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SN1 (substitution nucleophilic unimolecular) reactions proceed through a two-step mechanism: rate-limiting ionization of the substrate to produce a planar carbocation intermediate, followed by rapid nucleophilic attack from either face. Because the carbocation is sp2 hybridized and planar, attack from both faces is equally probable, producing a racemic mixture at the former stereocenter. SN1 favors tertiary > secondary substrates (reflecting carbocation stability), polar protic solvents (which stabilize ions through solvation), and weak nucleophiles. Carbocation rearrangements (hydride and methyl shifts) can complicate product prediction.
Draw energy-level diagrams for SN1 with two transition states flanking the carbocation intermediate, then compare with the single-transition-state SN2 diagram. Practice the four-factor analysis to choose between SN1 and SN2 under given conditions.
SN1 stands for Substitution Nucleophilic Unimolecular — the "unimolecular" label tells you the most important thing: the rate-limiting step involves only one molecule, the substrate. This contrasts sharply with SN2, where the nucleophile attacks at the same moment the leaving group departs. In SN1, the reaction happens in two separate steps: first the substrate ionizes to form a carbocation intermediate, then the nucleophile attacks. Because the nucleophile is not involved in the slow step, doubling its concentration does not speed up the reaction.
The key to understanding SN1 is carbocation stability. When the leaving group departs, it takes both electrons from the C–LG bond, leaving the carbon with only three bonds and a positive charge. That carbon becomes sp2-hybridized and planar. Alkyl groups stabilize carbocations by donating electron density through hyperconjugation and induction, so tertiary carbocations (three alkyl groups) are far more stable than secondary, which are far more stable than primary. This is why SN1 is practical only for tertiary — and some secondary — substrates: the carbocation intermediate for primary substrates is so unstable it barely forms.
Once the planar carbocation exists, the nucleophile can attack from either face with roughly equal probability — there is no longer a defined "front" or "back" face the way there is in SN2. This is why SN1 reactions at a stereocenter produce a mixture of enantiomers (racemization). In reality, the departing leaving group lingers as a solvent-caged ion pair and briefly blocks one face, so you typically see mostly racemized product with a small excess of the inverted stereoisomer rather than a perfect 50/50 split.
An additional complication is carbocation rearrangement. Once the carbocation forms, it can shift to a more stable structure via a 1,2-hydride shift or 1,2-methyl shift before the nucleophile attacks. This means the nucleophile sometimes bonds to a carbon that was not the original reaction site, yielding rearranged products. Whenever you draw a proposed SN1 mechanism, always check: is the intermediate carbocation adjacent to a hydrogen or methyl group on a carbon that would give a more stable carbocation after the shift? If yes, expect rearrangement.
Polar protic solvents (water, alcohols) are ideal for SN1 because they stabilize the charged species through hydrogen bonding and ion-dipole interactions. Weak nucleophiles are fine — and sometimes preferred — because strong nucleophiles would compete via SN2 or E2. The full decision tree for predicting substitution mechanisms always considers four factors together: substrate structure, nucleophile strength, solvent, and leaving group quality. SN1 wins when the substrate can form a stable carbocation and the conditions do not favor concerted attack.