ACID-BASE AND METAL-ION CATALYSIS IN THE ENOLIZATION OF 2-ACETYLIMIDAZOLE

The rates of iodination of 2-acetylimidazole (2AI) are independent of iodine concentration and the reaction is general base-catalysed. This shows that the rate-determining step is the enolisation or ionisation of the C-H bond adjacent to the carbonyl group. In dilute hydrochloric acid and in acetate buffers the enolisation of 2AI occurs with N-prot onation rather than O-protonation. Complex formation between 2AI and Z n2+ or Cu2+ ions leads to large rate increases, the ''water'' and the acetate-catalysed reactions of the complex being 5.3 x 10(2) and 2.3 x 10(2) (with Zn2+) and 8.1 x 10(3) and 1.2 x 10(3) (with Cu2+) times l arger than the corresponding reactions of the uncomplexed substrate. T he results for 2AI are compared with those previously obtained for ace tophenone and a number of acetyl heterocycles.