The mechanism of electrophilic substitution at a saturated carbon atom. Part IX. One anion- and two anion-catalysed unimolecular acidolysis of pyridiomethylmercuric chloride ions

Abstract

The water-soluble 4-pyridiomethylmercuric chloride ion is stable in dilute perchloric acid at 65°, but decomposes in the presence of added chloride ion to the 4-methylpyridinium ion. At any particular ionic strength and chloride ion concentration the rate of decomposition is independent of acid concentration in the range 0·01–2M-acid. At ionic strengths µ 0·21 and 0·51, the order of reaction with respect to chloride ion increases with chloride ion concentration to a maximum of 2, owing to the presence of concurrent reactions involving one and two chloride ions, the rate coefficients for which have been calculated for both values of the ionic strength. These reactions involve the slow unimolecular heterolytic dissociation of the di- and tri-chloro-4-pyridiomethylmercurate ions, respectively, followed by the rapid reaction of the pyridiomethyl zwitterion with the aqueous acid.

The reactions are subject to a large negative salt-effect and, at high ionic strength (µ 1·01 and 3·0) where activity coefficients may deviate significantly from unity, the apparent order of reaction with respect to chloride ion increases with increasing chloride ion concentration to values greater than 2. The possibility is considered that this implies a third type of reaction in which nucleophilic attack of chloride ion takes place at the mercury atom of the trichloro-4-pyridiomethylmercurate ion.

The proposed mechanism for the one and two chloride-ion catalysed reactions is supported by the fact that, whereas the 1-methyl-4- and the 2-pyridiomethylmercuric chloride ions react at similar rates to the 4-pyridiomethylmercuric chloride ion, the reaction of the 3-pyridiomethylmercuric chloride ion is too slow to measure under the same conditions.