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

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1,2-二苯乙醇酸催化脱水的机理

1,2-二苯基乙醇(l) 的酸催化脱水在 50-60 ℃ 很容易进行，得到反式二苯乙烯。活性 1,2-二苯基乙醇的消旋作用比脱水快 58 倍。脱水更快。k H / k D 为 1.83。1,2-二苯乙基阳离子随后限速质子损失得到反式二苯乙烯。硫酸，并且该反应在酸催化下非常剧烈，log k cs 的图。-Ho 的斜率为 1.32。在氘代硫酸中，2-氘代-1,2-二苯乙醇的脱水速率小于1，所有这些事实都与涉及酸催化醇脱水可逆形成研究的反应途径一致， A 作为烯烃酸催化水合的逆反应，提供了一种获得有关质子转移到不饱和系统机制的相关信息的方法。在这些实验室之前的研究中，已经检查了对羟基酸和对羟基酮的酸催化脱水，以与不饱和酸和不饱和酮的质子引发反应进行比较。相关的对，肉桂酸和对羟基-P-苯基丙酸，已被详细检查 4 并且表明这些反应的特征在于模仿质子转移到碳或从碳转移的速率。为了更深入地了解这些反应机制及其一般性，我们对芪与 1,2-二苯乙醇 (1) 的反应动力学和反应机理进行了研究。本报告的目的是提供证据，以合理详细地确定 1,2-二苯基乙醇的酸催化脱水机理。实验部分5 材料的制备。1,2-二苯基乙醇 (l)，mp 67.067.5"，是通过在甲醇中用硼氢化钠还原脱氧安息香来制备的。按照 Gerrard 和 Kenyon 的程序对 1 进行部分拆分。6 具有 [O(lz5D + 4.29" 用于确定 1 的外消旋化速率。按照 Curtin 和 Kell ~ m 的程序，~ 用氘化铝锂还原氧化顺二苯乙烯得到 t/zreo-2-deuterio-1,2-二苯基乙醇 (2) , mp 65.8-66.4" (M 7 mp 64.4-65.4")。分析。发现：6.87 原子过量 D，对应于 9 6 . 2 z 单氘化。类似地，用氘化锂铝还原反式二苯乙烯氧化物得到 eryf/iro-2-deuterio-1,2(1) 部分由美国国家科学基金会、GP 1572 和 GP 6133X 资助。美国化学学会管理的石油研究基金的赠款也提供了部分支持。对这些资金的捐助者表示感谢。(2) 这项工作的一部分已在初步通讯中报告：DS Noyce、DR Hartter 和 FB Miles, J。是。化学 SOC., 86, 3584 (1964)。(3) 壳牌化学研究员，1963-1964。(4) (a) DS Noyce 和 CA Lane, J。是。化学 SOC., 84, 1635 (1962); (b) DS Noyce、PA King、FB Kirby 和 WL Reed，同上，84, 1632 (1962)。(5) 分析由加利福尼亚大学伯克利分校的微量分析实验室进行。氘分析由伊利诺伊州厄巴纳 (Urbana) 的 Josef Nemeth 先生 (6) W. Gerrard 和 J. Kenyon, J 进行。化学 SOC., 2564 (1928)。(7) D.Y. 科廷和 DB 凯洛姆，J。是。化学 SOC., 75, 6011 (1 953)。二苯乙醇 (3), mp 66.0-66.2' (lit.' mp 64.4-65.4"). 分析。发现：6.98 原子% 过量 D，对应于 97.8% 单氘。红外光谱与科廷和凯洛姆报告的相同。核磁共振谱独特且与结构分配一致。l-Deuterio-1,2-二苯基乙醇（4）是通过用氘化铝锂还原脱氧安息香而制备的。脱氧安息香-a,ad?通过在碱性溶液中交换制备。将10g脱氧安息香在70ml纯化二恶烷中的溶液加入到70ml已溶解2g钠的D 2 0 中，混合物回流过夜。冷却时，形成两层。水层用三份100毫升的乙醚萃取，合并的有机部分在减压下浓缩。重复了交换。Deoxybenzoin-a,a-& 用氢化铝锂还原并分离出 1,2-二苯基乙醇-2,2-dz ( 5 )，mp 65.3-65.8' (来自己烷)。肛门。发现：13.80 原子过量 D，对应于 96.5 z 氘化。对于 5% 乙醇溶液中的所有研究，使用以下方法。向其中溶解有称量有机底物的 5 ml 95% 乙醇中，加入足够浓度的所需浓度的硫酸水溶液，使最终体积为 100 ml。最终溶液相对于标准碱滴定一式两份。为了确定酸度函数，使用了类似的程序，将哈米特指示剂溶解在原始乙醇中。使用类似程序制备20%乙醇-硫酸溶液和50%乙醇-硫酸溶液。