Reaction of 1,1,3,3-tetramethylthiourea with methyl iodide: kinetic and thermodynamic aspects

Abstract

In the reaction of 1,1,3,3-tetramethylthiourea with methyl iodide limiting forms of the rate law arise in solvents in which the concentration of free ions and ion-pairs is small in comparison with higher aggregates. The solvent effects on the rate of the forward reaction were linearly correlated with those on the Menschutkin reaction. The effects were quite close to those on the pyridine–methyl iodide reaction. The value (Δ₂V^‡/Δ₂°), which is expected to be an index of the position of the transition state along the reaction co-ordinate, is ca. 0.27 in acetonitrile and propylene carbonate. The position of the transition state, n_T, calculated from the enthalpy term, is ca. 0.30 for the present reaction and for the NN-dimethylaniline–methyl iodide reaction. These indices of the transition state position are qualitatively in agreement with each other. The results of CNDO/2 calculations, performed for the assumed transition state structures, are in accord with the results of the experimental observations on the solvent effects on reaction rates.