SN1

SN1, or substitution nucleophilic unimolecular, is a class of nucleophilic substitution reactions in which the rate-determining step is the loss of the leaving group to form a carbocation intermediate. The overall rate depends on the substrate concentration, not the nucleophile, giving a rate law of rate = k[substrate]. Nucleophile strength has little effect on the rate, though it determines the final product after carbocation formation.

Mechanism: The reaction proceeds by two steps. First, the leaving group departs, generating a carbocation. Second, the nucleophile attacks the planar carbocation to give the substitution product. The ionization step is favored by stable carbocations (tertiary, benzylic, allylic) and by good leaving groups. Polar protic solvents stabilize the charged intermediates and often participate as nucleophiles in solvolysis.

Stereochemistry and rearrangements: Because the carbocation is planar, attack occurs from either side, leading to racemization at a stereocenter. Carbocation rearrangements, such as hydride or alkyl shifts, can occur prior to capture by the nucleophile, altering the carbon skeleton. Resonance stabilization (benzylic, allylic) further promotes SN1.

Scope and context: SN1 is typical for tertiary alkyl halides and certain leaving-group derivatives (such as tosylates) in polar protic solvents, such as water or alcohols. It competes with E1 elimination, especially at higher temperature. By contrast, SN2 is bimolecular, favored by primary substrates, and proceeds with backside attack and inversion of configuration. Examples include tert-butyl chloride reacting with water to form tert-butanol.