Halogen Derivatives Notes for Class 12, IIT JEE & NEET

Haloalkanes have halogen atoms attached to sp³ carbon atoms in an aliphatic (non-aromatic) chain, such as CH₃Cl or C₂H₅Br. Haloarenes have halogen atoms attached to sp² carbon atoms in an aromatic ring, such as chlorobenzene (C₆H₅Cl). The C–X bond in haloarenes is shorter and stronger due to partial double bond character from resonance, making haloarenes far less reactive towards nucleophilic substitution than haloalkanes.

The highest-priority reactions for JEE are: SN1 and SN2 nucleophilic substitution (including stereochemistry and solvent effects), elimination vs substitution competition, Grignard reagent preparation and reactions with aldehydes/ketones/CO₂, the Finkelstein and Sandmeyer reactions, and Wurtz reaction for alkane synthesis. The SN1/SN2 mechanistic distinction — including carbocation stability, Walden inversion, and solvent polarity effects — is the single most tested concept.

 In nucleophilic substitution reactions, the reactivity order is RI > RBr > RCl > RF. This is because the leaving group ability follows the same order — iodide (I⁻) is the best leaving group as it forms the weakest C–I bond (despite being the largest halide). Fluoride (F⁻) is the worst leaving group because the C–F bond is the strongest.

SN1 occurs with tertiary alkyl halides, weak nucleophiles, polar protic solvents (water, alcohols), and at higher temperatures. It proceeds via a carbocation intermediate, giving a racemic mixture. SN2 occurs with primary alkyl halides (or methyl halides), strong nucleophiles (OH⁻, CN⁻, I⁻), polar aprotic solvents (DMSO, acetone), and at lower temperatures. It is a concerted one-step reaction with Walden inversion of configuration.

The Wurtz reaction involves treating an alkyl halide with sodium metal in dry ether to produce a higher alkane: 2R–X + 2Na → R–R + 2NaX. Its limitation is that it only works efficiently with two identical alkyl halides. When two different alkyl halides are used, a mixture of three products forms (R–R, R'–R', and R–R'), making it unsuitable for synthesising unsymmetrical alkanes.