Video narration
How can one person leave a doorway as another takes their place from behind, in a single instant? That is SN2. The nucleophile attacks the carbon from the side opposite the leaving group: backside attack. As it enters, the leaving group exits at the same time. Not two stages-a concerted, one-step process. The new C-Nu bond forms while the old C-X bond breaks, so the reacting carbon flips configuration: Walden inversion. And because both reactants participate, Rate = k[R-X][Nu−]. SN2 favors less crowded substrates: methyl, then primary, then secondary. Tertiary is usually extremely slow because bulky groups block backside approach. A strong nucleophile, a good leaving group-iodide, then bromide, then chloride-and a polar aprotic solvent speed the reaction. So why the doorway swap? The nucleophile enters from behind as the leaving group exits, in one step, causing inversion; the rate depends on both concentrations.