Least reactive towards ${\mathrm{SN}}^1$ (or) ${\mathrm{SN}}^2$

Aryl halides & vinyl halides are least reactive towards nucleophilic substitution $\left({\mathrm{SN}}^1\left(\text{or}\right){\mathrm{SN}}^2\right)$

Reasons for non-reactivity of Aryl and vinyl halides 

(1) Stabilize through resonance 

(2) ${\mathrm{CH}}_2=\stackrel{+}{\mathrm{C}}\mathrm{H}$ (or) $\stackrel{+}{\mathrm{ph}}$ cannot stabilize through resonance

(3) C - X bond acquires double bond character due to resonance

(4) Difficult to remove ${\mathrm{X}}^{-}$ from more EN ${\mathrm{sp}}^2-$carbon

(5) ${\mathrm{CH}}_2=\mathrm{CH}-$group (or) ${\mathrm{C}}_6{\mathrm{H}}_5-$group itself is electron rich