Nucleophilic Substitution and Elimination Mechanisms
Organic Chemistry mechanism questions in JEE Advanced require deep understanding of SN1, SN2, E1, and E2 pathways — including predicting which mechanism operates under given conditions and the resulting products and stereochemistry.
SN2 (bimolecular nucleophilic substitution): one-step, backside attack by the nucleophile simultaneously with leaving group departure. Rate = k[RX][Nu]. Favoured by: primary substrates (unhindered), strong nucleophiles (Br⁻, I⁻, CN⁻, RS⁻), polar aprotic solvents (DMSO, DMF, acetone), good leaving groups. Stereochemistry: Walden inversion — configuration inverts (R → S or S → R) at the stereocentre.
SN1 (unimolecular): two-step — leaving group departs forming a carbocation, then nucleophile attacks. Rate = k[RX]. Favoured by: tertiary substrates (stable carbocations), polar protic solvents (water, alcohols), weak nucleophiles. Stereochemistry: racemisation (nucleophile attacks planar carbocation from either face).
E2 (bimolecular elimination): concerted, anti-periplanar geometry required (H and leaving group must be 180° dihedral). Rate = k[RX][Base]. Favoured by: strong, bulky bases (t-BuOK), high temperature, secondary/tertiary substrates. Zaitsev's rule: more substituted alkene (Zaitsev product) forms predominantly. Bulky base gives Hofmann product (less substituted alkene) because it cannot reach the hindered β-H.
E1 (unimolecular elimination): involves same carbocation as SN1, then base removes β-H. Competes with SN1. Higher temperature favours E over S. Favoured conditions overlap with SN1.
Competition summary: primary + strong nucleophile + polar aprotic → SN2. Tertiary + polar protic solvent → SN1/E1. Secondary substrate + strong base → E2. Identify key conditions: substrate class, solvent polarity (protic/aprotic), nucleophile/base strength, and temperature.