Kinetics of reactions of Friedel–Crafts acylating agents. Part II. Effect of substituents in the acid chloride on the acylation of aromatic substrates

Abstract

The second-order rate constants for the aluminium chloride-catalysed acylation (p-XC₆H₄·COCl) of toluene in ethylene chloride are given at 0·0°, 20·0° for X = Me, H, and Cl and at 30·0° for X = Me and H. Similar second-order rate constants are given for PhX and RCOCl at 20·0° where X is H when R is Me, Et, and Ph and where X is Br when R is ClCH₂ and Et. The chloroacetylation of bromobenzene is shown to be independent of the concentration of an excess of acid chloride. The relative rates obtained are: toluoyl (0·2) < benzoyl (1·0) < chlorobenzoyl (2·5) with substrate toluene; benzoyl (1·0) propionyl (260) < acetyl (430) with substrate benzene and propionyl (1·0) chloroacetyl (ca. 100) with substrate bromobenzene. The oxonium complex, RCOCl, AlCl₃, and the ion pair [RCO]⁺[AlCl₄^–] have previously been suggested as acylating species; these orders of reactivity may be explained by either mechanism.