Decomposition thermique des acides peroxybenzoique et peroxydodecanoique en milieu non polaire: role de l'oxygene

Abstract

A study of the decomposition mechanisms of peroxydodecanoic (RCO 3H) and perbenzoic (φCO 3H) acids in cyclohexane at both reflux temperature and less than reflux temperature (i.e. with and without dissolved oxygene) and comparisons of all the products from these reactions with those obtained from the decomposition of dodecanoyl- and benzoyl-peroxydes under the same conditions, leads to the following conclusions: (a) in both cases there is a radical mechanism, the initiating step being the rupture of the O-O bonds giving RCO 2 and φCO 2, (b) under reflux (i.e. without oxygen) the radical RCO 2 undergoes rapid decomposition and the resulting R- gives rise to a chain reaction leading to the alcohol ROH. The φCO 2 radical undergoes much slower decomposition and through chain transfer with the solvent produces φCO 2H and cyclohexanol. These reactions are fast; (c) at less than reflux temperature (i.e. in presence of dissolved oxygen) R radicals or S (solvent) radicals combine with the oxygen giving peroxy and oxy radicals. These species are electrophilic and therefore do not lead to the breaking of the O-O bond of the peracid. Rather, the acid hydrogen is attacked, giving RCO 3 or φCO 3, radicals. Through a series of reactions involving aldehyde intermediates, the corresponding carboxylic acids are obtained. These reactions are much slower than those at reflux temperature.