Determination of acid dissociation constants of some phenol radical cations

Abstract

The variation of the ratio, R, of acid:base forms of some phenol radical cations with acid strength of the medium, is followed by means of e.s.r. spectroscopy.

The radical cations studied fall into two categories, first those for which R∝(h/W⁴) and secondly those for which R∝(h/W⁴)²; where h, 55.5 W are the number of moles of sulphuric acid and water, respectively, used to make up a litre of solution. The implications of these results are first, that it seems that four molecules of water are required to remove a proton from a phenol radical cation; second, that these molecules of water do not necessarily have to be “free” but may be water of solvation, and third, that when there are alkoxyl or hydroxyl substituents in the meta- or para-positions in a phenoxyl radical, two protons are necessary to produce the corresponding radical cation. The acid dissociation constants are given in terms of –log K′ where K′ is given by the equation K′=[base form]//_{[acid form]}(h//_W⁴) where n= 1 or 2 as appropriate.