Nonlinear Taft Polar Free Energy Relationship: Reactions of N-Substituted Benzyl Amines with Benzyl Bromide in Methanol



The rates of reactions of N-substituted benzyl amines with benzyl bromide were measured using a conductivity technique in methanol medium. The reaction followed a total second-order path. The end product of the reaction is identified as dibenzyl alkyl amine (C6H5CH2N(R)CH2C6H5). The rates increased with a decrease in the electron-donating capacity or with an increase in the Taft σ* value of electron-donating alkyl substituents (R) such as t-butyl (σ* = −0.3), i-propyl (σ* = −0.19), n-butyl (σ* = −0.13), and ethyl (σ* = −0.1) on nitrogen of the amine until the Taft σ* value becomes zero for the methyl group (math formula = 0.00), and then the rates decreased with an increase in the electron-withdrawing capacity or with an increase in the Taft σ* value of electron-withdrawing substituents (R) such H and C6H5 (math formula = 0.49 and math formula= 0.6). The locus of the Taft polar free energy relationship has a maximum near the point for N-methyl benzyl amine, showing that there is a sharp change in the rate-determining step. A mechanism involving formation of an SN2-type transition state between the amine nucleophiles and the benzyl bromide and its subsequent decomposition is proposed. Activation parameters were calculated and are discussed.