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.
Methods of Preparation of Alkyl Halides
1. 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.
2. 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)
Other 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
Methods 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
Reactions 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
Haloform 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