Class 12 Chemistry Chapter 6 Haloalkanes and Haloarenes – Revision Notes

Quick revision notes for Class 12 Chemistry Chapter 6 – Haloalkanes and Haloarenes, covering classification, nomenclature, preparation, substitution mechanisms, and polyhalogen compounds. Ideal for last-minute board exam revision.

Last Updated: September 23, 2026

Classification and Nomenclature

Halides are classified by the carbon bearing the halogen: alkyl (sp³ carbon, not adjacent to a ring/double bond), allylic (sp³ carbon adjacent to C=C), benzylic (sp³ carbon adjacent to an aromatic ring), vinylic (directly on an sp² C=C carbon), and aryl (directly on an aromatic ring carbon). Alkyl halides are further classed as 1°, 2° or 3° depending on the number of carbon groups attached to the C–X carbon.

Preparation and Physical Properties

Common preparation routes include: alcohols+HX/PX₃/PX₅/SOCl₂; alkenes+HX or X₂; free-radical halogenation of alkanes; and halide exchange (the Finkelstein reaction, RCl+NaI→RI+NaCl in dry acetone). Alkyl halides are polar but immiscible with water (no hydrogen bonding possible with water), and their boiling points and densities rise with the number and atomic mass of halogens present (I>Br>Cl>F). Dipole moment falls as more halogens are added to the same carbon due to increasing cancellation by symmetry (CH₃Cl>CH₂Cl₂>CHCl₃>CCl₄≈0).

Nucleophilic Substitution: S_N1 vs S_N2

S_N2: single-step, concerted backside attack; rate depends on both substrate and nucleophile concentration; causes inversion of configuration (Walden inversion); favoured by unhindered substrates (methyl>1°>2°>3°, reactivity decreasing with branching). S_N1: two-step, via a planar carbocation intermediate; rate depends only on substrate concentration; typically gives a racemic mixture; favoured by substrates forming stable carbocations (3°>2°>1°, and benzylic/allylic are especially favoured due to resonance stabilisation). Leaving-group ability follows I⁻>Br⁻>Cl⁻>F⁻. Elimination (E1/E2) competes with substitution, generally following Zaitsev’s rule (more substituted alkene favoured); aqueous KOH favours substitution, hot alcoholic KOH favours elimination.

Haloarenes

Haloarenes (e.g. chlorobenzene) are much less reactive towards nucleophilic substitution than haloalkanes, because the halogen’s lone pair is delocalised into the ring by resonance (giving the C–X bond partial double-bond character, shorter and stronger) and the sp² carbon resists backside attack. This same resonance donation makes halogens weakly deactivating but ortho/para-directing in electrophilic aromatic substitution.

Polyhalogen Compounds

Key uses (syllabus scope is limited to uses/environmental effects, not preparation): Freon-12 (refrigerant, aerosol propellant; ozone-depleting), DDT (insecticide; banned in most countries due to bioaccumulation), carbon tetrachloride (former solvent; can form toxic phosgene), and iodoform (former antiseptic, action due to liberated iodine).

One-Line Summary

Chapter 6 builds a complete picture of haloalkane and haloarene chemistry — from classification and preparation through the competing S_N1/S_N2 substitution mechanisms and their stereochemical consequences, to why haloarenes resist substitution, closing with the practical uses and environmental concerns of key polyhalogen compounds.

Written by Satish

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