nucleophilic substitution
Summary
Nucleophilic substitution replaces a leaving group on an electrophilic carbon with a nucleophile. It runs by SN2 (one concerted step, inversion, rate depends on both reactants) or SN1 (carbocation intermediate, racemization, rate depends on substrate only).
Detail
Meaning: A nucleophile (electron-pair donor) attacks an electrophilic carbon and a leaving group departs. SN2 is one concerted step with backside attack, giving inversion; it is favored by methyl or primary carbons, strong nucleophiles and polar aprotic solvents. SN1 loses the leaving group first, forming a planar carbocation that the nucleophile attacks from either face, giving racemization; it is favored by tertiary carbons, weak nucleophiles and polar protic solvents. Good leaving groups are weak bases such as I⁻, Br⁻ and tosylate.
Equation: SN2: rate = k[RX][Nu]; SN1: rate = k[RX]. Rate is in M/s, k is the rate constant (M⁻¹s⁻¹ for SN2, s⁻¹ for SN1), and [RX] and [Nu] are substrate and nucleophile concentrations in M (mol/L).
On the MCAT: Given a substrate, nucleophile and solvent, choose the mechanism, predict how the rate changes when a concentration doubles, or give the product stereochemistry. Energy diagrams show one barrier for SN2 and two barriers with an intermediate for SN1.
Common confusion: Inversion describes geometry, not the label. R/S usually flips in SN2 but can stay the same if priorities change, so reassign them. Tertiary halides are too crowded for SN2.
Sources
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.