Isotope effects radical rearrangements

Disproportionation (equation 13) is one of the side reactions that can occur in benzidine rearrangements. Shine and coworkers measured the nitrogen and carbon kinetic isotope effects for the disproportionation reaction of 4,4 -diiodohydrazobenzene, which only yielded disproportionation products, at 25 °C in 70% aqueous dioxane that was 0.376 M in perchloric acid29. The reaction was first order in hydrazobenzene and it has been assumed that an intermediate was involved in the disproportionation reaction. This intermediate must be one of a radical ion30 (equations 14 and 15), a jr-complex31 (equation 16) or a quinonoid structure32 (equation 17). 

The McLafferty rearrangement in certain carboxylic acids initially forms the ion (CH2 = C(OH)2)t, which then undergoes metastable ion decomposition to lose a hydroxyl radical. With the labelled ions [CD2 = C(OH)(OD)]t and [CH2=C(OH)(OD)]t, the isotope effect 7oh/ od has been measured as 0.38 [526, 753], 0.56 [342] and 0.42 [407], i.e. OD- loss was more probable than OH - loss. This observation has been interpreted [407, 526] in terms of the rate-determining step... 

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