Haloalkanes and Haloarenes Class 12 Chemistry Notes

Haloalkanes and haloarenes are hydrocarbons with one or more halogen atoms replacing hydrogen. They are classified by the number of halogens (mono-, di-, tri-, poly-) and by the nature of the carbon bound to the halogen (1°, 2° or 3°).

• By number of halogens: monohaloalkanes (CH₃Cl), dihaloalkanes (CH₂Cl₂), trihaloalkanes (CHCl₃), tetrahaloalkanes (CCl₄).
• By carbon class: primary (1°), secondary (2°), tertiary (3°) haloalkanes depending on the carbon carrying X.
• Vicinal vs geminal dihalides: two halogens on adjacent carbons vs the same carbon.
• Haloarenes: halogen bonded directly to the aromatic ring (aryl halide).

MCQ Questions

• From alcohols: R–OH + HX → R–X + H₂O (reactivity HCl < HBr < HI; with ZnCl₂ for 1° alcohols); or + PCl₅/PCl₃/SOCl₂ (purity, the SOCl₂ route leaves only HCl gas and SO₂).
• From alkenes: HX addition follows Markovnikov (2-bromopropane from propene); HBr + peroxide gives anti-Markovnikov 1-bromopropane.
• From alkanes: free-radical halogenation (useful for arenes via chlorination).
• Halogen exchange: Finkelstein (R–Cl + NaI/acetone → R–I) and Swarts (R–Br + AgF → R–F).
• From diazonium salts (arenes): Sandmeyer reaction (CuCl/CuBr) or KI provides haloarenes from aniline via benzene diazonium chloride.

MCQ Questions

• Boiling points: increase with molecular mass and number of halogens; for isomers, more branching → lower boiling point (less surface area).
• Density: haloalkanes are denser than water (halogens are heavy); density rises down the halogen family.
• Solubility: poorly soluble in water (no hydrogen bonding to X), freely soluble in organic solvents.
• C–X bond: polarised (δ⁺C–δ⁻X); polarity order C–F > C–Cl > C–Br > C–I; bond dissociation enthalpy decreases down the group: C–F > C–Cl > C–Br > C–I.

Halogen bonded to sp² carbon (vinyl, aryl) is held more tightly than on sp³ carbon; this is one reason haloarenes are sluggish in substitution.

MCQ Questions

• Reactivity: SN2 — 1° > 2° > 3°; SN1 — 3° > 2° > 1°.
• Carbocation stability: 3° > 2° > 1° > methyl; stabilised further by resonance (allyl, benzyl).
• Stereochemistry: SN2 inverts, SN1 racemises (with slight inversion since the leaving group shields one side).

MCQ Questions

Hot alcoholic KOH removes H and X from adjacent carbons to give an alkene (dehydrohalogenation). The preferred product is the most substituted alkene — Zaitsev's rule. Aqueous KOH favours substitution; alcoholic KOH favours elimination.

MCQ Questions

Haloarenes are far less reactive than haloalkanes in nucleophilic substitution. Resonance delocalises the halogen lone pair into the ring, giving the C–X bond double-bond character, and the sp² carbon forms a stronger C–X bond; moreover an aryl anion/carbanion intermediate would be highly unstable.

• Benzyne route: chlorobenzene + NaOH at 623 K (7 atm) → phenol via elimination–addition.
• Electrophilic substitution: X is ortho/para directing but ring-deactivating → chlorobenzene gives mainly o-/p-chloronitrobenzene on nitration, and o-/p- products in Friedel–Crafts alkylation, halogenation, sulphonation.
• DNP/DDT: a classic example is the insecticide DDT prepared from chlorobenzene and chloral.
• Uses: solvents (CHCl₃, CCl₄), anaesthetics, refrigerants (freons/CFCs), fire extinguishers, agrochemicals.

MCQ Questions

Exercises (NCERT Q1–Q3)

1. 1
   Arrange the following compounds in increasing order of reactivity towards SN2 displacement: 1‑Bromobutane, 2‑Bromobutane, 2‑Bromo‑2‑methylpropane.
2. 2
   Why is the dipole moment of chlorobenzene lower than that of cyclohexyl chloride?
3. 3
   What product is formed when chlorobenzene is treated with NaNH2 followed by water?

Answers at a glance

1. 1
   2‑Bromo‑2‑methylpropane < 2‑Bromobutane < 1‑Bromobutane (increasing SN2 reactivity).
2. 2
   Resonance in chlorobenzene gives the C–Cl bond partial double bond character, lowering its dipole moment compared to the single bond in cyclohexyl chloride.
3. 3
   Aniline is formed.

MCQ Questions

1 Mark Questions

1. 1
   Classify the following as primary, secondary, tertiary or aryl halide: 2‑Chloro‑2‑methylpropane.
2. 2
   What is ambident nucleophile? Give an example.
3. 3
   Write the structure of neo‑pentyl bromide.

2 Mark Questions

1. 1
   Arrange the following in increasing order of boiling point: CH3CH2CH2CH2Br, (CH3)3CBr, CH3CHBrCH3.
2. 2
   Explain why alkyl halides undergo nucleophilic substitution reactions whereas aryl halides do not under normal conditions.

3 Mark Questions

1. 1
   Discuss the mechanism of SN1 and SN2 reactions with energy profile diagrams.
2. 2
   How will you distinguish between 1° , 2° and 3° alcohols using Lucas test? Write the reactions involved.

5 Mark Questions

1. 1
   An alkyl halide (C4H9Br) (A) reacts with alcoholic KOH to give an alkene (B) which reacts with Br2 to give the compound C which is hydrolysed to give a diol D. Write the structures of A, B, C and D.
2. 2
   Explain the following: (i) p‑dichlorobenzene has higher melting point and lower solubility than o‑ and m‑isomers. (ii) Why is the thickness of oxide layer on metals like Al and Ti important for corrosion resistance?