SN2

SN2, or nucleophilic substitution bimolecular, is a reaction in which a nucleophile attacks a carbon bearing a leaving group from the opposite side in a single concerted step, displacing the leaving group and forming a new bond. No discrete carbocation intermediate is formed.

Because the reaction involves both substrate and nucleophile in the transition state, the rate law is second order: rate = k [substrate][nucleophile]. The mechanism is concerted, so the process is stereospecific and no intermediates are required.

In a chiral center, backside attack leads to inversion of configuration (Walden inversion). The reaction is thus stereospecific, with the outcome dependent on the approach of the nucleophile.

Substrate scope is heavily influenced by steric hindrance. Methyl and primary alkyl halides react readily; secondary halides can react with good nucleophiles; tertiary substrates react slowly or not at all in SN2. Leaving group ability follows I– > Br– > Cl– > F–; good leaving groups include tosylate, mesylate, and triflate.

Solvent and nucleophile effects also shape SN2. Polar aprotic solvents (for example DMSO, DMF, acetonitrile) stabilize cations while leaving nucleophiles relatively unsolvated, enhancing SN2. Strong, negatively charged nucleophiles such as cyanide, alkoxides, thiolates, and azide promote SN2. In protic solvents, nucleophilicity trends mirror solvation effects, with larger halides often better nucleophiles than fluoride.

SN2 contrasts with SN1, which proceeds via a carbocation and is favored by tertiary substrates and polar protic solvents. SN2 generally avoids rearrangements and is often used when stereochemical outcomes and avoidance of carbocation intermediates are important. Typical examples include substitution of alkyl halides with CN–, OH–, or SH– to form nitriles, alcohols, or thioethers.