Organic Compounds Containing Halogens

Halogen-containing organic compounds are categorized into two main groups:

• Alkyl Halides: These are aliphatic carbon chains where one or more hydrogen atoms are substituted by halogen atoms.
  Example: Chlorobutane.
• Aryl Halides: These are aromatic rings with halogen atom(s) directly attached to the ring.
  Example: Chlorobenzene.

1.0Methods of Preparation of Alkyl Halides

Grove’s Process

Involves replacing the -OH group in primary and secondary alcohols with a halogen (X) using zinc chloride (ZnCl₂).

R−OH + HX → ZnCl₂R−X + H₂O

Where:

• R-OH = Alcohol (Primary or Secondary)
• HX = Hydrogen halide (HCl, HBr, HI)
• ZnCl₂ = Catalyst
• R-X = Alkyl halide

The reaction follows the Sₙ2 mechanism when ZnCl₂ concentration is low.

Darzens Process

• Involves the reaction of thionyl chloride (SOCl₂) with primary alcohols.
• Can proceed with or without pyridine.

Reaction Steps:

With pyridine:

ROH + SOCl₂ → HCl + ROSOCl

HCl + C₅H₅N → C₅H₅NH⁺Cl⁻

• ROSOCl + Cl⁻ → RCl + SO₂ (Sₙ2 mechanism)

2.0Other Methods for Preparing Alkyl Halides

These reactions are fundamental in organic chemistry and widely applied in synthesis and industrial processes.

• Using Phosphorus Halides:
  ROH + PCl₅ → RCl + HCl + POCl₃
• Halogen Addition to Alkenes:
  R-CH=CH₂ + Br₂/CCl₄ → R-CH(Br)-CH₂Br
• Photohalogenation:
  CH₄ + Cl₂ + hv → CH₃Cl + HCl
• Halogen Exchange (Finkelstein Reaction):
  RCl + NaI → RI + NaCl
• Borodine-Hunsdiecker Reaction:
  RCOOAg + Br₂ → RBr + CO₂ + AgBr
• Hydrohalogenation of Unsaturated Hydrocarbons:
  Without peroxide:
  RCH=CH₂ + HBr → RCH(Br)CH₃
• With peroxide (Anti-Markovnikov’s Rule):
  RCH=CH₂ + HBr + Peroxide → RCH₂CH₂Br

3.0Methods of Preparing Aryl Halides

1. Halogenation

• Reaction:
  Ar-H+X₂+Lewis Base→Ar-X+HX

In this process, an aromatic hydrocarbon (Ar-H) reacts with a halogen (X₂) in the presence of a Lewis acid to form an aryl halide (Ar-X).

2. From Diazonium Salts

Aryl halides can also be synthesized from diazonium salts using the following reactions:

• Schiemann Reaction:
  C₆H₅N₂Cl+HBF₄→C₆H₅F
• Sandmeyer Reaction:
  C₆H₅N₂Cl + CuCl→C₆H₅Cl
• Gattermann Reaction:
  C₆H₅N₂Cl + Cu powder→C₆H₅Cl

Sₙ1 and Sₙ2 Mechanism Comparison

4.0Reactions of Alkyl Halides

• Hydrolysis:
  RX + OH⁻ → ROH + X⁻
• Williamson Synthesis:
  R-ONa + R'X → R-O-R' + NaX
• Reaction with Dry Silver Oxide:
  2R-X + Ag₂O → R-O-R
• Reaction with Sodio-Alkynides:
  R-C≡C-Na + X-R → R-C≡C-R + NaX
• Reaction with Potassium Cyanide:
  KCN + R-X → RCN + KX
• Reaction with Silver Cyanide:
  AgCN + R-X → RNC + AgX
• Reaction with Silver Nitrite:
  AgNO₂ + R-X → RNO₂ + AgX
• Reaction with Potassium Nitrite:
  KNO₂ + R-X → R-O-N=O + KX
• Friedel-Crafts Reaction:
  R-X + C₆H₆ + AlCl₃ → C₆H₅-R
• Malonic Ester Synthesis:
  R-X + CH(CO₂C₂H₅)₂ → R-CH(CO₂C₂H₅)₂ + HX
• Acetoacetic Ester Synthesis:
  R-X + CH(CO₂C₂H₅)₂ → R-CH(CO₂C₂H₅)₂ + HX
• Reaction with Ammonia:
  R-X + NH₃ → R-NH₂ + HX
• Wurtz Reaction:
  2R-I + 2Na → R-R + 2NaI
• Dehydrohalogenation:
  CH₃CH₂CH₂Br + alco.KOH → CH₃-CH=CH₂ + KBr + H₂O
• Reaction with Alcoholic AgNO₃:
  R-X + AgNO₃ → R⁺ + AgX↓ + HNO₃

5.0Haloform Reaction (Trihalide Reaction)

Preparation

Chloroform (CHCl₃) can be prepared from alcohols containing the –CH(OH)CH₃ group or from aldehydes and ketones with three α-hydrogen atoms, using halogen (X₂) and an alkali or Na₂CO₃.

Substitution / Elimination