: 200
Solvent choice is critical in determining whether SN1, SN2, E1, or E2 dominates.
Polar protic solvents (water, methanol, ethanol, acetic acid): Contain O-H or N-H bonds that can H-bond. They stabilise carbocations (ion-dipole) and leaving groups (H-bonding). Favour SN1/E1 pathways.
Polar aprotic solvents (DMSO, DMF, acetone, acetonitrile, THF): Lack O-H/N-H bonds. Dissolve ionic nucleophiles but don't solvate anions. Nucleophiles remain "naked" and highly reactive. Favour SN2.
Non-polar solvents (hexane, benzene, diethyl ether): Poor at stabilising charges. Generally not useful for ionic substitution reactions. Used for Grignard reagent preparation (ether coordinates to Mg).
Practical tip: Seeing "water" or "ethanol" in a JEE question usually signals SN1/E1. Seeing "DMSO," "DMF," or "acetone" signals SN2.