摘要:
610 CHEMICAL COMMUNICATIONS The Harnrnett 0-Value for the Forrnyl Group By A. A. HUMFFRAY, J. J. RYAN, J. P. WARREN, and Y. H. YUNG (Department of Physical Chemistry, University of Melbourne, Parkville, N2, Australia) ALTHOUGH it has been established1 that a con- group5 (a, 0.398, a, 0-522) the a,-value for the tinuous range of a,-values is required for correla- formyl group appears to be too small. We have ting the reactivity of an aromatic substituent in measured the PKa values, in water at 25” c, the para-position, the continued success2 of the for iso- and tere-phthalaldehydic acids, by the Hammett equation,3 using a single a,-value, method of Albert and Phillips,6 with the following justifies its application to kinetic and equilibrium data. If a single a,-value is to be used, it should preferably be based on the thermodynamic acidity constant for a substituted benzoic acid in water at 25” c4 Taking the pKa-value of benzoic acid as 4-20,? When the meta- and para-substituent constants the substituent constants for the formyl group are for the formyl group,6 CHO (a, 0.355, ap 0.216) a, 0.36 f 0.03, a, 0-43 & 0.03.The latter are compared with those for the acetyl group6 value was confirmed by conductance measure- (a, 0.306, a, 0~516)~ or for the ethoxycarbonyl ments on terephthalaldehydic acid solutions, results : Isophthalaldehydic acid PKa 3-84 f 0.03 Terephthalaldehydic acid PKa 3.77 f 0.03 1 H. van Bekkum, P. E. T‘erkade, and B. M. Wepster, Rec. Trav. chirn., 1959, 78, 815. 2H. H. JaffC and H. L. Jones, Adv. Heterocyclic Chenz., 1964, 3, 209.3 L. P. Hammett, “Physical Organic Chemistry”, McGraw-Hill Book Co., New York, 1940, p. 190. 6 H . H. Jaff6, Chern. Rev., 1953, 53, 191. G A. Albert and J. N. Phillips, J . Chern. Soc., 1956, 1294. 7 G. Kortum, W. Vogel, and K. Andrussow, “Dissociation Constants of Organic Acids in Aqueous Solution”, Butterworths, London, 1961, pp. 352-353. D. H. McDaniel and H. C. Brown, J . Org. Claem., 1958, 23, 420.NUMBER 23, 1965 611 which gave, using the treatment of IvesI8 pKa 3.75 f 0.03, 0, 0.45 f 0.03. The a,-value agrees well with the established value2 which, in Hammett's original tabulation3 was derived from the acid hydrolysis of aryl- sulphuric acid^.^ The low published U-value2 was derived from results of Tommila,lo on alkaline hydrolysis of substituted ethyl benzoates in 560/;, acetone; these results also give a a,-value for the forinyl group of 0.21.When an attempt was made to repeat Tommila's measurements, the solutions rapidly became yellow in colour, un- doubtedly due to the occurrence of a Claisen- Schmidt condensation between the solvent acetone and the aldehyde group, CHO Such a reaction should be favoured by the electron- attracting C02Et group, and hindered by a C0,- group, and hence should occur more readily before, than after, hydrolysis of the ester. Thus Tommila's results probably relate to the -CH=CH-COMe group, rather than to CHO. I t is of interest to note that the group -CH=CH-NO, has a a,-value of 0.26." Further confirmation for the view that acetone reacts with the formyl group in alkaline solution was provided by kinetic measurements on the alkaline hydrolysis of the ethyl esters of iso- and tere-phthalaldehydic acids in 850,; alcohol, under the conditions used by Evans, Gordon, and Watson.l2 KO change occurred in colour of the solutions, and the substituent values obtained for the formyl group were a, 0-38, a, 0.43. (Received, November 8th, 1965; Corn. 701.) 8D. J. G. Ives, J . Chem. Soc., 1933, 731. Q G. N. Burkhardt, C. Horrex, and D. I. Jenkins, J . Chem. Soc., 1936, 1654. 10 E. Tommila, Ann. Acad. Sci. Fennicae, 1941, A , 57, No. 13, 3. R. Stewart and L. G. Walker, Canad. J . Chem., 1957, 35, 1561. l2 D. P. Evans, J. J. Gordon, and A. B. Watson, J . Chem. SOC., 1937, 1430.
ISSN:0009-241X
DOI:10.1039/C19650000610
出版商:RSC
年代:1965
数据来源: RSC