The use of the tertiary alkyl tetraoxide–peroxyl equilibrium, ROOOOR ⇌ 2RO2˙, as a clean source of tertiary alkyl peroxyls

Abstract

Below 155 K there is a dynamic equilibrium between tertiary alkylperoxyls and their tetraoxide combination products. The system can be recycled repeatedly between 113 and 155 K without loss of radicals. The stationary peroxyl concentration depends solely on the temperature and the initial tetraoxide level. The equilibrium provides a clean source of peroxyls, since no other radicals such as alkoxyls are present. We have used the equilibrium to determine the mechanisms, and to estimate the rate constants for the reactions of tertiary alkylperoxyls with antioxidant and spin trap substrates. The method depends on rapidly mixing cold solutions of the substrates and solutions containing equilibrium concentrations of tertiary peroxyls. The mixing takes place in situ within an EPR cavity and the decay of the peroxyl signal is monitored continuously by EPR. The decay profiles have been kinetically analysed at three levels of approximation with the most accurate rate parameters being obtained by kinetic modelling of the total reaction scheme.The rate parameters for the reaction of peroxyls and the antioxidant, IONOL, are in good agreement with those determined by other kinetic EPR methods (KEPR). Studies of the reaction with the much used spin trap DMPO have shown that a rapid removal of peroxyls does not result in a paramagnetic mono-radical spin adduct, but yields a diamagnetic product probably containing two peroxyls added sequentially or concertedly to the spin trap.