Reaction rates of electrons in liquid methanol and ethanol: effects of pressure

Abstract

The value of the rate constant k1 for the reaction e−solv → RO−solv + H, [1], at 295 K and 1 bar is ≤1.4 × 105 s−1 in methanol and ≤8 × 104 s−1 in ethanol. The respective volumes of activation averaged between 1 bar and 2 kbar are ΔV1≠ ≤ −21 and ≤ −22 cm3 mol−1. A high concentration of potassium hydroxide (1 M) or water (5 M) decreases the apparent value of k1 somewhat but has little or no effect on the value of ΔV1≠. The effect of pressure on the rate constant of e−solv + S → product, [2], was also measured for a series of solutes that displays a wide range of reactivity. Experimental values of ΔV2≠ depend on the relative contributions of the effects of solvent density on the reactant diffusion rates, the concentrations of the actual reacting species, and the relative energies of the reactant and intermediate states. For reactions whose rates are near the diffusion controlled limit, k2 ≈ 1010 M−1 s−1 in methanol and ethanol, the values of ΔV2≠ are positive and similar to those for the diffusion of simple ions. ΔV≠(e−solv diffusion) = 4 cm3 mol−1 in methanol and 6 cm3 mol−1 in ethanol. Cadmium chloride is apparently not completely dissociated in alcohols, and k(e−solv + CdCl2) < k(e−solv + CdCl+) < k(e−solv + Cd2+). For a series of compounds with lower rate constants there is a correlation between log k2 and ΔV2≠, the latter being negative for very low values of k2. The products of electron capture by benzene, toluene, ethyl acetate, and possibly acetonitrile appear to be stabilized by protonation: [Formula: see text] S−solv + ROH → SH + RO−slov, [4]. The results indicate that the decomposition of e−solv in a pure alcohol occurs by protonation of the electron site, e−solv + ROH → H + RO−slov, [4′], rather than by electron transfer to an alcohol molecule followed by decomposition of the anion.