The $S_{N}2$ (Substitution Nucleophilic Bimolecular) mechanism involves a simultaneous bond-breaking and bond-forming event via a transition state.

• The nucleophile attacks from the back side (opposite to the leaving group), leading to an Inversion of Configuration (Walden Inversion).
• Because the nucleophile must approach the carbon center, the reaction is extremely sensitive to steric hindrance.
• Reactivity Order: $C{H}_{3}X>1^{\circ }>2^{\circ }\gg 3^{\circ }$.
• Primary halides are least crowded, offering less resistance to the incoming nucleophile, and thus react most rapidly.