Reaction of Hydroxyl Radicals with Alkyl Phosphates and the Oxidation of Phosphatoalkyl Radicals by Nitro Compounds

Abstract

The rate constants for reactions of hydroxyl radicals with a number of alkyl phosphates have been determined by competition with KSCN. Hydroxyl radicals react with alkyl phosphates preferentially by H-abstraction at the α-position of the phosphate functions. The resulting α-phosphatoalkyl radicals are not very efficient one-electron reducing agents towards nitro compounds. They react with tetranitromethane (TNM) by addition to form adduct intermediates with absorption maxima at about 300 nm. The rate constants for decay of these TNM adducts to produce the nitroform anion (NF⁻) and the corresponding α-phosphato-alcohols have been determined by optical and/or conductance detection. The stability of these TNM adducts varies considerably with the chain length (methyl > ethyl > isopropyl) and number (trialkyl > dialkyl > monoalkyl) of the alkyl substituents. Additional formation of proton during or after the decay of the TNM adducts has been tentatively attributed to the hydrolysis of the α-phosphato-alcohols. α-Phosphatoalkyl radicals derived from trimethyl, triethyl, triisopropyl, and diethyl phosphates react with p-nitroacetophenone (PNAP) very slowly (k < 5 × 10⁷ dm³ mol⁻¹ s⁻¹) possibly forming adducts. One-electron reduction of PNAP by these radicals to PNAP⁻ was not observed under pulse radiolysis conditions. The rate constants for the reactions of ^·OH with glycerol 1-phosphate and glycerol 2-phosphate have been redetermined by competition with KSCN. Using the radical scavengers NNN′N′-tetramethyl-p-phenylenediamine (TMPD) and TNM, the percentage of ^·OH attack at each carbon atom was obtained. Contrary to the simple alkyl phosphates described above, the α-position to the phosphate function is the least favoured (10–15% in glycerol 1-phosphate and 6% in glycerol 2-phosphate). These so-formed α-phosphatoalkyl radicals react with TNM also by forming adducts. The β-phosphatoalkyl radicals in both cases eliminate inorganic phosphate on formation (k > 10⁶ s⁻¹). The γ-phosphatoalkyl radical from glycerol 1-phosphate undergoes base-catalysed water elimination (k_obs = 1·8 × 10⁵ s⁻¹ at pH 10·6) to give an oxidizing radical. Products in the γ-radiolysis of N₂O-saturated solutions of glycerol 1-phosphate and glycerol 2-phosphate have been identified and their yields determined. The mechanisms for their formation are discussed.