DONALD S. KOYCEAND HARVEY S. AVARBOCK
1644
and then from water to yield (+)-P-phenyl-P-hydroxypropionic acid- a-dl, m.p. 115-1 18".'O d less Completely resolved sample of the ( - )-acid was also obtained, m.p. 113118'. Kinetic Procedures.-The kinetic procedures have been described in a previous paper.'" Product isolation studies \vere carried out as described previously.'" A. Racemization.-From a sample of ( +)-P-phenyI-Phydroxypropionic acid, there was isolated rac-P-phenyl-8hydroxypropionic-a-d acid, m.p. 90-92".
[COSTRIBUTION FROM
THE
Vol. SA
Ana1.22 Found: 10.05 & 0.3 atom yo excess D. B. Dehydration.-Samples of cinnalnic-a-d acid \\-ere isolated under the conditions described previously.'? Tlic cinnamic acid was purified by crystallization from heptane and sub~inlation. Anal. C k d . for c s H ~ D 0 2 : 12.5 atom % excess I). Found: 9.14, 9.46 from two separate preparations. (22) Deuterium analysis by Dr. C . W. Koch, Department of Chemist r y , University of California, Berkeley.
DEPARTMEST O F CHEMISTRY, USIVERSITY
OF
CALIFORNIA, BERKELEY, CALIF.]
The Effect of Substituents upon the Rate of Isomerization of Substituted cis-Cinnamic A~idsl-~ BY DONALD s. r\'OYCE
AND
HARVEY s. -kVL4RBOCK
RECEIVED SEPTEMBER 29, 1961 The rates of isomerization of cis-p-xiietlioxycinnainic acid and of cis-p-chlorociniiairiic acid as catalyzed by sulfuric acid have been studied. Each of the isomerizations parallels the acidity function Ho with unit slope. T h e corresponding Paryl-P-hydroxypropionic acid is a n intermediate. The rate of isomerization is sensitive t o the electronic nature of the substituent. Correlation with u + gives a p of -4.3. These data support a mechanism in which there is a large electron deficiency a t the benzylic carbon in the transition state.
Introduction The isomerization of cis-cinnamic acid as catalyzed by sulfuric acid4 has been shown to be closely involved with the behavior of the related hydroxy acid, P-phenyl-P-hydroxypropionic acid.1 ' 4 , 5 I t was also shown that the series of P-aryl-P-hydroxypropionic acid gave a very steep p-u+ correlation for the dehydration reaction, indicating the high degree of carbonium ion character a t the benzylic carbon.E In the present report we wish to present results on the kinetics of the acid-catalyzed isomerization of two substituted &-cinnamic acids. These results show that in this reaction also there is a high degree of positive charge a t the (benzylic) carbon. Experimental' Materials.- hirs-p-Methoxycinnamic acid \\-;is crystallized to coiistaiit i 1 i . p . ;tnd ultravinlet pectruiii froiii metlianol: i 1 i . p . 173.4 -174.8'; ncut. cqui\.. calcd. 178.2, found 17;. cis-P-Methoxycinnamic acid \ \ - a b prepared by ultraviolet irradiation of a sodium carbonate solution of the trans isomeiin a quartz flask. Separation of the cis and trans isomers was easily effected by fractional crystallization from benzene, the cis isomer being more soluble than the trans isomer by a factor of a t least 300 at 25'. The product obtained in 20ce yield was recrystallized to cullstaxit m.p. from a mixture of bcnzerie and petroleum ether; m.p. 69.2-69.4" (lit.* 66"). L l n u l . Calcd. fnr C l ~ f l l ~ , OC, d : 67.41; H, 5.66; neut. equiv., 178.2. Found: C, 67.51; H , 5.78; neut. equiv., 179. __ - ~ ~ ~ _ _ ( 1 ) Paper S V I I i n t h e Series Carbonyl Reactions; previous paper. 11. S . Soyce and C A . I.ane, J . A i n . ('Iioii k2) Sulii)urted i n part by rt grant f r u m the Petroleum Research F u n d administered b y the American Chemical Society. Grateful acknowlcdgmeut is hereby made t o the donors of this f u n d . ( 3 ) Presented in part a t the Eighth Conference on Organic Reaction Mechanisms. Princeton. S.J , Septembei, 1960. (4) D. S . S o y c e . P. A . King, F. B . Kirby and n'. I.. Reed, J . A m . CI7em Soc., 84, 1632 (1962). ( 5 ) D. S. Koyce and C . A , Lane, ibid., 84, 1636 (1962). (6) D . S Noyce, P. .4. King, C. A . 1-ane and W L. Reed, i b i d , 84, 1636 (1962). ( 7 ) .4nalyses ,ire by t h e Riicroanalyticzil Laboraturl- of t h e U n i v c r sity o f California. l l c l t i n g puints -re currecicd ( 8 ) \V. .1.R u t h and R . Stoermer, B m , 46, 260 (lQ13).
