Abstract:
Kinetic results for quaternization reactions of four n-alkyl bromides (C2, C3, C4, C6) and benzyl bromide, with poly(4-vinyl pyridine) in sulpholane, at a variety of concentrations and temperatures, have been analysed according to a neighbouring-group model. A particular system is characterized by the rate constants ki for reaction of a pyridyl group having i= 0, 1 or 2 already reacted neighbours. Values of k0 under comparable conditions are given. The ratios K=k1/k0 and L=k2/k0 for the series of reagents: C2(0.95, 0.54), C3(0.80, 0.37), C4(0.70, 0.31), C6(0.55, 0.27) and Bz(0.75, 0.34), indicate that retardation as reaction proceeds, in excess of that expected from second-order kinetics, is due predominantly to steric (rather than electrostatic) effects. All of the results for the time dependence of extent, ξ, of reaction are corrected on the supposition that 5 % of the polymer consists of unreactive impurity; previous studies have shown evidence of impurities, but these have not been allowed for in the few attempts at kinetic analysis. Activation energies E0 are fairly constant for C2 to C6: 68.8 ± 3.3, 66.2 ± 4.3, 65.8 ± 4.0 and 66.0 ± 3.5 kJ mol–1; cf. 52.6 ± 4.9 kJ mol–1 for benzyl bromide. The k1 and the k2 show the same temperature dependence, within experimental error, as the k0, i.e.E0=E1=E2.
