Kinetics, mechanism and thermodynamics of the reversible reaction of methylglyoxal (CH_3COCHO) with sulfur (IV)

Abstract

At pH ≤ 2 the following rate law for the formation of hydroxyacetylmethaneulfonate (HAMS) from methylglyoxal (MG) and S(IV) (H_2O•SO_2, SO_3^(2-))is obtained: d[HAMS]/dt = ((k_0α_1[H^+]/K_(a0)) + k_1α_1 + k_2α_2)[S(IV)][MG]_0)/(1 + K_d + K_d[H^+]/K_(a0)), where α_1 and α_2 are the fractional concentrations of HSO_3^- and SO_3^(2-), respectively; k_0, is the rate constant for the reaction of HSO_3^- with the carbocation aldehyde species (CH_3COC^+HOH); k_1, and k_2 are the rate constants for the reaction of unhydrated MG with HSO_3^- and SO_3^(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[HAMS]/dt = k_d[MG]/(l + K_d + _Kd [H^+]/K_(a0)), where k_d = k_w + k_H[H^+] + kOH[OH^-] + k_A[A] + kB_[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[S(IV)]/dt = ((k_(-0)[H^+] + k_)-1) + k_(-2)K_(a3)/[H^+])K_(a4)[H^+][HAMS])/[H^+]]^2 + K_(a4)[H^+] + K_(a3)K_(a4)) where k_(-0) k_(-1), and 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 sulfortate 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^* (and ΔS_(298)) on the relative magnitude of these constants.