trans-p-Chlorocinnamic acid was prepared from p-chlorobenzaldehyde and malonic acid following the procedure of Pandya and Pandya.Q A sample was crystallized to constant m.p. and spectrum from ethanol; m.p. 251.2-251.6" (lit. 247°.9 25OoIo); iieut. equiv. calcd. 182.6, found 183. cis-p-Chlorocinnamic Acid.-Ultraviolet irradiation of the trans isomer and separation of isomers in the manner described in the preparation of cis-p-Inethoxycinnanlic acid produced cis-p-chlorocinnamic acid in 259; yield. It was recrystallized to constant m.p. from distilled water; n1.p. 112.0-112.3' (lit." 113.8-116.2'). Anal. Calcd. for CsH502CI: C, 59.19; H, 3.86; C1, 19.42; neut. equiv., 182.6. Found: C, 58.93; H , 3.68; C1, 19.56; neut. equiv., 184. Product Isolation under Conditions of the Kinetic Experiments. A. p-Methoxycinnamic Acid.--X solution of 104 mg. of cis-p-methoxycinnamic acid in 1000 ml. of 43.5792 sulfuric acid was maintained at 25.0' for a period corresponding to eight half-lives; during the course of reaction, 75 riig. of material precipitated from solution. .ifter being worked up in the usual ~n:iiii~cr, it was recrystallized to coilstant 1 i i . p . froill iiiethaliiil; n l . p . 173.1--173.6'; lleut. cqui\-. calcd. 178.2, fouiid 177.0. -1 mixed rnelti~igImiiit with kiioivii t , n n s - p - i r ~ e t h i i \ ~ ~ i ~ i r i aacid ~ n i c4 1 ~ 1 w 111 d 1 tic~)ressioii. 1)ilution o f the reaction aolutim with water followed by extraction with ether on a continuous extractotyielded 1.5 ing. of material, m.p. 171-173". The infrared spectra of both portions was identical with that of pure trans-p-inethoxycintiamic acid. The combined yield was 90 mg., 877;. B. p-Chlorocinnamic Acid.--A solution of 109 tiig. of 62sp-chlorocitinamic acid in 160 ml. of 53.18% sulfuric acid was inaintaitied a t 90" for a period corresponding to 6 half-lives. During the cuurse of reaction, 96 mg. of material precipitated from solution. -1fter isolation in the usual nlanner, i t JYas recrystallized from ethanol; tn.p. 251.1-251.7'. The infrared spectrum was identical with that of pure truns-pclilor~~ci~ii~axriic acid. Extraction of the reaction solution with ether yielded t l i ? further product. The yield of 96 iiig. was 88Yc of theoretical. cis-trans Isomerization in Sulfuric Acid-dZ.--r), s u l u t i ~ uof 400 mg. of cis-p-chlorocinnamic acid in 216 g. of 517; sulfuric acid-dz was maintained at 90" for a period corresponding to 1 half-life. The solution was then immediately chilled a t - 10' and filtered through a sintered glass funnel. T h e precipitate was dissolved in 25 ml. of 1 M sodium hydroxide r Q J K . C . Pandya and R. B . Pandya, Proi I i t i l i ~ zA r a d . 11" (l!l-Ll), [ l o ) J . K . Kuchi, J , A J ~
