Structure and stability of free radicals

Free radicals are species that contain unpaired electrons. The octet rule notwithstanding, not all compounds have all of their electrons paired. Oxygen (O2) is the most familiar example of a compound with unpaired electrons it has two of them. Compounds that have an odd number of electrons, such as nitrogen dioxide (NO2), must have at least one unpaired electron. 

Nitrogen monoxide ( nitric oxide ) is another stable free radical. Although known for hundreds of years, NO has only recently been discovered to be an extremely important biochemical messenger and moderator of so many biological processes that it might be better to ask Which ones is it not involved in  

The free radicals that we usually see in carbon chemistry are much less stable than these. Simple alkyl radicals, for example, require special procedures for their isolation and study. We will encounter them here only as reactive intermediates, formed in one step of a reaction mechanism and consumed in the next. Alkyl radicals are classified as primary, secondary, or tertiary according to the number of carbon atoms directly attached to the carbon that bears the unpaired electron. 

The journal Science selected nitric oxide as its Molecule of the Year for 1992. 

Of the two extremes, experimental studies indicate that the planar sp model describes the bonding in alkyl radicals better than the pyramidal sp model. Methyl radical is planar, and more highly substituted radicals such as fert-butyl radical are flattened pyramids closer in shape to that expected for. sp -hybridized carbon than for sp.  

For more on the role of NO in physiology, see the boxed essay Oh NO It s Inorganic in Chapter 25. 

Bonding in methyl radical. Model (a) is more consistent with experimental observations. 

Of the two extremes, experimental studies indicate that the planar sp model describes the bonding in alkyl radicals better than the pyramidal sp model. Methyl radical is planar. 

Methyl and simple alkyl radicals are essentially planar with sp2 hybridization and the single electron in a p orbital, in contrast to the analogous silyl radicals which are pyramidal with approximately sp3 hybridization for all the valence orbitals three bonding and one singly occupied. The evidence is for this comes from ESR and stereochemistry studies. 

1 Coupling Constants for Radicals Centred on an Atom with a Nuclear Spin 

The interaction between the nuclear spin and the electron spin is expected to be proportional to the spin density of the unpaired electron at the nucleus of the central atom. Only s electrons have spin density at the nucleus (in fact this is the position of the highest value of the wave function i/0, whereas p electrons have a node at the nucleus corresponding to zero spin density. Thus the interaction between the nucleus and the electron, and thus the coupling constant, will be proportional to the s character of the orbital. The maximum coupling constants a for an electron in a 2s orbital in 13C and the 3s orbital in 29Si can be calculated to be 111 and 121 mT, respectively. For an electron in an sp3 orbital, the expected values will be 25% of these figures, i.e. 28 and 30 mT, respectively. For an electron in a p orbital, corresponding to sp2 hybridization of the central atom, the expected values of a will be zero in both cases. 

The near planarity of alkyl radicals and the non-planarity of silicon-centred radicals is supported by experiments on optically active compounds. In reaction (6.21) the chiral carbon centre in 25 is converted into a carbon-centred radical 26 with three different substituents, which in turn gives a chlorinated product 27 that has lost its optical activity. This implies that the radical 26 is either planar or, if pyramidal, it undergoes rapid inversion prior to capture to give the product. On the other hand, if the organosilane 28 is converted into 30 via the silicon-centred radical 29 (reaction 6.22), the product is optically active and can be shown to have retained its configuration, proving that, in this case, the intermediate radical has maintained its non-planar structure. 

Q The ESR spectrum of the benzene radical anion, [C6H(,], in I which the unpaired electron is in a molecular orbital delocalized round the benzene ring, shows a septet, with a coupling constant of 0.375 mT. Comment on the spectrum, in relation to the quartet I shown by the methyl radical, with n(C-H) = 2.30 mT. 

A radical is paramagnetic and so can be observed by electron spin resonance (esr) spectroscopy. Radicals are planar in configuration, but the energy difference between pyramidal and planar forms is very small. 

The stability of the radicals depends on the nature of the atom that is the radical centre and on the electronic properties of the groups attached to the radical. As in the case of carbocations, the order of stability of the free radicals is tertiary secondary primary methyl. This can be explained on the basis of hyperconjugation as in the case of carbocations. The stability of the free radicals also increases by resonance possibilities. Thus, benzylic and allylic free radicals are more stable and less reactive than the simple alkyl radicals. This is due to the delocalization of the unpaired electron over the Tr-orbital system in each case. 

The decreasing order of the stability of various radicals is as follows  

Benzyl Allyl Tertiary Secondary Primary Methyl 

The stability of a radical increases as the extent of potential delocalization increases. Therefore, Ph2CH is more stable than PhCH2, and PhsC is a reasonably stable radical. Adjacent functional groups, electron withdrawing or electron donating, both seem to stabilize radicals. 

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