Addition of e –aq, H· and ·OH to acrylamide in aqueous solution and reactions of the adducts

Abstract

A detailed study has been made of the transient species produced by the reaction of e^–_aq, H and OH radicals with acrylamide(m₁) in aqueous solution at natural pH (∼6) and at pH = 11, using the technique of pulse radiolysis. The intense transient absorption spectrum which was observed at natural pH has been assigned to the protonated monomer radical anion, ·m₁H, and the products Rm₁·, of the reaction of R (= H, OH) with monomer. Their decays are second order and values of the extinction coefficients and rate constants have been determined. The yields of ·m₁ H and Rm·₁ as a function of [m₁] have been obtained and are compared with other solute systems which scavenge e^–_aq in spurs. Thin layer chromatography has been used to identify adipamide and pionamide as two of the seven products obtained from pulse-irradiated solutions of acrylamide and an estimate of the total product yield has been made on the basis of these results.

At pH = 11 the transient absorption spectrum has been assigned to the monomer radical anion, ·m^–₁ and Rm₁·; the decay of the former was first order whereas that of Rm₁· was complex and occurred with the concomitant growth of a permanent species. The extinction coefficients and rate constants for the decay of ·m^–₁ and Rm₁· have been determined. The yields of ·m^–₁ and Rm₁· as a function of [m₁] have been obtained and compared with the corresponding yields of ·m₁H and Rm₁· at natural pH. Absolute rate constants (M^–1 s^–1) have been obtained for the reaction of acrylamide with H atoms, (1.8 ± 0.2 )× 10¹⁰, CO^–₂·, ∼4 × 10⁷, and ·m₁H, (2.5 ± 0.4)× 10⁵, for reaction of ·m₁H with Ag⁺, (1.1 ± 0.4 )× 10⁸, and for reaction of e^–_aq with propionamide, (3.0 ± 0.5)× 10⁷, α-chloropropionamide, (5.8 ± 0.5)× 10⁹, and β-chloropropionamide, (1.8 ± 0.2)× 10⁹.

E.s.r. spectra in α-methyl tetrahydrofuran glass at 77°K have been obtained for the radicals ĊH₂CH₂CONH₂, CH₃ĊHCONH₂ and the electron adduct of acrylamide. The yield and decay data presented in a previous publication for pulse radiolysed aqueous solutions of methacrylamide, N-t-butylacrylamide, acrylonitrile and acrylic acid have been recalculated in view of the yield dependence of (Gε)^·m1H₂₇₅ and (Gε)^Rm1·₃₇₀ observed for acrylamide.