The Friedel–Crafts acetylation of naphthalene in 1,2-dichloroethane solution. Kinetics and mechanism

Abstract

The Friedel–Crafts acetylation of naphthalene, using acetyl chloride and aluminium chloride in 1,2-dichloroethane solution, has been studied kinetically. The α/β isomer ratio changes as a function of concentration of reactants, and time, e.g. from an initial 4–5 to a final 0.7. The results point to a different dependence for the two positions on the concentration of acylating reagent, AcCl, AlCl₃, being second-order in this reagent for the α-reaction and first-order for the β-reaction. The rate of the α-reaction is impeded by the presence of free acetyl chloride, whilst the β-reaction is unaffected. The latter reaction exhibits activation parameters in the expected range, ΔH^‡=ca. 48 kJ mol^–1 and ΔS^‡=ca.–99 J K^–1 mol^–1, whereas for the α-reaction ΔH^‡=ca. 21 kJ mol^–1 and ΔS^‡=ca.–160 J K^–1 mol^–1 are both very low. Competitive and non-competitive kinetic hydrogen isotope experiments were carried out using [²H₈]naphthalene. The mechanism for β-naphthyl acetylation is believed to involve a two-stage process, the second (loss of proton) being rate-limiting. The α-acetylation is believed to proceed through a σ-complex, from which elimination of HCl to give products is prevented for steric reasons; the reaction instead proceeds through a second σ-complex, decomposition of which is usually at least partly rate-limiting.