The Beckmann rearrangement in concentrated sulfuric acid

The mechanism of the Beckmann rearrangement of acetophenone (a) and 1-phenyl-2-propanone (b) oximes in concd sulfuric acid was elucidated by means of NMR spectroscopy and the carbon-14 kinetic isotope effect. For (a), the postulated reactive species acetophenone oxime hydrogensulfate was detected, the absence of hydrolysis of the oxime during the course of the reaction being verified. Positive carbon-14 kinetic isotope effect at the phenyl-1, k12/k14 = 1.026 at 400C and 1.019 +- 0.005 at 600C, confirmed a definite change in bonding of the phenyl-1 carbon in the transition state of the reaction. Concertedness of the rearrangement was thus established. For (b), sulfonation of the benzene ring prior to the rearrangement was observed, kinetic isotope effect study being found to be useless. (author)