SN1, SN2, E1, E2: Mechanism Selection and Product Prediction
Four substitution and elimination pathways — SN1, SN2, E1, E2 — are among the highest-frequency JEE Advanced organic chemistry topics. Selecting the correct mechanism requires evaluating three factors simultaneously: substrate structure, nucleophile/base strength, and solvent polarity. SN2: favoured by primary substrates, strong nucleophiles (OH⁻, CN⁻, I⁻), and polar aprotic solvents (DMSO, acetone) — produces inversion of configuration (Walden inversion). SN1: favoured by tertiary (and secondary) substrates, weak nucleophiles, and polar protic solvents (water, ethanol) — proceeds via a planar carbocation intermediate, producing racemisation. E2: favoured by strong, bulky bases (t-BuO⁻), anti-periplanar geometry requirement — Zaitsev's rule predicts the more substituted alkene unless a bulky base favours the Hofmann product. E1: unimolecular elimination, same carbocation intermediate as SN1, proceeds in polar protic solvents with weak bases — competes with SN1. JEE Advanced specifically tests 'competing pathway' questions: given a substrate and reagent, predict whether SN1/SN2/E1/E2 dominates and what the major product is. Methyl and primary substrates: SN2 dominates. Tertiary substrates with strong base: E2 dominates. Tertiary substrates with weak nucleophile/heat: E1 competes with SN1.