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In the aminolysis of phenacyl bromides (YC 6 H 4 COCH 2 Br) with benzylamines (XC 6 H 4 CH 2 NH 2 ) in acetonitrile, the Bronsted β X (β n u c ) values observed are rather low (β X = 0.69-0.73). These values are similar to those (β X ≅ 0.7) for other aminolysis reactions of phenacyl compounds with anilines and pyridines, but are much smaller than those (β X = 1.1-2.5) for the aminolysis of esters with benzylamines which are believed to proceed stepwise with rate-limiting expulsion of the leaving group. The relative constancy of the β X values (β X ≅ 0.7) irrespective of the amine, leaving group and solvent can be accounted for by a bridged type transition state in the rate-limiting expulsion of the leaving group. Thus the aminolysis of phenacyl derivatives are proposed to proceed stepwise through a zwitterionic tetrahedral intermediate (T ′ ), with rate-limiting expulsion of the leaving group from T ′ . In the transition state, the amine is bridged between the carbonyl and α-carbons, which leads to negligible effect of amine on the leaving group expulsion rate.

Kinetics and Mechanism of the Aminolysis of Phenacyl Bromides in Acetonitrile. A Stepwise Mechanism with Bridged Transition State