The Photo-sensitized oxidation of ethanol

Abstract

Certain anthraquinonoid derivatives act as photosensitizers of the autoxidation of ethanol. In this quantitative investigation the process has been studied in aqueous solution under non-alkaline conditions using anthraquinone 2:6 disodium sulphonate. This sensitizer operates by an extremely efficient cyclic mechanism, abstracting hydrogen from the ethanol to give a semi-quinone radical which is rapidly reconverted to the original quinone by reaction with molecular oxygen. The reaction products are acetaldehyde, acetic acid and hydrogen peroxide; it has been demonstrated that the acetic acid is a primary product. Autoxidation proceeds by a non-chain mechanism involving radicals of the peroxy type, H I CH 3 .COO.. | OH A detailed kinetic analysis of the proposed reaction scheme leads to expresisions relating the product yields to the experimental conditions which are in accordance with the observed results. Consideration of the limiting product yields at high oxygen pressures provides evidence regarding the mode of interaction of two peroxy radicals. The mechanism is applicable in general to compounds containing primary or secondary alcoholic groupings and provides a new interpretation of the inhibitory action of alcohols. Although the autoxidation of ethanol differs both in kinetic characteristics and in the non-peroxidic nature of some of the primary products from the olefinic and aldehydic systems which have been the subject of intensive study in recent years, the same basic principles of hydrogen abstraction followed by oxygen addition persist.