THE ALPHA-CYANO-ALPHA-PHENYLACETIC ACID KETO-ENOL SYSTEM - FLASH PHOTOLYTIC GENERATION OF THE ENOL IN AQUEOUS-SOLUTION AND DETERMINATION OFTHE KETO-ENOL EQUILIBRIUM-CONSTANTS AND ACID DISSOCIATION-CONSTANTS INTERRELATING ALL KETO AND ENOL FORMS IN THAT MEDIUM

The enol of alpha-cyano-alpha-phenylacetic acid was observed as a shor t-lived transient species formed upon flash photolytic generation of p henylcyanoketene by a photo-Wolff reaction of alpha-cyano-alpha-diazoa cetophenone followed by hydration of the ketene. Rates of ketonization of the enol were measured in dilute hydrochloric acid solutions and a cetic acid, biphosphate ion, tris(hydroxymethyl)methylamine, and ammon ia buffers; this gave a rate profile with turning points from which th e first and second acid ionization constants of the enol were determin ed: pK1 = 0.99 and pK2 = 8.70. Rates of enolization of the keto tautom er were also determined by bromine scavenging, and combination of thes e with rates of ketonization of the enol gave the keto-enol equilibriu m constant pK(E) = 7.22, relating the un-ionized carboxylic acid to th e un-ionized enol, and the additional keto-enol equilibrium constant p K(E)' = 6.49, relating the carboxylate ion to the enolate monoanion. T he results also provided the acidity constants pK(a) = 1.68 for the ca rboxylic acid ionizing as an oxygen acid to give the carboxylate ion, pK(a)K = 8.22 for the carboxylic acid ionization as a carbon acid to g ive the enolate monoanion, and pK(a)'K = 15.19 for the carboxylate ion ionizing as a carbon acid to give the enolate dianion; the latter agr ees well with pK(a)K = 15.25 obtained directly by monitoring the exten t of carbon acid ionization of the carboxylate ion in concentrated pot assium hydroxide solutions. The magnitudes of these equilibrium consta nts are discussed, and comparisons with other keto-enol systems are ma de.