Thermodynamic and kinetic analysis of the dimerization of aqueous glyoxal

Equilibria and rates of interconversion between monomeric and dimeric glyoxal were measured in aqueous solution. The equilibrium constant [G₂]/[G₁]² was 0.56 M^?1 at 25°C, and was hardly affected by changes of ionic strength and pH but increased rapidly with increase of temperature. The rate of depolymerization was first-order in dimer, with the pseudo first-order rate coefficient in the pH range 1.3–7.8being of the form b₁[H₃O⁺] + b₂ + b₃[OH^?]/(1 + b₄[OH^?]) + b₅[OH^?]. Coefficients b₁ and b₂ were more strongly affected by changes of temperature, though [OH^?] was much the more effective catalyst. This rate form has not previously been observed for monomer-dimer inter-conversion of α-hydroxycarbonyls and α-dicarbonyls or for related reactions such as mutarotations and hydrations. Equivalent rate forms arisefrom reactions where an intermediate at steady state and low concentration is produced.