30μl 乙酸-硫酸溶液通过混合称重部分的乙酸和标准化硫酸来制备。其他动力学方法之前已经描述过。8 动力学测量通常使用 10 厘米的池进行。5 乙醇中的酸度函数。芪在硫酸水溶液中极低的溶解度决定了使用混合溶剂系统作为动力介质。上述混合介质中酸度函数的测量以通常的方式进行，这些测量的结果记录在表 I 中。值得注意的是，从 10% 到 40% 的硫酸，酒精溶液的酸性略低于硫酸水溶液；以上 40 交换 tra/is-Stilbene-a-di。rrans-Stilbene-a-dl采用Curtin和H arri的方法制备。~ 通过用硫酸水溶液将 70 ml 95% 乙醇稀释至 200 ml 的总体积来制备混合溶剂。向该溶液中加入 40 mg rrans-芪-a-dl，将溶液保持在 45.00"，然后加入冷水淬灭。用乙醚萃取分离芪。萃取液用无水硫酸钠干燥并在 L'UCUO 中蒸发至干。固体残留物，30-40mg，通过色谱在中性氧化铝上纯化。剩余的氘分数通过红外光谱测定，使用 Perkin-Elmer 421 型分光光度计，通过在扩展的刻度设置上测量 2235-cm-l 波段。使用已知的反式二苯乙烯和 trunsstilbene-cu-dl 的混合物来构建校准曲线。溶液的制备和动力学方法。硫酸 5% 的酒精溶液酸性更强。(8) D.S. 诺伊斯和 M. J . 乔根森，同上，84, 4312 (1962)。(9) DY Curtin 和 EE Harris，同上，73, 4519 (1951)。Noyce, Hart ter , Pollack 1 Dehydration of 1,2Diphenylethanol Dow nlo de by TARB IA TMODARRESUN IV on July 1 6, 2 00 9 Pu bl is he d on May 1, 2 00 2 on h ttp :/ / pubs .acs .o rg | do i: 10 .1 02 1/ ja 01 01 6a 03 4 3792 表 I. 5% EtOH-95 z HaO-HaSOd HaSOd, Ho HaSO4, Ha 7z (alcoholp Indicatorc AH,' z (alcohol) Indicatorc AHo 的 Ho 值' 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 -3.803 d .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 -810.10 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 +0.6.4 7 g 0.33 78 -7.41 g -0.38 42 -2 .56 c, d 0 76 -7.08 g -0.37 a Ho 在 5% EtOH-95% HsSOd 系统中。MA Paul 和 FA Long [Clrern. Rev., 57, 1 (1957)l 用于含水酸的最高 6 0 z HsS.04；60% 以上的硫酸，MJ Jorgenson 和 DR Hartter 的值 [ J . 是。陈。SOC., 85, 878（使用了 196311。H2O（酒精）测定使用以下指示剂：a，2-硝基-4-氯苯胺；b、2,5-二氯-4-硝基苯胺；c、2-氯-6-硝基苯胺；d, 2,4-二氯-6-硝基苯胺；e, 2,4-二硝基苯胺；f, 2,6-二硝基苯胺；g, 2,6-二硝基-4-氯苯胺。* AH, = HO（酒精）Ho。结果和讨论 1,2-二苯基乙醇的脱水可以通过观察反式二苯乙烯的特征紫外吸收的出现而方便地进行动力学跟踪。在硫酸水溶液中，反式二苯乙烯的极有限溶解度产生了一些实验问题，通过使用少量有机助溶剂（我们决定使用 5z 添加乙醇）和使用更稀的溶液（约 M ) 在 10 厘米的单元格中。为了获得出色的一阶行为，必须仔细清洁玻璃器皿。1,2-二苯基乙醇的脱水是非常剧烈的酸催化。从 46% 到 62% 的硫酸，反应速率提高了 100 倍以上。反应基本上进行到完成。在 50z 硫酸中仔细测量反式二苯乙烯的最终产量给出了 98.7 和 0.5% 脱水的值。测得的脱水率（遵循稳定的“无穷大”光谱）尚未针对非常少量的逆反应进行校正。在 25 和 45 O 下测得的速率列于表 11 中。值得注意的是，反应速率的增加比介质酸度的增加更快。当我们绘制伪一阶速率常数的对数时。- 何，斜率为 1.32。像这样的高斜率似乎是碳正离子中间体，尤其是苄基阳离子进行的反应的特征。在脱水器中观察到类似的斜率