THE ACTIVATION ENERGY FOR THE DISPROPORTIONATION OF THE HO₂ RADICAL IN ACID SOLUTIONS¹

Absolute rate constants for hydrocarbon autoxidation. V. The hydroperoxy radical in chain propagation and termination

“…T h e products from 1,4-dihydronaphthalene which were formed in this work depend 6In the gas phase, the entropy change in reaction [8] (--30 cal/mole "K (29)) corresponds t o a free energy change of about 10 kcal/mole. The entropy change should be smaller in solution.…”

“…The the values which have been obtained for the self-reaction of HOO' radicals in aqueous systems a t this temperature (6)(7)(8)(9). Pulse radiolysis of concentrated aqueous solutions of hydrogen peroxide and its oxidation with ceric ion give kt values in the range 1-3 X lo6 M-I s-I.…”

“…For reaction [3], With saturated alkyl radicals AHrO for reaction [8] is about -2s kcal/mole (e.g. O .…”

“…, a n d 0. respectively (30)). Forthese radicals, therefore, reaction [8] is strongly favored over reaction [l].j Since the cyclohexadienyl peroxy radical is unstable with respect to benzene and HOO', the heat of reaction [8] T h e balance between reaction [l] (HOO' formation) and [S] (ROO' formation) is extremely sensitive to hydrocarbon structure (see Table 111). Thus, the HOO' chain is unimportant in the autoxidation of 9,lOdihydroanthracene and yet the gain in resonance energy on aromatization of this molecule6 (-11 kcal/mole (35)) is scarcely less than for the aromatization of 1,4-cyclohexadiene (-12 kcal/mole (31)).…”

“…Of particular interest is the very high rate constant for the chain-termination process involving two hydroperoxy radicals in nonpolar media compared with its value in aqueous media (6)(7)(8)(9). …”