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

Article abstract of DOI:10.1021/j100402a043

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/dt = (k₁α₂ + (k₂ + k₃K_H- (H⁺))α¹)_t where α₁ = ->/, α₂ = 2->/, and K_H is the proton association constant of benzaldehyde.The rate-limiting steps of each term appeared to be the nucleophilic attack of SO3^2- on the carbonyl carbon of benzaldehhyde, the attack of HSO3^- on the carbonyl carbon, and the attack by HSO3^- on the protonated carbon of the carbocation, C6H5C⁺H(OH), respectively.Over the pH range of most natural systems, only the k₁ and k₂ steps contribute to adduct formation while the k₃ term becomes important for pH < 1.At 25 deg C and μ = 1.0 M, the intrinsic rate constants were determined to be k₁ = (2.15 +/- 0.09) * 10⁴ M^-1 s^-1, k₂ = (0.71 +/- 0.03) M^-1 s^-1, k₃ ca./= 2.5 * 10⁷ M^-1 s^-1.Para-substitution on the benzaldehyde ring resulted in a slight increase in reactivity for p-NO2- and p-Cl-, and a decrease for p-OH-, p-OCH3-, and p-CH3-C6H5CHO.The equilibrium association constant, K = ->/-> , at 25 deg C was determined to be 4.8 (+/-0.8) * 10³ at μ = 0.1 M and 0.98 (+/-0.11) * 10³ M^-1 at μ = 1.0 M. ΔH deg and ΔS deg were determined to be -64.6 kJ mol^-1 and -146 J mol^-1 deg^-1, respectively.