Substitution électrophile aromatique dans l'anhydride sulfureux liquide. Influence de la concentration en bromure et rôle du solvant sur la réactivité des anisoles au cours de la réaction de bromation

Electrophilic Aromatic Substitution in Liquid Sulfur Dioxide. Kinetic Dependance of Rate on the Bromide Concentration and Influence of the Solvent during the Course of the Reaction On the reported data for bromination of anisole and eleven of its derivatives in liquid SO₂, it was shown that, with a large excess of bromide, the rate of reaction, obeys a first-order law. Rate constants thus obtained do not discriminate between the two different forms of bromide, e.g. Br₂ and Br^?₃ present as the A⁺Br^?₃ form, and corrections were made by use of the apparent equilibrium constant K′ for tribromide formation. The variations of rate constants with initial concentration of bromide has been studied and the effect results in a retardation of the bromination rate. Moreover, the ratio Br₂] A⁺Br^?]_T, which is constant during an experiment, varies with initial bromide concentrations, this variation affecting the total rate. To account for the bromide effect on the reactivity, variations of k^o,p_g {1 + K′A⁺Br^?]_T}VSA⁺Br^?]_T were studied over a 0.01 to 1M range of bromide concentration. The mechanism proposed shows that liquid SO₂ helps the reactive intermediate to be deprotonated and because of solvation of reactive species this step would probably be rate determining. Bromination by molecular bromine is more sensitive to substituent effects in liquid SO₂ than in water. This result is ascribed to the +M effect of the methoxy group which increase the conjugation of ortho-substituted derivatives (p⁺_p = ?7.83; p⁺_o= ?10.47).