Kinetics and mechanism of the Beckmann rearrangement of acetophenone oximes in sulphuric acid

Abstract

The kinetics of the Beckmann rearrangement of acetophenone oximes have been measured over the acidity range 70–98% sulphuric acid. A study of the rearrangement of ortho-substituted acetophenone oximes has shown that an N-arylnitrilium ion is an intermediate in the rearrangement. Comparison of the rates of the rearrangement of 2,4,6-trimethylacetophenone oxime in sulphuric and perchloric acids indicates that the reactive species in concentrated sulphuric acid (>70%) is the oxime O-sulphonic acid. Isotopic labelling experiments have shown that the migration of the oxygen function is intermolecular. The transition state of the rearrangement is postulated to be a phenonium ion in which the leaving oxygen function is still partially bonded to the nitrogen atom.