Mechanism of electrophilic substitution at a saturated carbon atom. Part XVI. Rates of reaction of metallic electrophiles with the 2-, 3-, and 4-pyridiniomethylchromium(III) ions

Abstract

The kinetics of reactions of the penta-aquo-2-, -3-, and -4-pyridiniomethylchromium(III) ions with mercury(II) perchlorate and thallium(III) perchlorate have been studied as a function of the acidity and chloride ion concentration at constant ionic strength. As with the higher chloro-mercury(II) and -thallium(III) species studied earlier, the reaction is believed to involve the bimolecular displacement of the aquated chromium(III) cation by attack of the metallic electrophile at the saturated carbon atom. Individual rate coefficients for the reactions of the species Hg²⁺, Tl³⁺, TlOH²⁺, TlCl²⁺, and HgCl⁺ have been estimated. These results, together with those for the higher chloro-mercury(II) and -thallium(III) species, show that the rate of substitution at the 2-position is less than that at the 3-position by approximately the same factor, irrespective of the charge or the character of the electrophile. Any significant bond breaking prior to bond formation in the transition state can be ruled out, and the direct influence of the charge on the pyridinium ion on the incoming charged electrophile is the same for each position of substitution. The influence of ion-pairing on the rates of reaction is discussed.