New insight into solvent effects on the formal HOO + HOO reaction.

The 2,2'-azobis(isobutyronitrile)(AIBN)-induced autoxidation of g-terpinene (TH) at 50 8C produces pcymene and hydrogen peroxide in a radical-chain reaction having HOOC as one of the chain-carrying radicals. The kinetics of this reaction in cyclohexane and tert-butyl alcohol show that chain termination involves the formal HOOC + HOOC self-reaction over a wide range of g-terpinene, AIBN, and O2 concentrations. However, in acetonitrile this termination process is accompanied by termination via the cross-reaction of the terpinenyl radical, TC, with the HOOC radical under conditions of relatively high [TH] (140-1000 mm) and low [O2] (2.0-5.5 mm). This is because the formal HOOC + HOOC reaction is comparatively slow in acetonitrile (2k~8107m1 s1), whereas, this reaction is almost diffusion-controlled in tert-butyl alcohol and cyclohexane, 2k~6.5108 and 1.3109 M1 s1, respectively. Three mechanisms for the bimolecular self-reaction of HOOC radicals are considered: 1) a head-to-tail hydrogen-atom transfer from one radical to the other, 2) a head-to-head reaction to form an intermediate tetroxide, and 3) an electron transfer between HOOC and its conjugate base, the superoxide radical anion, O2 C. The rate constant for reaction by mechanism (1) is shown to be dependent on the hydrogen bond (HB) accepting ability of the solvent; that by mechanism (2) is shown to be too slow for this process to be of any importance; and that by mechanism (3) is dependent on the pH of the solvent and its ability to support ionization. Mechanism (3) was found to be the main termination process in tert-butyl alcohol and acetonitrile. In the gas phase, the rate constant for the HOOC + HOOC reaction (mechanism (1)) is about 1.8 109m1 s1 but in water at pH2 where the ionization of HOOC is completely suppressed, this rate constant is only 8.6105m1 s1. The very large retarding effect of water on this reaction has not previously been explained. We find that it can be quantitatively accounted for by using Abrahams HB acceptor parameter, bH2 , for water of 0.38 and an estimated HB donor parameter, aH2 , for HOOC of about 0.87. These Abraham parameters allow us to predict a rate constant for the HOOC + HOOC reaction in water at 25 8C of 1.2106m1 s1 in excellent agreement with experiment.