Polyhalogen Compounds

Uses and environment effects

Dihalogen Derivatives

In this reaction simple Halogenation of alkenes is done to get dihalogenated compounds

Dichloromethane (CH₂Cl₂) is widely used as a solvent, as a propellant in aerosols. Direct contact of dichloromethane in humans causes intense burning and milk redness of the skin.

Trihalogen Derivatives

1. Chloroform [Trichloromethane, CHCl₃]

Methods of preparation

Haloform Reaction

[Chlorination]

Properties

1. Oxidation of CHCl₃ gives poisonous gas phosgene (carbonyl chloride).

phosgene

To avoid this oxidation CHCl₃ is stored in dark brown bottles and filled to the brim. 1% ethanol is added to chloroform which converts harmful phosgene gas into diethyl carbonate.

2. CHCl₃ is widely used in the production of freon refrigerant R-22.

3. On nitration, it gives tear producing insecticide substance chloropicrin. It follows free radical mechanism.

4. On dehalogenation, it gives C₂H₂ (acetylene)

5. When subjected to hydrolysis, it gives formate.

2. Iodoform (tri-iodornethane, CHl₃)

Iodoform is prepared by iodoform reaction.

Compounds containing either CH₃CO^- or CH₃CH(OH) group form yellow colour iodoform with I₂ and NaOH.

Iodoform when comes in contact with organic matter, decomposes easily to free iodine, an antiseptic. Due to its objectionable smell, it has been replaced by other formulations containing iodine.

Polyhalogen Derivatives

1. Tetrachloromethane (Carbon Tetrachloride, CCl₄)

Preparation

In Methane gas initially Chlorine is added via free radical mechanism due to action of sunlight and thus Carbon Tetrachloride is formed. This done by taking methane gas in chlorine gas atmosphere in presence of sunlight.

Sometimes if Chloroform is available then also by same mechanism carbon tetra chloride is made by taking less amount chlorine.

CCI₄ is a colourless, non-inflammable, poisonous liquid, soluble in alcohol and ether.

Carbon tetrachloride is used

1. as a solvent for oils, fats, resins

2. in dry cleaning

3. as fire extinguisher under the name ‘pyrene’.

2. Freons

The chlorofluorocarbon compounds of methane and ethane are collectively known as freons. These are usually produced for aerosol propellants, refrigeration and air conditioning purposes. Carbon tetra chloride when reacts with antimony trifluoride in the presence of SbCl₅ as catalyst, dichlorofluromethane (freon) is obtained.

3. DDT (p, p’-Dichlorodiphenyltrichloroethane)

DDT is the first chlorinated organic insecticide. Its stability and fat solubility’is a great problem.

It is prepared from chloral and chlorobenzene in the presence of conc. H2SO4·

4. Perchloroethane (C₂Cl₆)

It is used as moth repellant and is also known as artificial camphor.