New Insight into Solvent Effects on the Formal HOO~(centre dot) + HOO~(centre dot) Reaction

摘要

The 2,2'-azobis(isobutyro-nitrile)(AIBN)-induced autoxidation of gamma-terpinene (TH) at 50 deg C produces p-cymene and hydrogen peroxide in a radical-chain reaction having HOO~(centre dot) 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 HOO~(centre dot) + HOO~(centre dot) self-reaction over a wide range of gamma-terpinene,AIBN,and O_2 concentrations.However,in aceto-nitrile this termination process is accompanied by termination via the cross-reaction of the terpinenyl radical,T',with the HOO~(centre dot) radical under conditions of relatively high [TH] (140-1000 mM) and low [O_2] (2.0-5.5 mM).This is because the formal HOO~(centre dot) + HOO~(centre dot) reaction is comparatively slow in acetonitrile (2k approx 8x 10~7M~(-1)s~(-1)),whereas,this reaction is almost diffusion-controlled in tert-butyl alcohol and cyclohexane,2k approx 6.5xl0~8 and 1.3xl0~9 M~(-1)s~(-1),respectively.Three mechanisms for the bimolecular self-reaction of HOO~(centre dot) 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 HOO~(centre dot) and its conjugate base,the superoxide radical anion,O_2~(-centre dot).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 HOO~(centre dot) + HOO~(centre dot) reaction (mechanism (1)) is about 1.8x10~9 M~(-1)s~(-1) but in water at pH<=2 where the ionization of HOO~(centre dot) is completely suppressed,this rate constant is only 8.6xl0~5M~(-1) s~(-1).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 Abraham's HB acceptor parameter,beta_2~H,for water of 0.38 and an estimated HB donor parameter,alpha_2~H,for HOO~(centre dot) of about 0.87.These Abraham parameters allow us to predict a rate constant for the HOO~(centre dot) + HOO~(centre dot) reaction in water at 25 deg C of 1.2xl0~6M~(-1) s~(-1) in excellent agreement with experiment.