Quantitative Assessment of Solvent-Sorting Effects. 1. Menschutkin Reaction in Mixed Solvents

Article abstract of DOI:10.1021/j100338a046

The second-order rate constant k_app for the reaction between (C2H5)3N and CH3I has beeen determined at 30.0 deg C in cyclohexane and in mixtures of cyclohexane with variable amounts (up to ca. 0.3 M) of the following cosolvents (S): N,N-dimethylacetamide, cyclohexanecarbonitrile, cyclohexyl methyl ketone, butanone, tetrahydrofuran, ethyl acetate, cyclohexyl chloride, methyl cyclohexanecarboxylate, cyclohexyl methyl ether, dibutyl ether, nitrobenzene, N,N-dimethylbenzamide, benzonitrile, acetophenone, benzophenone, methyl benzoate, diphenylmethane, anisole, chlorobenzene, fluorobenzene, 1,2-diphenylethane, triphenylmethane, benzene, 1,4-dichlorobenzene, toluene, and p-xylene.Within the limits of experimental error, the following have been found: (1) The activity coefficients of the reagents are practically unaffected by the added cosolvent. (2) For low cosolvent concentrations, k_app is related to the concentration of S by the equation k_app = k₀ + k_c, where k₀ is the reaction rate in pure cyclohexane. (3) In all cases, the catalytic effect of S is quite large: it amounts to ca. 50percent of the lowering of the activation free energy of the reaction, observed on going from pure cyclohexane to pure S. (4) This major effect is largely underestimated by the Onsager-Kirkwood model. (5) The catalytic efficiency of aromatic cosolvents of low or zero dipolarity is over 1 order of magnitude larger than predicted by this model.This is traced to electrostatic interactions involving higher multipoles and to London forces. (6) Although a fraction of the catalytic effect can be attributed to a "general dielectric" contribution, these and other results strongly suggest that this reaction can proceed through a true termolecular channel.