Free Radicals

Free Radicals

A free radical is a species which has one or more unpaired electrons. In the species where all electrons are paired the total magnetic moment is zero. In radicals, however, since there are one or more unpaired electrons. There is a net magnetic moment and the radicals as a result are paramagnetic. Free radicals are usually defected by electron spin resonance, which is also termed electron paramagnetic resonance.

Simple alkyl radicals have a planar (trigonal) structure i.e., these have sp2 bonding with the odd electron in a p orbital. The pyramidal structure is another possibility when the bonding may be sp3 and the odd electron is in an sp3 orbital. The planar structure is in keeping with loss of activity when a free radical is generated at a chiral centre. Thus, a planar radical will be attacked at either face after its formation with equal probability to give enantiomers unlike carbocations, the free radicals can be generated at bridge. This shows that pyramidal geometry for radicals is also possible and that free radicals need to be planar

Stability

As in the case of carbocation, the stability of free radicals is tertiary > secondary > primary and is explained on the basis of hyperconjugation. The stabilizing effects in allylic radicals and benzyl radicals is due to vinyl and phenyl groups in terms of resonance structures. Bond dissociation energies shows that 19 kcal/mol less energy is needed to form the benzyl radicals from toluene than the formation of methyl radical from methane. The triphenyl methyl type radicals are no doubt stalbilized by resonance, however the major cause of their stability is the steric hindrance to dimerization.

Ease of formation of alkyl free radicals, benzyl > 30 > 20 > 10 > > Vinyl

ELECTRONIC DISPLACEMENT IN COVALENT BONDS

The following four types of electronic effects operates in covalent bonds

• Inductive effect            
• Electromeric effect
• Resonance and mesomeric effect    
• Hyperconjugation