THE FRIEDEL-CRAFTS ACETYLATION OF NAPHTHALENE IN 1,2-DICHLOROETHANE SOLUTION - KINETICS AND MECHANISM

The Friedel-Crafts acetylation of naphthalene, using acetyl chloride and aluminium chloride in 1,2-dichloroethane solution, has been studied kinetically. The alpha/beta 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 alpha-reaction and first-order for the beta-reaction. The rate of the alpha-reaction is impeded by the presence of free acetyl chloride, whilst the beta-reaction is unaffected. The latter reaction exhibits activation parameters in the expected range, DELTA-H double-ended dagger = ca. 48 kJ mol-1 and DELTA-S double-ended dagger = ca. -99 J K-1 mol-1, whereas for the alpha-reaction DELTA-H double-ended dagger = ca. 21 kJ mol-1 and DELTA-S double-ended dagger = ca. -160 J K-1 mol-1 are both very low. Competitive and non-competitive kinetic hydrogen isotope experiments were carried out using [H-2(8)]naphthalene. The mechanism for beta-naphthyl acetylation is believed to involve a two-stage process, the second (loss of proton) being rate-limiting. The alpha-acetylation is believed to proceed through a sigma-complex, from which elimination of HCl to give products is prevented for steric reasons; the reaction instead proceeds through a second sigma-complex, decomposition of which is usually at least partly rate-limiting.