The a-Cyano-a-phenylacetic Acid Keto-Enol System. Flash Photolytic Generation of the Enol in Aqueous Solution and Determination of the Keto-Enol Equilibrium Constants and Acid Dissociation Constants Interrelating All Keto and Enol Forms in That Medium

The enol of a-cyano-a-phenylacetic acid was observed as a short-lived transient species formed upon flash photolytic generation of phenylcyanoketene by a photo-Wolff reaction of a-cyano-a-diazoacetophenone followed by hydration of the ketene. Rates of ketonization of the enol were measured in dilute hydrochloric acid solutions and acetic acid, biphosphate ion, tris(hydroxymethyl)methylamine, and ammonia buffers; this gave a rate profile with turning points from which the first and second acid ionization constants of the enol were determined: pK1 = 0.99 and pK2 = 8. 70. Rates of enolization of the keto tautomer were also determined by bromine scavenging, and combination of these with rates of ketonization of the enol gave the keto-enol equilibrium constant pKE = 7 .22, relating the un-ionized carboxylic acid to the un-ionized enol, and the additional keto-enol equilibrium constant pK'E = 6.49, relating the carboxylate ion to the enolate monoanion. The results also provided the acidity constants pK. = 1.68 for the carboxylic acid ionizing as an oxygen acid to give the carboxylate ion, pr. = 8.22 for the carboxylic acid ionization as a carbon acid to give the enolate monoanion, and pl(K = 15.19 for the carboxylate ion ionizing as a carbon acid to give the enolate dianion; the latter agrees well with pK~ = 15.25 obtained directly by monitoring the extent of carbon acid ionization of the carboxylate ion in concentrated potassium hydroxide solutions. The magnitudes of these equilibrium constants are discussed, and comparisons with other keto-enol systems are made.