SN2

Meaning: In an SN2 reaction the nucleophile attacks the electrophilic carbon from the side opposite the leaving group (backside attack) while the leaving group departs at the same time. There is one step, one transition state with partial bonds to both the nucleophile and the leaving group, and no intermediate. A stereocenter is therefore inverted (Walden inversion). Crowding around the carbon raises the transition state energy, so reactivity runs methyl > primary > secondary, and tertiary carbons essentially do not react by SN2.

Equation: rate = k[R–X][Nu] where k is the second-order rate constant (M⁻¹ s⁻¹), [R–X] is the substrate concentration and [Nu] is the nucleophile concentration (both mol/L).

On the MCAT: Passages give rate data in which doubling either reactant doubles the rate, which points to SN2. You may be asked to choose conditions that favor it: a strong, unhindered nucleophile, a good leaving group (I⁻, Br⁻, Cl⁻, tosylate, but not OH⁻), a polar aprotic solvent (DMSO, acetone, DMF), and a methyl or primary substrate. You may also be asked to draw the product with inverted stereochemistry.

Common confusion: Inversion describes the spatial arrangement, not the label. An R reactant often gives an S product, but the CIP priorities can change, so assign the label again. Do not mix this up with SN1, which has two steps, goes through a carbocation, gives racemization, favors tertiary substrates and polar protic solvents, and has a rate that depends only on the substrate.