Secondary Kinetic Isotope Effects in Bimolecular Nucleophilic Substitutions. IV. The Temperature Dependence of the Deuterium Effect for the Reaction of N,N-Dimethyl-d₆-aniline and Methyl p-toluenesulfonate. Correlation of Isotope Effects on Activation Parameters

Abstract

The temperature dependence of the isotope effect for the reaction of dimethylaniline and dimethyld6-aniline with methyl p-toluenesulfonate in nitrobenzene solvent has been measured, yielding the The significance of the correlation is discussed and a possible rationalization, in terms of mechanism and particularly the role of the solvent is given.Canadian Journal of Chemistry, 49. 439 (1971) Introduction Kaplan and Thornton (1) have studied the effect of deuterium substitution for the methyl hydrogens of N,N-dimethylaniline on the rate of the quaternization reaction of the amine with methyl p-toluenesulfonate in nitrobenzene solvent at 51". From frequency change considerations and force constant model calculations, they concluded that steric interactions yielded the most satisfactory explanation of their results. This implies that the isotope effect is a rate change brought about primarily by a difference between the enthalpies of activation forthe normal and deuterated species, rather than a difference between the entropies of activation. The only temperature dependence study of a secondary deuterium isotope effect on this type of reaction was carried out by Leffek and MacLean (2) who found that the rate change resulting from the deuteration of methyl iodide in its reaction with 2-picoline in benzene was due entirely to an entropy effect. It seemed of interest, therefore, to carry out a temperature dependence study on the isotope effect for N,N-dimethyl-d6-aniline and methyl p-toluenesulfonate.

Secondary Kinetic Isotope Effects in Bimolecular Nucleophilic Substitutions. VI. Effect of α and β Deuteration of Alkyl Halides in their Menschutkin Reactions with Pyridine in Nitrobenzene

“…This has been interpreted in zero-point energy terms, assuming that the A H + isotope effect dominates that of A S f , which is true for several SN2 reactions (12)(13)(14). The argument is that a tight SN2 transition state, such as that for a methyl compound leads to an increase in the C-H bending frequencies during activation, which in turn leads to a faster rate for the deuterium compound than for the protium compound (15).…”

“…except for those involving methyl bromide. A calibration curve was constructed for each alkyl halide reaction and they were all very similar in form to that for trimethylphenyiammonium tosylate in nitrobenzene (14). The conductivities were converted to concentrations and the second-order rate constants were calculated from the integrated second-order equation.…”