Photochemistry of phosphorus compounds. Part IX. Photolysis of trimethyl phosphate in aqueous solution

Abstract

The products of irradiation (254 nm) of aqueous trimethyl phosphate, in the presence of argon or of oxygen, have been analysed. In accord with the very low absorption of aqueous trimethyl phosphate at 254 nm, almost no reaction takes place in an argon atmosphere. Mass spectrometry indicates that some dimer (MeO)2P(O)OCH2CH2OP(O)(OMe)2 is formed, probably via the reactive intermediate (MeO)2P(O)OCH2·.The photolysis proceeds much faster in an oxygen atmosphere to give, as initial products, dimethyl hydrogen phosphate, formaldehyde, and possibly formic acid. Prolonged irradiation leads to (i) stepwise further degradation of dimethyl hydrogen phosphate to methyl dihydrogen phosphate and subsequently to orthophosphoric acid; and (ii) formation of carbon monoxide and carbon dioxide as major reaction products, coinciding with the build-up of steady state concentrations of formaldehyde and formic acid. Carbon dioxide is probably formed by secondary photolysis of formic acid, whereas photolysis of formaldehyde might give carbon monoxide.The quantum yields for the formation of dimethyl phosphate were determined, both in oxygen (Φ= 0·11) and in argon (Φ⩽ 0·01) atmospheres.