FLASH PHOTOLYTIC GENERATION AND STUDY OF KETENES - ACID-CATALYZED HYDRATION IN AQUEOUS-SOLUTION

Flash photolysis was used to generate CH2 = C = O by fragmentation of cyclobutanone and (CH3)3CCH = C = O, (CH2)5C = C = O and PhC(CH3) = C = O by the photo-Wolff reaction of the corresponding diazoketones; the rates of hydration of these ketenes were measured in wholly aqueous solutions of sodium perchlorate, perchloric acid and phosphoric and acetic acid buffers. Uncatalysed and acid-catalysed reactions were observed for the aliphatic ketenes, but only an uncatalysed reaction was observed for the aromatic derivative. A mechanism for the acid-catalysed process, involving rate-determining proton transfer from the catalyst to the terminal carbon atom of the ketene functional group, is indicated by the occurrence of general acid catalysis and hydronium ion isotope effects in the range k(H+)/k(D+) = 2.7-3.3. The absence of acid catalysis in the hydration of PhC(CH3) = C = O is explained in terms of a phenyl-substituent effect that retards the acid-catalysed process but accelerates the uncatalysed reaction.