Nucleophilic Substitution Reactions of Haloalkanes (SN1 and SN2 Overview)

It is a reaction where a nucleophile replaces the halogen atom on a haloalkane. Example: CH3Br + OH⁻ → CH3OH + Br⁻. The OH⁻ (nucleophile) takes the place of Br. The word 'substitution' just means one group is swapped for another on the same carbon. This is the single most important reaction of haloalkanes for NEET.

The halogen (F, Cl, Br, I) is more electronegative than carbon, so it pulls the shared electrons toward itself. This makes the carbon slightly positive (δ+) and the halogen slightly negative (δ-). A nucleophile is electron-rich, so it is drawn to the electron-poor δ+ carbon. That is why the attack always happens at the carbon bearing the halogen.

A nucleophile is a species that brings a pair of electrons to attack a positive centre. NCERT says: 'A reagent that brings an electron pair to the reactive site is called a nucleophile (Nu:).' Common ones are OH⁻, CN⁻, NH3, RO⁻ (alkoxide), and H2O. Most carry a lone pair or a negative charge.

The leaving group is the atom or group that departs with the bonding electron pair. In haloalkanes it is the halide ion (Cl⁻, Br⁻, I⁻). A good leaving group is a weak base that is stable on its own. This is why I⁻ is the best leaving group and F⁻ the worst — I⁻ is the most stable halide ion.

Both are nucleophilic substitutions, but the path differs. SN2 is one concerted step: the nucleophile attacks as the halogen leaves (rate depends on both reactant concentrations). SN1 is two steps: first the halogen leaves to form a carbocation, then the nucleophile attacks (rate depends only on the substrate). The full comparison is a separate topic — see the SN1 vs SN2 page below.

Aqueous KOH supplies OH⁻ acting as a nucleophile — it substitutes the halogen and gives an alcohol (substitution). Alcoholic KOH supplies OH⁻ acting as a base — it removes H and X (dehydrohalogenation) and gives an alkene (elimination). Same reagent, different solvent, different job. NEET tests this exact contrast often.

Nucleophilic substitution is the general reaction. SN1 and SN2 are the two mechanisms (pathways) by which it can happen. All SN1 and SN2 reactions are nucleophilic substitutions, but you still choose which path a given haloalkane follows.

The carbon in the C-X bond is electron-poor (δ+), so it attracts electron-rich nucleophiles, not electrophiles. Electrophiles need an electron-rich site to attack, which haloalkanes do not offer at that carbon.

Iodides react fastest because the C-I bond is the weakest and I⁻ is the best leaving group. The order of reactivity is R-I > R-Br > R-Cl > R-F.

No, not easily. Haloarenes (like chlorobenzene) are much less reactive because of C-X bond resonance and the sp² carbon. They need harsh conditions or a benzyne pathway. This is a separate NEET topic.