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Narrative
In S_N2 reactions the nucleophile attacks the alkyl halide from the backside of (side opposite) the leaving group.

Discussion
In S_N2 (substitution nucleophilic second order) reactions the nucleophile attacks the alkyl halide from the side opposite the leaving group by a direct displacement leading to inversion of configuration. This reaction is very sensitive to steric hindrance and is fastest for primary halides. With tertiary halides, approach of the nucleophile from the backside is blocked.