Kinetics, Thermodynamics, and Mechanism of the Formation of Benzaldehyde-sulfur(IV) Adducts

The kinetics and mechanism of the formation of α-hydroxyphenylmethanesulfonate (HPMS) by the addition of bisulfite to benzaldehyde were studied at low pH. A three-term rate law was observed as d[HPMS]/dt = (k_1α -2 + (k_2 + k_3K_H^- {H^+})α_1)[S(IV)]_t[C_6H_5CHO] where α_1 = [HSO_3^-]/[S(IV)], α_2 = [SO_3^(2-)/[S(IV)], and K_H is the proton association constant of benzaldehyde. The Rate-limiting steps of each term appeared to be the nucleophilic attack of HSO_3^- on the carbonyl carbon of benzaldehyde, the attack of HSO_3^- on the carbonyl carbon, and the attack by HSO_3^- on the protonated carbon of the carbocation, C_6H_5C^+H(OH), respectively. Over the pH range of most natural systems, only the k_1 and k_2 steps contribute to adduct formation while the k_3 term becomes important for pH < 1. At 25 ºC and µ = 1.0 M, the intrinsic rate constants were determined to be k_1 = (2.15 ± 0.09) X 10^4 M^(-1)s^(-1), k_2 = (0.71 ± 0.03) M^(-1) s^(-1), k_3 ≅ 2.5 x 10^7 M^(-1) s^(-1). Para-substitution on the benzaldehyde ring resulted in a slight increase in reactivity for p-NO_2- and p-Cl-, and a decrease for p-OH-, p-OCH_3-, and p-CH_3-C_6H_5CHO. The equilibrium association constant, K = [C_6H_5CH(OH)SO_3^-]/[HSO_3-] [C_6H_5CHO], at 25 ºC was determined to be 4.8 (±0.8) x 10^3 at µ = 0.1 M and 0.98 (±0.11) x 10^3 M^(-1) at µ = 1.0 M. ΔHº and ΔSº were determined to be -64.6 kJ mol^(1-) and -146 J mol^(-1) deg^(-1), respectively.