In dioxane-water mixtures rich in dioxane, the hydrolysis of benzoyl f
luoride is catalysed by hydrogen ions by two mechanisms, one dominant
at low, and the other at high, values of [H3O+]. In purely aqueous sol
utions, and in water-rich dioxane-water mixtures, the only catalysis o
bserved is that at high acid concentrations. The effect of temperature
, and of p-substituents suggests this later catalysis involves an A1 m
echanism. We tentatively assign an AB(AC)3 mechanism to the catalysis
found at low acid concentrations in dioxane-rich media. The hydrolysis
of phenylacetyl fluoride in dioxane-water exhibits a behaviour patter
n similar to that found for benzoyl fluoride.