The reaction complex in proton transfer

Abstract

Catalytic coefficients for hydrolysis of diphenyldiazomethane by a long range of neutral oxygen acids (mostly phenols and carboxylic acids) in 80% DMSO + 20% water form a curved Brönsted plot which appears to reach a limiting rate of not more than 10 M^–1 s^–1. This plot can be fitted to the Marcus theory of proton transfer, yielding a W^r value of about 70 kJ mol^–1 and a λ/4 value of about 6 kJ mol^–1.

For 4 of the neutral acids, Hammett ρ values were obtained by determining k_HA for three ring-substituted diphenyldiazomethanes in addition to the parent compound. From these the Hammett ρ associated with W^r, ρw, and that for λ, ρλ, can be determined. The latter is close to zero, while ρw is –1.5.

The primary hydrogen isotope effect for the neutral acids is around 4, and appears to be invariant under change of pK_HA up to a change of almost 7 units. Phenols other than nitrophenols form a homogeneous series with carboxylic acids as judged by k_HA, α, ρ_HA, and k_HA/k_DA values, but H⁺ is not a member of the series.

These observations can be rationalized by a model in which a reaction complex with a strong hydrogen bond is formed with a very substantial net input of work. The proton transfer itself then takes place with very little additional activation energy, beyond the work required to reach the level of the product complex.