The basicities of a number of acyl chlorides in concentrated sulphuric acid have been determined using the conductometric method. It has been found that aliphatic acid chlorides are weak bases with Kb25 in the range (2·4–5·4)× 10–2 mole l.–1, whereas substituted benzoyl chlorides are somewhat weaker bases with Kb25 in the range (1·6–3·3)× 10–2 mole l.–1. Oxalyl chloride and dichloroacetyl chloride behave as non-electrolytes, the latter also undergoing a slow reaction with sulphuric acid. Phenylacetyl chloride undergoes rapid sulphonation. The Hammett plot for substituted benzoyl chlorides (against σ+ constants) yielded a line of very low slope, ρ= 0·3. In general, the range of basicities of acyl chlorides is very small compared with the range of basicities of the corresponding carboxylic acids. The reduction in basicity upon substitution of the hydroxyl in the carboxyl group by chlorine is discussed.
