Reactivity of ions and ion pairs in the nucleophilic substitution reaction on methyl p-nitrobenzenesulfonate

Nucleophilic substitution on methyl p-nitrobenzenesulfonate has been studied with a series of chloride salts with different structures and solvations: Bu4NCl, PPNCl or KCl complexed by 18-crown-6 or Kryptofix 2,2,2 as well as with a bromide Salt (PPNBr) for comparison purposes. The rate constants and the activation parameters are in accordance with an S(N)2 mechanism. The treatment of the data, following the Acree equation, shows that the process takes place by way of two reaction paths: the first,due to the chloride ion, has the same rate for all the salts, while the second slower path, involving the ion pair, has a rate related to the dissociation constant of the salts.