Protonation kinetics of acrylate radical anions

Abstract

The radical anions of acrylamide, methacrylamide and 2-hydroxypropyl acrylate were produced by pulse radiolysis of moderately acidic aqueous solutions well below the pK_a of the radical anion. The rate coefficients of protonation were determined by UV kinetic spectroscopy at wavelengths where the protonated and ionic forms have considerably different absorbances. The rate coefficients of protonation were measured as (6.0±0.9)×10¹⁰, (6.6±1.0)×10¹⁰ and (5.0±1.0)×10¹⁰ mol⁻¹ dm³ s⁻¹. Using these rate coefficients and the pK_a values of the radical anion, 7.9, 8.0 and 5.5, the rate coefficients of deprotonation were determined as 7.5×10², 6.6×10² and 1.6×10⁵ s⁻¹. The results clearly showed that in acidic solution the protonation occurred with H₃O⁺ and not with H₂O. The rate coefficients determined were discussed and compared with relevant data in the literature.