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

Abstract

ABSTRACTThe 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 ( = 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 ( = 0.49 and = 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.