SN2Nukleophilsubstitution

SN2 nucleophiles participate in bimolecular nucleophilic substitution reactions, in which a nucleophile attacks an electrophilic carbon bearing a leaving group in a single concerted step, displacing the leaving group. The reaction rate is second order, depending on both nucleophile and substrate: rate = k [Nu−][RX]. SN2 is favored by strong, unhindered nucleophiles and by substrates with little steric hindrance, typically methyl and primary alkyl halides; secondary halides react more slowly, and tertiary halides are generally not subject to SN2 due to steric hindrance and instead favor SN1 or elimination.

Nucleophile requirements: a good SN2 nucleophile is typically a charged, reactive anion or a strongly donating neutral molecule. Common SN2 nucleophiles include hydroxide, alkoxides, cyanide, azide, thiolates, and halide ions (Cl−, Br−, I−) in appropriate solvents. In polar aprotic solvents (such as DMSO, DMF, acetonitrile), anions remain largely unsolvated and are highly reactive nucleophiles; in protic solvents, smaller anions are heavily solvated and less nucleophilic, though larger, softer nucleophiles can still perform SN2 effectively.

Leaving groups: effective SN2 substrates have good leaving groups; iodide, bromide, and chloride are common, with tosylates and related sulfonate esters being highly favorable leaving groups in many contexts.

Stereochemistry: when the carbon undergoing substitution is chiral, SN2 proceeds with backside attack, causing inversion of configuration. Competing pathways include E2 elimination or SN1, depending on substrate, base, solvent, and temperature.

Examples: CH3Br with CN− gives CH3CN; benzyl chloride with thiolate yields benzyl thioether; primary alkyl chlorides with hydroxide yield alcohols.