Aqueous Solubility and Reaction Kinetics of Hydroxymethyl Hydroperoxide

Article abstract of DOI:10.1021/j100180a051

The aqueous-phase equilibria and kinetics of the formation of hydroxymethyl hydroperoxide (HMP) and of bis(hydroxymethyl) peroxide (BHMP) from formaldehyde and hydrogen peroxide, i.e., H2CO + H2O2 <*> HOCH2O2H (1, -1), and HOCH2O2H + H2CO <*> HOCH2O2CH2OH (2) were studied using an amperometric technique which is highly sensitive and selective for H2O2.The equilibrium constants of reactions 1 and 2 between 5 and 35 deg C were determined to be K₁ = 2.35*10^-2 exp(2610/T) M^-1 and K₂ = 1.04*10^-3 exp(2780/T) M^-1, respectively, both independent of pH between 4.0 and 8.4.The rate coefficients of (1) and (-1) determined at pH 7.07 +/- 0.02 between 5 and 35 deg C are k₁ = 6.0*10¹³ exp(-9450/T) M^-1 s^-1 and k_-1 = 1.0*10¹⁵ exp(-11800/T) s^-1.Both k₁ and k_-1 are base-catalyzed and are linearly dependent on pH between 4.0 and 8.2, namely, k_-1 = (5.0 +/- 0.3)*10^-10/ s^-1 at 25.8 +/- 0.1 deg C.The Henry's law constants of HMP and BHMP, determined by measuring their corresponding gas and aqueous concentrations at phase equilibrium, are 5.0^+1.6_-0.9*10⁵ and 6⁺³_-2*10⁵ M atm^-1 at 22.0 +/- 0.1 deg C, and 6.2^+1.2_-0.9*10⁵ and 20⁺¹⁸_-7*10⁵ M atm^-1 at 10.0 +/- 0.1 deg C, respectively.The reaction kinetics of HMP with S(IV) was studied by a competition technique using the H2O2-S(IV) reaction as the reference; the reaction is acid-catalyzed, with an effective second-order rate constant of (2.2*10⁷) +/- 15percent M^-1 s^-1 for the pH range 3-4 at 22.0 +/- 0.1 deg C.These results indicate that gas-phase HMP in the atmosphere is efficiently removed by wet scavenging processes and would be quantitavely detected by peroxide instruments involving gas-liquid scrubbers, provided that the scrubbed HMP is stabilized.The time constant of the dissociation of dissolved HMP to H2O2 is fairly short, being ca. 100 min at pH 5.5, shorter at higher pH.Consequently, HMP is expected to be stable and detected in atmospheric liquid water only at pH <*>5.5.