Influence of pressure on the rates of deuteration of formate and acetate ions in liquid D₂O

Abstract

We have examined the effects of hydrostatic pressures to 8 kbar on the rates of deuteration of formate and acetate ions in pure D₂O at temperatures between 160 and 200°C. The formate reaction is only slightly accelerated by pressure (the activation volume ΔV_o^?, at zero pressure, is -2.5 cm³ mol^-1). This fact disproves mechanisms that have been proposed previously for the reaction and we propose an alternative, concerted, molecular mechanism that fits the facts. By contrast, the acetate reaction is significantly accelerated by pressure (ΔV_o^? = -10.5 cm³ mol^-1), which is consistent with its occurring by mixed molecular and carbanionic mechanisms.