CONCURRENT O-18 EXCHANGE ACCOMPANYING THE ACID-CATALYZED HYDROLYSIS OF ANILIDES - IMPLICATIONS FOR THE LIFETIMES OF REVERSIBLY FORMED INTERMEDIATES

Hydrolysis of acetanilide and N, 2, 4-trimethylacetanilide in acid is accompanied by oxygen exchange. The log khyd and log kex vs -log [H+] profiles are parallel and show no significant divergence. The deuterium solvent kinetic isotope effects on exchange and hydrolysis are unity. A possible alternative mechanism for exchange in acetanilides involving enolization and elimination to give a transient keteniminium ion has been ruled out by the absence of concurrent deuterium incorporation into the acetyl group and shown to be thermodynamically prohibited. The deuterium solvent kinetic isotope effects (KIE) allow two mechanisms for hydration: the most probable is an attack of water with concerted loss of a proton to a second water; less likely is an attack of water with concerted transfer of a proton by way of a second water to the nitrogen. The solvent KIE allows only one mechanism for amine expulsion, concerted loss of amine from TN and loss of a proton to solvent water. A decision on the mechanism of hydration can be made by the use of estimated energies of the intermediates and a two-dimensional extension of Marcus theory. These calculations rule out the mechanism leading directly to TN and favor one leading to T°. © 1990, American Chemical Society. All rights reserved.