RUSSELL termination step

The last dimerisation reaction is probably assumed to resemble the Russell termination step (p. 23) but it must be said that is an unlikely candidate for the typically high quantum yields of the luminol reaction. Direct reaction [76] with oxygen is observed and, instead of diazaquinone, its addition product with oxygen radical anion is assumed. An endoperoxide such as (47) is not proposed as an intermediate. 

In chain reactions involving three termination steps (two uncrossed and one crossed) the quantity = 1c /(1c 1c )1/2 is frequently used to interrelate the cross-termination constant with the two uncrossed termination constants. For many different types of radical < is found to be about 1 (or alternatively, if the statistical factor of 2 favoring the crossed termination process is ignored in the definition of the rate constants, < 2). In the present reaction system —3-6, in agreement with the value obtained by Russell at 90°C. (26). The crossed termination constant itself is somewhat less than half the value found for kt. This seems reasonable since only one hydrogen atom will be available for transfer in the crossed termination, compared with the two that are available in the self-reaction of two tetralylperoxy radicals. In addition, steric hindrance to reaction should be greater for the crossed termination than for Reaction 8. The products are presumably cumyl alcohol, a-tetralone [3,4-dihydro-l(2H)naphthalenone], and oxygen (28). 

Thus the proportionality constant conveniently cancels out. The validity of this technique rests on the requirement that the proportionality in eqn. (86) holds for each hydrocarbon. In oxidation of primary or secondary C—H bonds, the light emitted apparently comes from excited carbonyl produced in the termination step (89) [54] following the mechanism originally suggested by Russell [55]... 

The termination step occurs by the Russell mechanism [1] via the unstable tetraoxide (5). The products are a ketone, oxygen and an alcohol as shown. The conservation of spin in this assumed concerted reaction, as previously discussed, leads, to formation of the ketone in the triplet excited state. Although singlet oxygen may be found in low yield, the carbonyl excited state certainly predominates. 

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