Non-linear Taft correlation

What is it about?

The rates of reactions of N-substituted benzyl amines with benzyl bromide were measured using conductivity technique in methanol medium. The reaction followed a total second order path. The end product of the reaction is identified as di-benzyl alkyl amine (C6H5CH2N(R)CH2C6H5). The rates increased with decrease in electron donating capacity or with increase in the Taft * value of electron donating alkyl substituents (R) like t-butyl (* = -0.3), i-propyl (* = -0.19), n-butyl (* = -0.13), ethyl (* = -0.1), on nitrogen of the amine till Taft * value becomes zero for methyl group ( = 0.00) and then the rates decreased with increase in electron withdrawing capacity or with increase in Taft * value of electron withdrawing substituents (R) like H and C6H5 ( = 0.49 and = 0.6). The locus of the Taft polar free energy relationship (TPFER) 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 discussed.

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