Question

What is meant by radical? And how it forms?

Answer 1

`"A radical species is a residue with a single unpaired electron......."`

Explanation:

Radicals are especially reactive in that upon reaction, they generate ANOTHER radical to continue the chain of reaction. Radical halogenation of alkanes is the classic organic reaction, which is usually accomplished by the use of UV light on bromine........

These are usually separated into intitiation, propagation, and termination steps:

`i.` `X_2 + hnu rarr 2dotX`

`hnu` represents UV light.............

`ii.` `dotX + H_3C-Ar rarr XH + H_2dotC-Ar`

`iii.` `H_2dotC-Ar + X_2 rarr H_2XC-Ar + dotX`

And thus formation of the product, propagates the reaction; i.e. the mechanism generates another reactive halogen radical........

The reaction finally terminates with the coupling of two radical species, for instance:

`2H_2dotCAr rarr ArCH_2CH_2Ar`

or by the coupling of the two halide radicals...........

`2dotClrarrCl-Cl`.

The presence of such `C-C` coupled products is good evidence in support of the radical mechanism (`C-C` coupling reactions are VERY RARE in organic chemistry, and their occurrence is quite a big deal).