91Ó°ÊÓ

An atom or group of atoms containing odd or unpaired electron is known as the free radical. The unpaired electron is represented by a single unpaired dot in the formula. Free radicals are electrically neutral. They are highly reactive species formed by homolytic fission of a covalent bond.

It may be defined as the amount of enthalpy required to break a bond homolytically in such a way that each bonded atom retains one of the bond’s two electrons.

Mathematically,

Δ H^o= ∑ (BDE of bond broken)-∑ (BDE of bond formed)

A single structural formula sometimes cannot explain all the properties of a compound that is given. In such cases, the compound may be represented by two or more structural formulae which differ from each other only in the arrangement of electrons. None of this structural formula alone can explain all the observed properties of the compound. The compound is then said to show resonance. The various structures are known as resonating structures. The true structure of the molecule is not represented by any of the resonating structures but is considered to be a resonance hybrid of the various resonating structures.

The greater bond dissociation enthalpy implies that the radical is less stable.

(most stable) (least stable)

radicals in decreasing order of stability

The resonance structures of benzylic radicals are also shown.

resonance forms of benzylic radical