Nucleophilic displacement reactions of benzyl chlorides.

Abstract

The rates of reactions of methoxide and a series of substituted thiophesoecides with 2-methyl, 2,6-dimethyl, 4-methyl, 3-nitro and unsubstituted benzyl chloride have been measured. The pKa values of nine substituted thiophenols were determined in methanol. The magnitude of the London interaction energy in the transition state for 2,6-dimethylbenzyl chloride was found to be approximately twice that of the observed value for 2-methylbenzyl chloride. This suggests that the effect of London interaction may be additive. The chemical shifts of the CH2-groups of benzyl chloride, 2-methylbenzyl chloride and 2,6-dimethylbenzyl chloride measured in various mixed solvents indicate that ground-state solvation of the reaction center is similar for the first two chlorides. The transition-state activity coefficients for reactions of 2-methyl, 2,6-dimethyl and unsubstituted benzyl chloride with methoxide were obtained from treatment of the rate constants measured in MeOH and DMF. The results indicate that differences in transition-state solvation are small. Bunnett's adjusted rate ratio for the evaluation of London energy between the ortho substituant and the approaching nucleophile at the transition-state remains constant in various solvents and is discussed in terms of the ground state and the transition-state solvation of reactions. The modified reaction constants, PN, are estimated by plotting the logarithms of the rate constants for substituted benzyl chlorides against the corresponding values for unsubstituted chloride. The similarity of the reaction constants between o-methylbenzyl chloride and p-methylbenzyl chloride can be attributed to the polar influence which predominates over the steric effect for the ortho substituent. However, systematic deviations for the nucleophiles of m-nitrothiophenoxide and 3,5-dinitrothiophenoxide were observed in the plots mentioned above. The reactivities for these two nucleophiles increase as the electron-donating ability of the substituent of benzyl chloride increases. The kinetic parameters were determined. Charge-transfer complex stabilization at the transition-state is proposed for the accelerating effect of m-nitro and 3,5-dinitrothiophenoxide with benzyl chlorides. A linear relation between substituent effects and the degradation of the molecular ion (RSSR)·+ for five substituted aryl disulfides has been found.