Smaller the R group reactivity will be higher towards S_N2 reaction.For alkyl halides containing similar alkyl group better will be the leaving group, more facile is the nucleophilic substitution reaction.

Amongst the halide ions, the order in which the leaving groups depart follows the sequence :

I^– > Br^– > Cl^– > F^–

It is because of this reason that the order of reactivity of haloalkanes follows the sequence:

iodoalkanes > bromoalkanes> chloroalkanes > fluoroalkanes

In the first case the reaction gives side chain substitution product which is easier in A. In the second case the reaction will proceed by benzyne mechanism for which ortho position w. r. to Cl must have H-atoms.