Methods of Preparation of Haloalkanes

NCERT Reference:Chapter 6 – Haloalkanes and Haloarenes – Page 155–157

Quick Notes

Haloalkanes can be prepared by three major methods:
- From alcohols using halogenating agents.
- From hydrocarbons via free radical halogenation or addition of halogens/hydrohalides.
- By halogen exchange reactions such as Finkelstein and Swarts reactions.

Full Notes

Haloalkanes from Alcohols

Alcohols are commonly converted to haloalkanes by replacing the –OH group with a halogen (X). This can be achieved using various halogenating agents:

NCERT 12 Chemistry schematic showing conversion of alcohols R–OH to haloalkanes R–X using reagents such as HX, PX3, PX5, and SOCl2.

(a) Using HX (Hydrogen Halides)

General reaction: R–OH + HX → R–X + H₂O
Works best with tertiary alcohols > secondary > primary.
Reaction is catalysed by anhydrous ZnCl₂ (Lucas reagent) in case of HCl.

Example Formation of chloroethane

CH₃CH₂OH + HCl (with ZnCl₂) → CH₃CH₂Cl + H₂O

(b) Using NaX (Sodium Halides) + Conc. H₂SO₄

General reaction: R–OH + NaX + H₂SO₄ → R–X + H₂O

Example Preparation of bromoethane

CH₃CH₂OH + NaBr + H₂SO₄ → CH₃CH₂Br + NaHSO₄ + H₂O

(c) Using Phosphorus Halides (PX₃, PX₅)

With PBr₃ (prepared in situ): 3R–OH + PBr₃ → 3R–Br + H₃PO₃
With PCl₅: R–OH + PCl₅ → R–Cl + POCl₃ + HCl
With red phosphorus and X₂: 3 R–OH + P + 1.5 Cl₂ → 3 R–Cl + H₃PO₃

(d) Using Thionyl Chloride (SOCl₂)

R–OH + SOCl₂ → R–Cl + SO₂ + HCl
Preferred method because both by-products (SO₂ and HCl) are gases and leave the reaction mixture.

Haloalkanes from Hydrocarbons

Haloalkanes can also be prepared by halogenation or addition reactions involving hydrocarbons.

Free Radical Halogenation of Alkanes

General reaction: RH + X₂ → R–X + HX (in presence of UV light or heat).

Occurs via free radical chain mechanism and often gives a mixture of isomers due to lack of selectivity.

NCERT 12 Chemistry diagram of free radical halogenation of alkanes showing initiation, propagation, and termination steps under UV light.

Example Chlorination of methane

CH₄ + Cl₂ (hν) → CH₃Cl + CH₂Cl₂ + CHCl₃ + CCl₄

Addition of Hydrogen Halide (HX) to Alkenes

Alkenes react with HX to form alkyl halides via electrophilic addition following Markovnikov’s Rule.

NCERT 12 Chemistry mechanism sketch showing electrophilic addition of HBr to an alkene giving a Markovnikov product.

Example Hydrobromination of ethene

CH₂=CH₂ + HBr → CH₃CH₂Br

Addition of Halogens (X₂) to Alkenes

Alkenes react with halogens to form vicinal dihalides.

NCERT 12 Chemistry schematic of bromine addition to ethene forming 1,2-dibromoethane via anti addition.

Example Dihalogenation of ethene

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

Haloalkanes Using Halogen Exchange

This method involves replacing one halogen with another.

Finkelstein Reaction

Substitution of Cl or Br by I⁻ using NaI in dry acetone. Works well for primary alkyl halides.

Example Halide exchange to alkyl iodide

R–Cl + NaI → R–I + NaCl (ppt)

Swarts Reaction

Replacement of halogen (usually Cl) by fluorine using metallic fluorides.

Example Preparation of alkyl fluorides

R–Cl + SbF₃ (or Hg₂F₂, SbF₅) → R–F

Summary

Alcohols give haloalkanes using HX, PX₅, PX₃ or SOCl₂.
Hydrocarbons yield haloalkanes via radical halogenation or electrophilic addition.
Finkelstein uses NaI in dry acetone for R–I and Swarts uses metal fluorides for R–F.
Choice of reagent and conditions depends on substrate type and desired halogen.