Explain the mechanism of nucleophilic bimolecular substitution (SN²) reaction with one example?

In this rate of reaction depends upon concentration of both alkyl halide and nucleophile.
Eg: CH₃Cl reacts with OH^– ion to form CH₃OH and Cl^–1 ion
${\mathrm{CH}}_3\mathrm{Cl}+{\mathrm{OH}}^{-}\to {\mathrm{CH}}_3\mathrm{OH}+{\mathrm{Cl}}^{-}$
mechanism - this takes place in one step. 
In SN² reaction the nucleophile attacks carbon from back side of leaving halogen. During this reaction complete inversion product is formed.
Equation :


Rate ∝ [alkylhalide] [nucleophile]