Why are aryl halides less reactive towards nuclophilic substituition reaction than alkyl halides? How can be enhance the reactivity of aryl halides?

Explanation

In haloarenes, the `sp^(2)`- hybridised carbon with a greater s-character is more electronegative and can hold the electron pair of C-X bond more tightyl than `sp^(3)`-hybridised carbon in haloalkane with lesser s-character.
Thus, the C-X bond of haloarenes is less than that of haloalkanes. Polarity of C-X bond of haloarenes is further decreased due to involvenment of lone pair of electrons of halogen atom in delocalisation with the `pi`-electron system of the aromatic ring.

Due to such delocalisation, the C-X bond in haloarenes has parial double bond character, thereby making the C-X bond stronger. In a nucleophilic substitution reaction the approaching nucleophilie is also repelled by the `pi`- electron system of the aromatic ring. Thus aryl halides are less reactive towards nucleophilic substitution reaction than alkyl halides. the reactivity of aryl halides can be enhanced by the introduction of electron-withdrawing -R groups (e.g., `-NO_(2), -CN`) at ortho or para positions with respect to the halogen atom. In presence of such groups, the stability of anionic intermediate increases and hence, the reaction - rate increases.