Formation kinetics, mechanism, and thermodynamics of glyoxylic acid-sulfur(IV) adducts

Abstract

A spectrophotometric study of the reversible addition of glyoxylic acid and S(IV) was conducted over the pH range of 0.7-2.9. Formation rates of the adduct, 2-hydroxy-2-sulfcethanoiacc id (HSEA), and its conjugate base were consistent with the following rate expression: -d[S(IV)]/dt = (k_1α_1β_1+ k_3α_2β_1)[C_2H_2O_3][S(IV)], where [C_2H_2O_3]= [CHOCO_2H]+ [CHOCO_2^-] + [CH(OH)_2CO_2H] + [CH(OH)_2CO_2^-], [S(IV)] = [H_2O•SO_2] + [HSO_3^-] + [SO_3^(2-)]ɑ_1 = [HSO_3^-]/[S(IV)], α_2 = [SO_3^(2-)]/[S(IV)], and β_1 = [CHOCO_2H]/[C_2H_2O_3]. The rate constants k_1 and k_3 correspond to the respective addition steps of HSO_3^- and SO_3^(2-) with CHOCO_2H. A study of the pH dependence gave k_1 = 443 (±21.2) M^(-1) s^(-1) and k_3 = 1.98 x 10^7 M^(-1) s^(-1). Temperature studies of the reaction showed that the ΔS^* for the addition of bisulfite and glyoxylic acid is significantly more negative than activation entropies for other low molecular weight aldehydes. A transition state was proposed that involves intramolecular hydrogen bonding. Adduct stability constants were determined as ^cK_1 = [HO_2CCH(OH)- SO_3^-]/([CHOCO_2H][HSO_3^-]) 7.09 X l0^7 M^(-1) and ^cK_2 = [^-O_2CCH(OH)SO_3^_]/([CHOCO_2^-][HSO_3^_]) = 5.27 x 10^6 M^(-1) at 25 oC and µ = 0.2 M. Correlations of these and other α-hydroxyalkanesulfonate stability constants with the Taft σ^* parameter and with carbonyl hydration constants are presented. The acid dissociation constant for the HSEA carboxyl group was estimated by potentiometric titration to be p^cK_(a5) = 2.99. The importance of glyoxylic acid as a reservoir for S(IV) in atmospheric water droplets was evaluated on the basis of these thermodynamic constants.