Aromatic Acylation in Homogeneous solution and in the Gas Phase: the Mechanistic Relevance of the Mesitylene/durene Reactivity Ratio

By means of a mixed acetic-trifluoroacetic anhydride a scale of relative substrate reactivity for aromatic acetylation is obtained, which spans over eight powers of ten from benzene to 2-methylthiophene. Correlation of these reactivity data with relevant substrate parameters is attempted, to gain an understanding of the possible nature of the electrophilic substitution mechanism at work, i.e. either an ionic (two-electron) or a single-electron route. The study includes gas-phase reactivity data on the reaction of free acetylium ion, which has been generated by a radiolytic method. From this comparative approach a 'conventional' ionic mechanism for the aromatic acetylation reaction is substantiated.