Question

Mention any three methods of preparation of haloalkanes from alcohols.

Answer 1

Haloalkanes are prepared by the following methods.

From alcohols: Alcohol can be converted into halo alkenes by reacting it with any one of the following reagents.

1. Hydrogen halide
2. Phosphorous halides
3. Thionyl chloride.

• Reaction with hydrogen halide:

\[\ce{\underset{\text{Ethanol}}{CH3CH2}OH + HCl ->[Anhydrous ZnCl2][\Delta] \underset{\text{Chloroethane}}{CH3CH2Cl + H2 O}}\]

Mixture of con. HCl and anhydrous ZnCl₂ is called Lucas Reagent.

The order of reactivity of halo acids with alcohol is in the order HI > HBr > HCl.

The order of reactivity of alcohols with halo acid is tertiary > secondary > primary.

• Reaction with phosphorous halides:

Alcohols react with PX₅ or PX₃ to form haloalkanes.

Example:

\[\ce{\underset{\text{Ethane}}{CH3CH2}OH + PCl5 ->\underset{\text{Chloro ethane}}{CH3CH2Cl +}POCl3 + HCl}\]

\[\ce{\underset{\text{Ethanol}}{3CH3CH2OH} + PCl3 -> \underset{\text{Chloro ethane}}{3 CH3CH2Cl +} H3PO3}\]

• Reaction with Thionyl chloride(Sulphonyl Chloride)

\[\ce{\underset{\text{Ethanol}}{CH3CH3OH} + SOCl2 ->[Pyridine] \underset{\text{Chloro ethane}}{CH3CH2Cl +} SO2↑ + HCl↑}\]