The electrophilic substitution reaction between benzene and ethanoyl chloride What is acylation? An acyl group is an alkyl group attached to a carbon-oxygen double bond. If R represents any alkyl group, then an acyl group has the formula RCO-.he facts The most reactive substance containing an acyl group is an acyl chloride (also known as an acid chloride). These have the general formula RCOCl. Benzene is treated with a mixture of ethanoyl chloride, CH3COCl, and aluminium chloride as the catalyst. A ketone called phenylethanone is formed.