Haloalkanes and Haloarenes covers organic compounds where a halogen is bonded to an sp3 alkyl carbon or an sp2 aromatic carbon, including their classification, IUPAC naming, the polar C-X bond, and methods of preparation. The heart of the chapter is reaction mechanisms, namely nucleophilic substitution by the SN1 and SN2 routes, elimination giving alkenes by the Saytzeff rule, and why aryl halides resist substitution. It is a high-yield NEET chapter because mechanism, stereochemistry, and reactivity-order questions appear almost every year.
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Classification & Nomenclature
Halogen on an sp3 carbon makes a haloalkane; on an sp2 ring carbon, a haloarene.
Halogen is a ‘halo-‘ prefix; number the chain for the lowest locant set.
- By carbon type: 1°, 2°, 3° (X on C bonded to 1, 2, 3 other carbons)
- Allylic/benzylic = X next to C=C / benzene → very reactive
- Vinylic/aryl = X on sp2 C → unreactive; this split drives the chapter
The Polar C–X Bond
X is more electronegative than C, so carbon is δ+ and is the site nucleophiles attack.
Down the group bond length increases and strength decreases; C–I breaks easiest.
- C–I weakest → iodides are the most reactive
- Insoluble in water (no strong H-bonds), soluble in organic solvents
- Same R: b.p. R–I > R–Br > R–Cl > R–F; branching lowers b.p.
Making Haloalkanes
Alcohols are the most common starting point for haloalkanes.
SOCl2 is best — by-products escape as gases, giving pure RCl.
- With HX: reactivity HI > HBr > HCl (HCl needs ZnCl2 / Lucas reagent)
- Also PCl3 and PCl5 convert ROH to RCl
- Alkene + HX follows Markovnikov; HBr + peroxide → anti-Markovnikov (Kharasch)
Finkelstein & Swarts
Halogen-exchange reactions swap one halide for another.
NaCl precipitates in acetone, pulling Finkelstein forward.
- Finkelstein: R–Cl/R–Br + NaI in dry acetone → R–I
- Swarts: R–Br + AgF → R–F + AgBr (route to fluorides)
- Haloarenes: arene + Cl2/Br2 with anhydrous FeCl3 (Lewis acid)
SN2 Substitution
A nucleophile attacks the back side in one concerted step as X leaves.
Back-side attack flips the centre → inversion (Walden inversion).
- Reactivity 1° > 2° > 3° (least steric hindrance wins)
- Gives complete inversion of configuration
- Favoured by strong nucleophiles and polar aprotic solvents
SN1 Substitution
The C–X bond ionises first to a carbocation, then the nucleophile is captured.
Planar carbocation is attacked from both faces → racemic mixture.
- Reactivity 3° > 2° > 1° (more stable carbocation wins)
- Gives partial racemisation, not clean inversion
- Favoured by polar protic solvents; allylic/benzylic react fast
Elimination (β-Elimination)
A strong base pulls off H and X from adjacent carbons to form an alkene.
The favourite NEET trap: ‘alcoholic = alkene’, ‘aqueous = substitution’.
- Saytzeff (Zaitsev): more substituted, more stable alkene is the major product
- 2-bromobutane + alc. KOH → mainly but-2-ene
- Reactivity 3° > 2° > 1°; bulky base + 3° + heat favours elimination
Haloarenes: Low Reactivity
Aryl halides resist nucleophilic substitution far more than alkyl halides.
Phenol from chlorobenzene needs ~623 K and 300 atm (forcing conditions).
- Resonance + sp2 carbon hold the C–X bond tighter (stronger, shorter)
- In EAS, halogen is deactivating but o,p-directing
- –NO2 at ortho/para boosts reactivity (addition-elimination)
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- Alcohols, Phenols and Ethers Class 12 Notes
- Aldehydes, Ketones and Carboxylic Acids Class 12 Notes
- Amines Class 12 Notes
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Frequently Asked Questions
Haloalkanes are compounds in which one or more halogen atoms are bonded to an sp3 hybridised carbon of an alkyl group, while haloarenes have a halogen bonded directly to an sp2 carbon of an aromatic ring. They are formed by replacing hydrogen atoms of hydrocarbons with halogen atoms.
SN2 is a single-step bimolecular reaction with rate proportional to both the halide and the nucleophile, and it gives inversion of configuration, favoured by primary halides. SN1 is a two-step unimolecular reaction with rate depending only on the halide, going through a carbocation and giving a racemic mixture, favoured by tertiary halides.
Alcoholic KOH causes beta-elimination (dehydrohalogenation) to give an alkene, while aqueous KOH causes nucleophilic substitution to give an alcohol. This contrast is a very common NEET trap and the Saytzeff rule decides the major alkene in elimination.
In haloarenes the halogen lone pair delocalises into the ring by resonance, giving the C-X bond partial double-bond character so it is shorter and stronger. The sp2 carbon is also more electronegative and holds the bonding electrons tightly, so aryl halides resist nucleophilic substitution and need very harsh conditions.
Yes, it is part of the NEET Class 12 organic chemistry syllabus and is a reliably high-yield chapter. Questions on SN1 versus SN2, reactivity order of halides, Saytzeff elimination, and named reactions like Finkelstein and Swarts appear frequently.