Reactions of the hydroxy peroxy radicals HOCH2O2 and CH3CHOHO2 in aqueous solution

Abstract

The products formed from HOCH₂O₂(RO₂) radicals during (a) radiolysis of oxygen saturated aqueous solutions of methanol by γ-rays and 1.8 MeV electrons, and (b) photolysis of similar solutions containing H₂O₂ are HCOOH, HCHO, CH₂OHO₂H and H₂O₂. Some of the HCHO is formed initially in the keto form and is subsequently hydrated to CH₂(OH)₂. The major organic product at pH 2 is HCOOH. At low radiation intensities G(HCOOH)/G(HCHO) and ϕ(HCOOH)/ϕ(HCHO) decrease to zero with increasing pH in the range 3 to 6.5; at high dose rates this range extends from pH 3 to pH 8.5.

These observations and those of an earlier pulse radiolysis study by Stockhausen et al.¹ are interpreted quantitatively in terms of reactions of the species HOCH₂O₄CH₂OH(A), HOCH₂O₄CH₂O^–(B^–), and OCH₂O₄CH₂O⁼(C⁼) formed by bimolecular reactions of RO₂. At low dose rates HCOOH is formed by unimolecular decomposition of A; at high dose rates it is formed also by bimolecular reactions of B^– with B^– and its yield is controlled by both the equilibria A ⇌ B^–+ H⁺(pK ∼ 4.6) and B^–⇌ C⁼+ H⁺(pK ∼ 7.1) or HOCH₂O₂⇌ O₂CH₂O^–+ H⁺(pK ∼ 6.8). HCHO results partially from unimolecular decomposition of RO₂ into HCHO + HO₂ with k≃ 9.9 s^–1, and also from reactions of C⁼, B^– and A. Reactions of HOC₂H₄O₂ are similar.

An unusual reaction occurs at high dose rates in the pH region where B^– and C⁼ co-exist and causes the product yield to exceed G(RO₂); its nature and mechanism are discussed.