Radical fission

The C—H bond of the aHyl position easily undergoes radical fission, especially in the case of aHyl ethers, reacting with the oxygen in the air to form peroxide compounds. 

Besides such dissociation into long-lived radicals in solution, numerous examples are known of radical cleavage in the gas phase into unstable or reactive radicals. Two factors, the strain of hydrocarbon molecules and the stability of the radicals, are suggested as the major controlling factors for radical fission (Riichardt and Beckhaus, 1980, 1986). 

In the photochemistry of larger molecules the same physical principles apply and the sequence of events is essentially the same, although descriptions are more complex and less precise. Opposite to diatomic, the polyatomic molecules can yield a multitude of different sets of products. To establish the photodissociation mechanism, the nature of the elementary chemical process undergone by an electronically excited molecular entity (primary photoreaction) yielding primary photoproducts should be known, eg in the case of alkanes both the radical fission... 

Excited state fission products Radical fission products... 

The amide N-H may also be halogenated, oxidized and nitrosated. A -Bromosuccinimide (NBS), like a number of other iV-halo compounds, readily undergoes a radical fission to give a bromine radical. This provides a useful reagent for radical bromination at, for example, allylic or benzylic positions. In the presence of acid, NBS is also a mild source of the halonium ion, which is used for the addition of hypobromous acid (Scheme 3.74) to alkenes or for the bromination of reactive aromatic rings. 

A study of the photochemical fission of the stannanes (199) has reported the efficiency of the process yielding, for example, from benzyl stannane (199a), bibenzyl and the alkyl tin derivative (200). The reaction in benzene is best explained. by a radical fission affording the tin and the benzyl radicals. ... 

Briefly, the shock tube data give A-factors for radical fission which are about 1 order of magnitude lower than that calculated from the A-factor for the reverse reaction (which is radical recombination) and the estimated entropy of the over-all reaction. In addition, the shock tube activation energies are from 3 to 6 kcal/mole higher than the bond dissociation energies for branched radicals. 

Formation of the latter materials would appear to proceed from cyclopropyl radical fission in the trimethylenemethane species responsible for the interconversion of the two methylenespiropentanes. 

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