halo alkane 3

Preparation of Haloalkanes and Haloarenes

Module 3: Preparation of Haloalkanes and Haloarenes

(From CBSE Class 12 Chemistry, Chapter: Haloalkanes and Haloarenes)

Haloalkanes and haloarenes can be prepared by several methods. These methods primarily include substitution, addition, and other reactions that introduce halogen atoms into the organic molecule. Below is a detailed note:

Preparation of Haloalkanes

• From Alcohols: Alcohols react with halogenating agents to form haloalkanes. For example:
  • With Hydrogen Halides (HX): Alcohol + HX → Haloalkane + Water
  • Using PCl₅: ROH + PCl₅ → RCl + POCl₃ + HCl
  • With Thionyl Chloride (SOCl₂): ROH + SOCl₂ → RCl + SO₂ + HCl (This method is preferred as the by-products are gaseous.)
• From Alkanes: Free radical substitution occurs when alkanes react with halogens in the presence of UV light.
  • CH₄ + Cl₂ → CH₃Cl + HCl (Monochlorination)
• From Alkenes:
  • Addition of HX to alkenes (Markovnikov's Rule): CH₂=CH₂ + HCl → CH₃CH₂Cl
  • Anti-Markovnikov addition in the presence of peroxides: CH₂=CH₂ + HBr (peroxide) → CH₃CH₂Br

Preparation of Haloarenes

• From Arenes:
  • Electrophilic substitution reaction: Benzene reacts with halogens in the presence of a Lewis acid catalyst such as FeCl₃ or AlCl₃.
  • C₆H₆ + Cl₂ (FeCl₃) → C₆H₅Cl + HCl
• From Diazonium Salts: Aryl diazonium salts undergo Sandmeyer reactions to form haloarenes.
  • C₆H₅N₂⁺Cl⁻ + CuCl → C₆H₅Cl + N₂

Applications of Preparation Methods

• These methods allow controlled synthesis of specific halo compounds required in industries.
• The choice of method depends on the substrate, the desired halo compound, and environmental considerations.

1. Which reagent is used for the preferred preparation of alkyl chlorides from alcohols?

2. What type of reaction forms haloalkanes from alkanes?

3. Which rule is followed in the addition of HBr to alkenes in the absence of peroxides?

4. Haloarenes can be prepared from diazonium salts by which reaction?

5. Which catalyst is required for electrophilic substitution in benzene?