LINEAR FREE ENERGY RELATIONSHIPS CONCERNING REACTION RATES IN MODERATELY CONCENTRATED MINERAL ACIDS

Abstract

Linear relationships exist between log kψ + H0 (for reactions of weakly basic substrates) or log kψ (for reactions of strongly basic substrates) and (H0 + log [H+]). These are linear free energy relationships. For weakly basic substrates, the correlations obtained are better than in plots of (log kψ, + H0) versus log [Formula: see text] or of log kψ versus −H0. The slopes in plots of log kψ or (log kψ + H0), as appropriate, against (H0 + log [H+]) are taken as parameter, [Formula: see text], which characterizes the response of the reaction rate to changing mineral acid concentration. Values of [Formula: see text] for reactions of strongly basic substrates reflect only relationships between protonated substrate and transition state, and may be related to reaction mechanism. [Formula: see text] values for reactions of weakly basic substrates reflect both equilibrium protonation of the substrate and transformation of protonated substrate to transition state, and are therefore less directly related to mechanism. However, the [Formula: see text] values for the two steps are additive and that for the latter step can be obtained by subtraction if the overall [Formula: see text] value and that for equilibrium protonation are known.