Q.Toluene --Cl2/hv--> A. 'A' is

Free-Radical Halogenation of Alkanes

Alkanes are largely inert, but they do undergo halogenation (substitution of an H by a halogen) with Cl2 or Br2 under light or heat -- fluorination is too violent to control and iodination too slow to be useful. The reaction proceeds by a three-stage FREE-RADICAL CHAIN MECHANISM, illustrated for methane + chlorine:

Initiation: light (or heat) homolytically cleaves the weak Cl-Cl bond into two chlorine radicals (the weakest bond present is always the one that breaks; C-H and C-C bonds are much stronger).

Propagation (a self-sustaining two-step cycle): (a) a chlorine radical abstracts an H from the alkane, generating an alkyl radical and HCl; (b) that alkyl radical attacks a fresh Cl2 molecule, giving the haloalkane product and regenerating a chlorine radical, which re-enters step (a) -- so one initiation event can drive thousands of product-forming cycles. …