,2-dz ( 5 ) was isolated, mp 65.3-65.8' (from hexane). Anal. Found: 13.80 atom excess D, corresponding to 96.5 z deuteration. For all studies in 5 % ethanolic solutions, the following method was used. To 5 ml of 95% ethanol in which a weighed quantity of the organic substrate was dissolved, sufficient aqueous sulfuric acid of the requisite strength was added to give a final volume of 100 ml. The final solution was titrated in duplicate against standardized base. For determination of the acidity function, a similar procedure was used, dissolving the Hammett indicators in the original ethanol. A like procedure was used to prepare 20 % ethanol-sulfuric acid solutions and 50 % ethanol-sulfuric acid solutions. The 30 z acetic acid-sulfuric acid solutions were prepared by mixing weighed portions of acetic acid and standardized sulfuric acid. Other kinetic methods have been described previously.8 Kinetic measurements were generally made using 10-cm cells. Acidity Function in 5 Ethanol. The extremely low solubility of stilbene in aqueous sulfuric acid dictated the use of a mixed solvent system for the kinetic medium. Measurements of the acidity function in this mixed medium, described above, were carried out in the usual manner, and the results of these measurements are recorded in Table I. It is to be noted that from 10 to 40% sulfuric acid, the alcoholic solution is slightly less acidic than aqueous sulfuric acid; above 40 Exchange of tra/is-Stilbene-a-di. rrans-Stilbene-a-dl was prepared by the method of Curtin and H a r r i ~ . ~ A mixed solvent was prepared by diluting 70 ml of 95% ethanol with aqueous sulfuric acid to a total volume of 200 ml. To this solution was added 40 mg of rrans-stilbene-a-dl, The solution was maintained at 45.00", and then quenched by the addition of cold water. Stilbene was isolated by extraction with ether. The extracts were dried over anhydrous sodium sulfate and evaporated to dryness in L'UCUO. The solid residue, 30-40 mg, was purified by chromatography on neutral alumina. The fraction of deuterium remaining was determined by infrared spectroscopy, using a Perkin-Elmer Model 421 spectrophotometer, by measuring the 2235-cm-l band on an expanded scale setting. Known mixtures of trans-stilbene and trunsstilbene-cu-dl were used to construct a calibration curve. Preparation of Solutions and Kinetic Methods. sulfuric acid the 5 % alcoholic solution is more acidic. (8) D. S . Noyce and M. J . Jorgenson, ibid., 84, 4312 (1962). (9) D. Y. Curtin and E. E. Harris, ibid., 73, 4519 (1951). Noyce, Hart ter , Pollack 1 Dehydration of 1,2Diphenylethanol D ow nl oa de d by T A R B IA T M O D A R R E S U N IV o n Ju ly 1 6, 2 00 9 Pu bl is he d on M ay 1 , 2 00 2 on h ttp :// pu bs .a cs .o rg | do i: 10 .1 02 1/ ja 01 01 6a 03 4 3792 Table I. Ho Values for 5 % EtOH-95 z HaO-HaSOd HaSOd, Ho HaSO4, Ha 7z (alcoholp Indicatorc AH,’ z (alcohol) Indicatorc AHo‘ 10 -0.29 a 0 44 -2.76 c, d -0.01 12 -0.44 a +0.01 46 -2.98 c, d -0.02 14 -0.59 a +0.01 48 -3.22 c, d -0.03 16 -0.73 a +0.02 50 -3.42 c, d -0 .04 18 -0 .86 a 1-0.03 52 -3.64 d -0.04 20 -0.98 a, b +0.05 54 -3.83 d, e -0 .04 22 -1.10 a , b $0.06 56 -4.08 e -0.07 24 -1.23 a, b +0.07 58 -4.33 e -0 .09 26 -1.37 a, b +0.07 60 -4.58 e, f -0.10 28 -1.51 b 1-0.07 62 -4.80 e, f -0 .10 30 -1.64 b, c +0.08 64 -5.07 e, f -0.13 32 -1.80 b, c +0.06 66 -5.40 f -0 .20 34 -1.93 b, c + O . 06 68 -5 .74 f -0.24 36 -2.08 b, c $0.05 70 -6.07 f, g -0.27 38 -2.23 b, c $0.03 72 -6.41 g -0.31 40 -2.39 b, c +0.02 74 -6.74 g -0.33 78 -7.41 g -0.38 42 -2 .56 c, d 0 76 -7.08 g -0.37 a Ho in the 5 % EtOH-95 % HsSOd system. The HO values of M. A. Paul and F. A. Long [Clrern. Rev. , 57, 1 (1957)l for aqueous acid were used up to 6 0 z HsS.04; above 60% sulfuric acid, the values of M. J. Jorgenson and D. R. Hartter [ J . Am. Chern. SOC., 85, 878 (196311 were used. HO (alcohol) determinations used the following indicators: a , 2-nitro-4-chloroaniline; b, 2,5-dichloro-4-nitroaniline; c, 2-chloro-6-nitroaniline; d, 2,4-dichloro-6-nitroaniline; e, 2,4-dinitroaniline; f, 2,6-dinitroaniline; g, 2,6-dinitro-4-chloroaniline. * AH, = HO (alcohol) Ho. Results and Discussion The dehydration of 1,2-diphenylethanol may be conveniently followed kinetically by observing the appearance of the characteristic ultraviolet absorption of trans-stilbene. In aqueous sulfuric acid the extremely limited solubility of trans-stilbene creates some experimental problems which are overcome by carrying out studies with a small fraction of organic cosolvent (we settled on the use of 5z added ethanol) and by using more dilute solutions (about M ) in 10-cm cells. Scrupulous care to have the glassware clean was necessary in order to obtain excellent first-order behavior. The dehydration of 1,2-diphenylethanol is very sharply acid catalyzed. The reaction rate increases more than 100-fold in changing from 46 to 62% sulfuric acid. The reaction proceeds essentially to completion. Careful measurements in 50z sulfuric acid of the ultimate production of trans-stilbene gave values of 98.7 i 0.5% dehydration. The measured rates of dehydration (which were followed to a stable “infinity” spectrum) have not been corrected for the very small amount of back reaction. Measured rates at 25 and 45 O are presented in Table 11. It is to be noted that the reaction rate increases more rapidly than the increase in the acidity of the medium. When the logarithm of the pseudo-first-order rate constant is plotted us. -Ho, the slope is 1.32. A high slope such as this appears to be characteristic of reactions proceeding cia carbonium ion intermediates, particularly benzyl cations. Similar slopes were observed in the dehydrat