Computational study of electrophilic addition to acrylate anion:cyclic halonium is the transition structure for degenerate rearrangement of alpha-lactones.

The cyclic chloronium or bromonium carboxylate obtained by addition of Cl+ or Br+ to acrylate anion is shown by PCM/B3LYP/6-31+G* calculations to be not an intermediate but a transition structure for interconversion of equivalent halomethyl oxiranones.