Kinetics and Mechanism of the Acid-Catalyzed Bromination of Ring-Substituted Acetophenones in Methanol. Thermodynamics of the Ketone-Acetal-Enol Ether System in Mathanol and Water

Article abstract of DOI:10.1021/jo00371a026

The kinetics of HBr-catalyzed bromination of ring-substituted acetophenones in equilibrium with acetals have been studied in methanol containing small amounts of water at very low halogen concentration (ca. 10^-6 M).Under these conditions, the rate is zeroth order with respect to the halogen and first order with respect to the CH3OH2⁺ ion and the overall carbonyl compound concentrations.The relative concentration of acetal at equilibrium (x_eq = _eq/(_eq + _eq)) has been varied by increasing the water concentration; the lyonium ion catalytic constant depends linearly on x_eq with an x_eq = 0 intercept corresponding to bromination via intermediate enol formation and with an x_eq = 1 intercept corresponding to bromination via enol ether.In the case of 3-nitroacetophenone (1a), these extrapolated rate constants are identical with those directly measured from the rates for bromination of 1a and of 1a dimethyl acetal, before ketone-acetal equilibration.In most cases, the enol ether path is highly predominant.Substituent effects on the constants associated with the two paths are examined.The rate constants for enol ether formation are combined with rate constants for the reverse process and with acetal formation equilibrium constants to provide acetal-to-enol ether and ketone-to-enol ether equilibrium constants in methanol.Taking into account data of Gibbs free energies of transfer from methanol to water for the different species involved, equilibrium data in water are also deduced.