SN2: one-step backside attack
SN2 (bimolecular nucleophilic substitution) is a one-step process where a nucleophile attacks the carbon bearing the leaving group from the back side (opposite to the leaving group). The C-X bond breaks and the C-Nu bond forms simultaneously. The nucleophile must have unhindered access to the backside, so SN2 is fastest for primary alkyl halides and slowest for tertiary, where the bulky groups around the carbon block attack. The reaction inverts the stereochemistry at the substituted carbon (Walden inversion).
SN1: two-step ionization via carbocation
SN1 (unimolecular nucleophilic substitution) is a two-step process. The C-X bond breaks first to form a carbocation intermediate, which is then attacked by a nucleophile in the second step. Carbocations are stabilized by alkyl groups, so SN1 is fastest for tertiary alkyl halides and slowest for primary. The nucleophile can attack from either face, so the reaction yields a mixture of inverted and retained stereochemistry (racemization). The carbocation may also rearrange to a more stable form before the nucleophile attacks, potentially producing rearranged products.
Predicting which mechanism dominates
SN2 is favored by primary substrates, strong nucleophiles, and polar aprotic solvents like DMSO or acetone. SN1 is favored by tertiary substrates, weak nucleophiles, and polar protic solvents like water or ethanol. Secondary substrates are ambiguous and can undergo either mechanism, often yielding a mixture of products. Leaving group ability also matters; better leaving groups (iodide, tosylate) favor ionization and SN1. Understanding the interplay of substrate, nucleophile, and solvent is essential for controlling substitution reactions and predicting products.