Transition state sulphites

Chloride ion is a less effective leaving group than chloro-sulphite or acetylsulphate (p. 261), so that the transition state for dehydrochlorination must involve relatively greater stretching of the C H and less stretching of the C-X bond. The correspondingly more anionic character of the transition state for dehydrochlorination means that loss of the 4/9-proton would tend to be assisted by its proximity to an electron-attracting substituent at C(3). 

Chelation by the 0-bonded sulphite ion is postulated to involve the formation of a seven-co-ordinate intermediate (or transition state) which readily loses the cis-ligand (X ). Catalysis by nitrate ion is also found to obey equation (34) and a mechanism for the loss of X analogous to that described above is favoured. 

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