SN1

Meaning: In the slow first step the leaving group departs, forming a planar carbocation. In the fast second step the nucleophile attacks it. The rate tracks carbocation stability, so tertiary, allylic and benzylic substrates react well, while methyl and primary substrates typically do not. Polar protic solvents favor SN1 by stabilizing the ions. The nucleophile can attack either face, so a chiral center typically gives a mixture of enantiomers (racemization). Carbocations may rearrange by hydride or alkyl shifts.

Equation: rate = k[R–X], where k is the first-order rate constant (s⁻¹) and [R–X] is the substrate concentration (mol/L). The nucleophile does not appear.

On the MCAT: Passages give a substrate, solvent and nucleophile and ask you to choose SN1 or SN2. Others show a reaction coordinate diagram with two peaks and an intermediate valley between them, the first peak higher. Doubling the substrate typically doubles the rate; doubling the nucleophile does not change it. Stereochemistry questions expect racemization.

Common confusion: Unimolecular does not mean one step. SN1 has two steps, and the 1 refers to the single species in the rate-determining step. A strong nucleophile does not speed SN1; with an unhindered substrate it typically pushes the reaction toward SN2.