Kinetics, mechanism and thermodynamics of glyoxal-sulfur(IV) adduct formation

The reversible addition of glyoxal (ethanedial) and S(IV) to form glyoxal monobisulfite (GMBS) was studied spectrophotometrically over the pH range of 0.7-3.3. Far from equilibrium, the rate of GMBS formation is given by d[GMBS]/dt (k_(1,app)^ɑ1 + k_(2.app)ɑ_2)[C_2H_2O_2][S(IV) where [C_2H_2O_2 = [(CH(OH)_2)_2] + [CH(OH)_2CHO] + [CHOCHO], [S(IV)] = [H_2O-SO_2] + [HSO_3^-] + [SO_3^(2-)], ɑj = [HSO_3^-]/[S(IV)], and ɑ_2 = [S0_3^(2-)]/[S(IV)]. The apparent rate constants, k_(l,app) = 0.13 M^(-l) s^(-l) and k_(2,app)= 2.08 X 10^3 M^(-1) s^(-1), are pH independent functions of the dehydration equilibrium constants of (CH(OH)_2)_2 and CHOCH(OH)_2, and intrinsic rate constants for the reaction of HSO_3^- and SO_3^(2-) with unhydrated and singly hydrated glyoxal. Glyoxal dibisulfite (GDBS) and GMBS were shown to dissociate with a rate given by d[S(IV)]/d_t = k’_(-1) + k"(_1)K_(D3) + (k’_(-2)K'_(a2) + k"_(-2)K"_(a2)K_(D3))/[H^+]][GMBS]_t + [k_(-3) + k_(-4)K_(a3)/{H^+}}[GDBS]_t, where k_(-1) and k_(-2) correspond to the release of bisulfite and sulfite, respectively, from unhydrated and hydrated GMBS species; k_(-3) and k_(-4) correspond to the release of bisulfite and sulfite from GDBS; K_(D3) is the dehydration constant for GMBS; and K’_(a2) K"_(a2), and K_(a3) are acid dissociation constants. Stability constants for the formation of GMBS and GDBS were determined to be ^CK_1 = [CH(OH)_2CH(OH)S0_3^-]/([CH(OH)_2)_2][HSO_3^-]) = 2.81 X 10^4 M^(-1) and ^cK_2 = [(CH(OH)SO_3^-)_2]/([CH(OH)_2CH- (OH)SO_3^-][HSO_3^-]) = 1.45 X 10^4 M^(-1) at 25 ºC and µ = 0.2 M.