Kinetics, Mechanism, and Thermodynamics of the Reversible Reaction of Methylglyoxal (CH3COCHO) with S(IV)

Article abstract of DOI:10.1021/j100295a074

At pH =/<2 the following rate law for the formation of hydroxyacetylmethanesulfonate (HAMS) from methylglyoxal (MG) and S(IV) (H2O*SO2, HSO3^-, SO3^2-) is obtained: d/dt = (((k₀α₁+>/K_a0) + k₁α₁ + k₂α₂)₀)/(1 + K_d + K_d+>/K_a0), where α₁ and α₂ are the fractional concentrations of HSO3^- and SO3^2-, respectively; k₀ is the rate constant for the reaction of HSO3^- with the carbocation aldehyde species (CH3COC⁺HOH); k₁ and k₂ are the rate constants for the reaction of unhydrated MG with HSO3^- and SO3^2-, respectively; k_d is the dehydration constant of hydrated MG; and K_a0 is the acid dissociation constant of the carbocation.At pH =/>4 the rate of formation of HAMS is determined by the rate of dehydration of the diol form of (hydrated) MG: d/dt = k_d/(1 + K_d + K_d+>/Θ_a0), where k_d = k_w + k_H+> + k_OH-> + k_A + k_B, and k_w is the intrinsic (water) rate constant; k_H and k_OH are the specific acid and base rate constants; and k_A and k_B are the general acid (A) and base (B) rate constants.Between pH 2 and 4, biexponential kinetics are observed because, under our conditions, the rates of dehydration and of S(IV) addition become comparable.Over the pH range 0.7-7.0, the dissociation of HAMS follows the rate law: d/dt = ((k_-0+> + k_-1 + k_-2K_a3/+>)K_a4+>)/(+>² + K_a4+> + K_a3K_a4), where k_-0, k_-2 are the reverse of the analogous forward rate constants defined above and K_a3 and K_a4 are the acid dissociation constants of the sulfonate anion and the sulfonic acid, respectively.Experiments to determine the effect of temperature on the rate (and equilibrium) constants indicate a marked effect of ΔS(excit.) (and ΔS₂₉₈) on the relative magnitude of these constants.