THE HYDRATION OF MESITYLKETENE IN AQUEOUS-SOLUTION - DETECTION OF ACID CATALYSIS FOR AN AROMATIC KETENE

Mesitylketene was generated flash photolytically in aqueous solution b y the photo-Wolff reaction of 2,4,6-trimethyldiazoacetophenone and als o by rearrangement of mesitylynol obtained through photodecarbonylatio n of mesitylhydroxycyclopropenone, and rates of hydration of this kete ne were measured in dilute perchloric acid, sodium perchlorate, and so dium hydroxide solutions as well as in concentrated sodium perchlorate and perchloric acid solutions. In dilute solution only an uncatalyzed reaction and a sodium-hydroxide-catalyzed process were observed, both of which could be attributed to nucleophilic attack, by water and by hydroxide ion, respectively, at the ketene carbonyl carbon atom. In co ncentrated sodium perchlorate solutions, a mild decrease in reaction r ate with increasing salt concentration was observed, as expected on th e basis of decreasing water activity and a consequent slowing of the u ncatalyzed reaction. A similar mild decrease was found in perchloric a cid solutions up to [HClO4] congruent to 1 M, but this then gave way t o a rate increase that became dominant above [HClO4] congruent to 2 M. This appearance of acid catalysis indicates a change in reaction mech anism from nucleophilic attack of water to an electrophilic process in volving rate-determining protonation on the P-carbon atom of the keten e group. Analysis of the acid-catalyzed reaction rate by the Cox-Yates method gives the catalytic coefficient k(H)+ = 4.7 M(-1) s(-1). This, when compared with k(H)+ = 1.01 X 10(4) M(-1) s(-1) for ketene itself , shows that the mesityl group retards acid-catalyzed hydration by a f actor of 2200, and consequently the acid-catalyzed reaction of this, a nd other aromatic ketenes as well, becomes apparent only under strongl y acidic conditions.