At pH ≤2 the following rate law for the formation of hydroxyacetylmethanesulfonate (HAMS) from methylglyoxal (MG) and S(IV) (H2O·SO2, HSO3-, SO32-) is obtained: d[HAMS]/dt = (((k0α1[H+]/Ka0) + k1α1 + k2α2)[S(IV)][MG]0)/(1 + Kd + Kd[H+]/Ka0), where α1 and α2 are the fractional concentrations of HSO3- and SO32-, 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 SO32-, respectively; Kd is the dehydration constant of hydrated MG; and Ka0 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 = kd[MG]/(1 + Kd + Kd[H+]/Ka0), where kd = kw + kH[H+] + kOH[OH-] + kA[A] + kB[B], and kW is the intrinsic (water) rate constant; kH and kOH are the specific acid and base rate constants; and kA and kB 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-2Ka3/[H+])Ka4[H +][HAMS])/([H+]2 + Ka4[H+] + Ka3Ka4), where k-0, k-1, and k-2 are the reverse of the analogous forward rate constants defined above and Ka3 and Ka4 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‡ (and ΔS298) on the relative magnitude of these constants.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry