Binary and Ternary Catalytic Mechanisms in the Hydration of Acetaldehyde

Abstract

IN a recent paper¹, C. G. Swain has suggested that reactions which are catalysed by acids and bases normally take place by a ternary mechanism, in which the substrate is attacked simultaneously by an acid and a base. Such a mechanism accounts for the product term k′ [HAc][Ac⁻] found by Dawson and Spivey² in the expression for the velocity of iodination of acetone in acetate buffers. It has commonly been held that the magnitude of this term was much too small to justify the assumption that the reaction goes primarily by a ternary mechanism involving water molecules³. However, Swain (loc. cit.) has shown clearly that this criticism of the ternary picture is not a valid one, since it ignores the experimental equivalence of pairs of kinetic terms such as [HAc][H₂O] and [Ac⁻][H₃O⁺], or [Ac⁻][H₂O] and [HAc][OH⁻]. Swain is, in fact, able to show that the magnitude of the term K′ [HAc][Ac⁻] observed by Dawson and Spivey is close to that predicted by the ternary hypothesis on the basis of the other observed catalytic coefficients. Similarly, Swain shows that if the mutarotation of glucose also involves a ternary mechanism, the term in [HAc][Ac⁻] will not con-tribute more than a few per cent of the total velocity, and would thus escape detection.