Reactions in strongly basic media. Part III. Kinetic acidity-dependences in methanolic solutions of sodium methoxide

Abstract

Kinetic measurements for nine reactions in concentrated solutions of sodium methoxide provide no example of a direct correlation between reaction rate and either the H_ function or stoicheiometric base concentration. No relation between kinetic acidity-dependence and mechanism is found among examples of E2 and E1cB β-eliminations, S_N2 and S_N1cB substitutions, and mechanisms of base-catalysed hydrogen exchange; reactions with the same mechanism show different acidity-dependences and reactions with different mechanisms show the same acidity-dependence. It is shown that the acidity-dependences can be expressed as free-energy relationships by plotting log k₂ against H_– pK_MeOH– log (C_MeO–/C_MeOH). The plots so obtained show satisfactory linearity and their slopes may be related semiquantitatively or qualitatively to the transition-state structures for the reactions. The possibility that diffusion-controlled reactions display a unique acidity-dependence is noted.