Ammonolysis of aryl toluenesulphonate esters: evidence for the concerted displacement of the aryl oxide group

Abstract

Rate constants have been measured for the ammonolysis of aryl toluenesulphonate esters in aqueous solution. The Brønsted-type β value of –1.08 coupled with the β_eq for the overall reaction indicates a large increase in negative effective charge on the leaving oxygen in the transition state of the rate-limiting step. Data for β_nuc for attack of amines on sulphonates taken from the literature indicate a large increase in positive effective charge on the attacking nitrogen; this coupled with the present data excludes a stepwise mechanism for the displacement of the aryl oxide group. The large changes in effective charge on attacking and leaving atoms are consistent with a concerted S_N2(S) displacement mechanism in this case.