The radiolysis of alcohols at low temperatures—II. The precursors of hydrogen in glassy alcohols up to n-hexanol including allyl alcohol. The hydrogen formation in irradiated glassy alcohols at 77 K

Abstract

The mechanism of hydrogen formation in the radiolysis of glassy ethanol, n-propanol, isopropanol, n-butanol, n-pentanol and n-hexanol is analogous to the mechanism found in methanol, previously studied by Teplý and Janovský. The precursors of hydrogen are trapped electrons and hydrogen atoms, and there is also unscavengeable hydrogen. The yields of trapped electrons are approximately equal ( G ∼ 2) as are those of the unscavengeable hydrogen ( G ∼ 1·3), while the yields of hydrogen atoms show greater fluctuations about the value ∼1. The difference in the hydrogen yields after optical and thermal bleaching in ethanol and propanol are similar to that in methanol; on optical bleaching probably a photochemical decay of the trapped electron occurs while on thermal bleaching a reaction of the migrating electron with the protonated molecules of the alcohols takes place. In the first case, the hydrogen atom formed abstracts another hydrogen from a molecule of alcohol, in the second one it abstracts to a lesser extent or not at all. The difference between the hydrogen yield on optical and thermal bleaching gradually disappears with increasing chain length of the alcohol. Allyl alcohol behaves quite differently: the electrons are not trapped and they take part in the formation of hydrogen only to a small extent ( G ∼ 0·7); the yield of the unscavengeable hydrogen is ∼0·4.