Vol. 74
'1.392 [COSTRIBUTIOS FROM
THE
DEPARTMEST OF CIIEMISTKY O F BARNARD COLLEGE]
Ionization Constants and Rates of Ester Hydrolysis in the Benzylidenepyruvic Acid Series BY EMMA DIETZSTECHER AND HELENFRANCES RYDZR R E c t m m MARCH5, 1952
Ioiiization constants have been determined at 23 ' for benzylidenepyruvic acid and its +bromo-, +methyl- and p-methoxy derivatives. These are very strong acids ( K ' in \\:iter = 1.1 X 1 0 F ) and show no significant variation of K ' with substituent groups. Rates of hydrolysis have been determined at 25' for the corresponding methyl esters in 5Or6 dioxane using hydrochloric acid as the catalyst. The rates are first order with respect t o ester and hydrochloric acid concentrations ( k ' = 5.7-6 1 X lW3hr.-'). Absorption spectra are reported for the methyl esters.
In the benzoic acid series, ~ n u c his k n o w n about the variation of ionization constant with the intr(iduction of substituent grLjups on the benzene ring. I The rates of acid-catalyzed hydrolysis of the corresponding methyl benioates also show a sniclll but definite variation with substitution.? I n the cinnamic acid series, the variations of PK with substituent groups are smaller but in the same direction." Cinnamic esters hydrolyze much more rapidly than benzoic esters and substituent effects are In general these differences have been explained in terms of polarization of the niolecule through inductive and resonance efiects.'"s2 I t seemed of interest to obtain information concerning the effect of substituents on ben7ylidenepyruvic acid (I, X = 13) antl its methyl ester. This Irap-SC,II,CH-=Cf-ICOCOOII s = H . Ilr CII?, OCII I
vides a system in which the benzene ring is not directly conjugated with the carboxyl group as it is in benzoic arid cinnamic acids, b u t with the c.trbony1 group. Since i t is this carbonyl group which makes pyruvic acid much stronger thari acetic acid, one might expect detectable substituent effects. The present study reports the measurement of the ionization constants of five benzylidenepyruvic acids, three of which are substituted in the para position arid one in the meta position, In a second series of experiments, the methyl esters of the para substituted acids were hydrolyzed, using hydrochloric acid as the catalyst, and the rates of hydrolysis were measured.
Experimental Preparation of Compounds.--Table I lists the benzylitienepyruvic acids arid esters prepared for this study together with their melting points. 1)etermination of the equivalent weights of both acids and esters showed them to lie 99.6-99.9yo pure. Benzaldehyde, anisaldehyde a r i d pyruvic acid were Eastman Kodak Co. products. p-Tolualdehyde, b.p. 1C)3-105° a t 21 m m . , was prepared according to a known method.' I n the preparation of P-bro~nobenzaldehyde~ only 60% of the suggested amount of chromium trioxide was used, and yields of 65-8OY0 of the aldehyde diacetate were obtained. For the preparation of the benzylidenepyruvic acids i t w:is found necessary t o observe certaili precuiitioils not de,cribed i n ttic. piihlislit.(l mt.thorls.6 A ~Vpiciil procetlrirc .
. ..
~ . .
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r)ippy, Chznz. R V V C 2. 5 , 1.i1 ( I ! I : ? ~ ) : f h ) J. P. Ilnrnmett, "Phyiical Organic C h e m i s t r y , " h l c c r a w -Trill B . , I n r , New York, N. Y ,I!l-10, I, 84 ff , I, 18-1R (2) E . \ V Tirnm and C. S.llinshelwooti, J . Cham Soc.. 8li2 IlY38). ( 3 ! K . Kindler, A n n . , 453, b(I (I!J27). (-1) G. 11. Coleman and D. Craig, "Org.inic Synthrsr;." C i h i ! i l l 1,
1-aluesobtained for the series of benzylidenepyruvic acids and for two reference acids are summarized in Table Y. I t will be noted that the benzylidenepyruvic acids are about 200 times as strong as benzoic or cinnamic acid. This high degree of ionization is attributable to the pyruvic acid struc3.2 X ture (S' for pyruvic acid = 5.6 X 10--3(12), ( l a ) j . Allso, within the unavoidable, large experimental error of &lo% of the K' value for strong acids, the benzylidenepyruvic acids are all of the same strength (K' in water = 1.0 to I .1 0.1 x lo-?; in .iOc?, methanol-0.2 -51 LiC1, K' = 2.7 to 3 . 3 f 0.3 X There may well he small group effects less than 10% in magnitude, which we have not detected, as are found with phenylacetic acid.? In p-methoxybenzylidenepy1-uvic acid one would expect a resonance form of the
*
0t
type CH30=-C6H4 CHCH-=CCOOH to place partial negative charge in the neighborhood of thc carboxyl group, thus retarding ionization. Clair and Wiesneris report the following ionization constants for other substituted pyruvic acids : phenylpyruvic acid 2.09 X 1(1 --:i, 3,4-dimethoxyphenylpyruvic acid 7.9-4 X 10--,$, Here the two electronrepellant methoxyl groups reduce the ionization considerably even though they are conjugated with the carbonyl rather than the carboxyl group. We expected larger substituent effects in our acids but before proposing a theory we plan to investigate the problem further by studying other derivatives, as well as the series p-XCGH4COCOOH. Table VI summarizes the hydrolysis rate constants for all of the esters studied. The rates of hydrolysis of the four methyl benzylidenepyruvates in 5070 dioxane using hydrochloric acid as the cat(11) L. F. Fieser, ref. 10, p. 364. To avoid the formation of purple gelatinous by-products, drying with phosphorus pentoxide was reduced minutes X. I l o n t z a c h i m d A >lioluti. Z p i i j A i k . L./rrm., [ A ] 10, X (189 ) A I . €1. RoesPken, I.. \V. I I a n e n a n t 1 S . 11. Bertram, RI-C I? 36, R 1 - : ' 1 ' 1 1 1 ~ 1 , I: ('.
