Kinetics of quaternization of 4-methyl and 4-ethyl pyridine with n-propyl and n-butyl bromide in sulpholane

Abstract

Kinetic measurements have been carried out in sulpholane to provide a basis for comparison with quaternization reaction of poly(4-vinyl pyridine), and to clarify the roles of steric and inductive effects. Activation energies, and rate constants at 319.2 K, taken from Arrhenius plots of about ten points for each reaction are: (A) 4-methyl pyridine+n-propyl bromide E/kJ mol^–1= 66.0±1.1, 10⁵k/(dm³ mol^–1 s^–1)= 8.21; (B) 4-ethyl pyridine+n-propyl bromide 63.3±2.2, 8.70; (D) 4-methyl pyridine+n-butyl bromide 65.0±0.7, 7.10; (E) 4-ethyl pyridine+n-butyl bromide 63.0±1.5, 7.71. Concentrations of base and reagent were nominally 0.2 and 0.6 mol dm^–3. The ethyl pyridine has a k-value slightly greater than that for the methyl pyridine, and k for propyl bromide is 10–15 % greater than that for butyl bromide. The ratios k(A)/k(B)= 0.94 and k(D)/k(E)= 0.92 are remarkably constant. Reactions (C) and (F) for n-propyl bromide and n-butyl bromide and poly(4-vinyl pyridine), reported previously, give the ratios k(C)/k(A)= 0.80, k(C)/k(B)= 0.75, k(F)/k(D)= 0.81, k(F)/k(E)= 0.74. A linear dependence of ln k on (ε–1)/(2ε+ 1) was found for system (B) where ε is the dielectric constant, which depends on the concentrations of reactants.