Benzene radical stabilisation

The shape of Ph3C- (49) is a matter of some interest as it has a bearing on the extent to which delocalisation of the unpaired electron, with consequent stabilisation, can occur. The radical carbon atom is certainly sp2 hybridised in (49), i.e. the bonds joining it to the three benzene nuclei all lie in the same plane but maximum stabilisation will only occur if all three benzene nuclei can be simultaneously coplanar (49a),... 

The rearranged radical (119) is more stable than the original one (117) not only because the former is tertiary and the latter primary, but also because (119) is stabilised by delocalisation of the unpaired electron over the n orbital system of a benzene nucleus. It is significant that only Ph migrates in (117), despite the fact that migration of Me... 

The dehydrodimerization reaction involving aromatic radical-cations is fast only when electron donating substituents are present in the benzene ring. These substituents stabilise the a-intermediate. Benzene, naphthalene and anthracene radical-cations form a a-sandwich complex with the substrate but lack the ability to stabilise the a-intermediate so that radical-cation substrate reactions are not observed. The energy level diagram of Scheme 6.4 illustrates the influence of electron donating substituents in stabilising the Wheland type a-intermediate. 

With anisole, the SOMO/HOMO interaction (B) is strong, and with nitrobenzene the SOMO/LUMO interaction (A) is strong, but with benzene neither is stronger than the other. Product development control can also explain this, since the radicals produced by attack on nitrobenzene and anisole will be more stabilised than that produced by attack on benzene. However, this cannot be the explanation for another trend which can be seen in Table 7.1, namely that a p-nitrophenyl radical reacts faster with anisole and benzene than it does with nitrobenzene. This is readily explained if the SOMO of the p-nitrophenyl radical is lower in energy than that of the phenyl radical, making the SOMO/HOMO interactions (C and D) strong with the former pair. 

In nearly all cases where there is an alkyl group that is joined to an aromatic ring, the abstracted hydrogen will be on the a-carbon of the alkyl side chain, because such a bond is relatively weak and the resultant radical is stabilised by delocalisation of the unpaired electron around the aromatic ring. The aromatic ring is very rarely attacked to form an aryl radical. Thus, benzene reacts much more slowly to form the C6H- radical. Suggest why this is so. 

Bromination of alkyl side chains. Al-Bromosuccinimide (NBS) can brominate at the benzylic position via a radical chain mechanism (i.e. at the carbon atom attached to the benzene ring). It should be noted that the intermediate benzylic radical is stabilised by resonance (i.e. the radical can interact with the Jl-electrons of the benzene ring). 

Non-polar solvents, such as hexane and benzene, produce high yields of excited states via ion recombination, and relatively low yields of radical ions. In contrast, polar solvents like methanol, acetonitrile and water support high yields of radical ions with low excited state yields, due to solvation and stabilisation of the initial ions, particularly the electrons, leading to a slow rate of ion recombination. In intermediate polarity solvents, such as acetone, approximately equal amounts of radicals and excited states are generated. Hence, generally it is better to study solute excited states with pulse radiolysis in non-polar solvents and solute radicals or radical ions in polar solvents. This is often not possible due to insolubility in the preferred solvent, but if the transients are being monitored via transient absorption spectroscopy and they have high molar absorption coefiicients then low yields need not be problematic. 

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