Substituent effects on the spontaneous cleavage of benzyl-<i>gem</i>-dichlorides in aqueous solution

2145-2149The α-chlorobenzyl cations are produced in aqueous solution by chemical initiation (solvolysis) of benzyl-gem-dichlorides. The solvolysis reactions of benzyl-gem-dichlorides in water proceed by a stepwise mechanism through α-chlorohalobenzyl carbocation intermediates, which are captured by water to give the corresponding benzaldehydes as the sole detectable products. Rate constant ratio kc1/ks (M-1) for partitioning of the carbocation between reaction with chloride ion and reaction with water are determined by analysis of chloride common ion inhibition of the solvolysis reaction. The rate constants ks (S-1) for the reaction of the cation with solvent water are determined from the experimental values of kc1/ks and ksolv, for the solvolysis of the benzyl-gem-dichlorides, using kcl = 5×109 M-1 S-1 for diffusion- limited reaction of chloride ion with benzyl carbocations. Calculation and discussion of selectivities, kc1/ks, of the α-chlorobenzyl cations, activation parameters for solvolysis step and for addition of water to the cation are some of the salient features not reported earlier. Effect of substituents is also discussed in terms of Hammett equation for both the steps