Base-catalysed hydrogen-exchange of substituted phenylacetylenes

Abstract

Rates of detritiation have been measured for seventeen tritiated phenylacetylenes, X·C₆H₄·CC·³H, and tritiated mesitylacetylene in a mixture of methanol (1 vol.) and an aqueous buffer solution (pH ∼ 8·05)(4 vol.) at 25°. For meta- and para-substituents the results correlate well with their Hammett sigma-constants (ρ= 0·77).

The values of the activation energy and entropy have been determined for the compounds with X = H, m-Cl, and p-OMe, and show that the ease of the reaction is associated with a highly favourable entropy change. It is suggested that the entropy of activation, ca. 42 e.u., is numerically fairly close to the absolute entropy of solvation of the hydroxide ion in this medium.