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Reaction / Rate Law:
$$\text{Rate} = k[\text{R--X}][\text{Nu}^-]$$
$$\text{Nu}^- + \text{R--X} \to [\text{Nu}\cdots\text{C}\cdots\text{X}]^\ddagger \to \text{Nu--R} + \text{X}^-$$
Concept: A concerted, single-step mechanism passing through a five-coordinate transition state. The nucleophile attacks from the back side (opposite to the leaving group), forcing a complete inversion of stereochemical configuration (Walden Inversion).
Reactivity Order: $$\text{CH}_3\text{X} > 1^\circ > 2^\circ > 3^\circ$$ (governed by steric hindrance).
Solvent Effect: Favored by polar aprotic solvents (e.g., DMSO, DMF, Acetone) which solubilize cations without solvating nucleophiles, leaving nucleophiles naked and highly reactive.
