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Polyhalogen Compounds

Carbon compounds containing more than one halogen atom are called polyhalogen compounds. Many of these compounds are useful in industry and agriculture. Some important polyhalogen compounds are described as follows:

1. Dichloromethane or methylene chloride, CH₃Cl₂

Dichloromethane is prepared industrially by direct chlorination of methane. The mixture of CH₃Cl, CH₂Cl₂, CHCl₂ and CCl₄ so obtained is separated by fractional distillation.

Properties

Dichloromethane is a sweet smelling volatile liquid. Its boiling point is 313 K.

Dichloromethane harms the human central nervous system. Exposure to lower levels of dichloromethane in air can lead to slightly impaired hearing and vision. Higher levels of dichloromethane in air can cause dizziness, nauses and numbness in the fingers and toes. Direct skin contact with methylene chloride causes intense burning and mild redness of the skin. Direct contact with eyes can burn cornea.

Uses

1. Because of its low boiling point and low inflammability it is used as extraction solvent in pharmaceutical and food industries. It is also used as solvent for carrying out many organic reactions in research laboratories.

2. It is used as a metal finishing and cleaning solvent.

3. It is also used as a propellent in aerosols.

2. Trichloromethane (Chloroform), CHCl₃

Preparation

(i) From methane: chloroform is manufactured by chlorination of methane in the presence of light or catalysts.

The mixture of CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄ can be separated by fractional distillation.

(ii) From chloral hydrate, pure chloroform can be obtained by distilling chloral or chloral hydrate with concentrated aqueous solution or KOH solution.


(iii) Laboratory method: In this method chloroform is obtained from ethanol or acetone by reaction with a paste of bleaching powder and water.

In this reaction, bleaching powder serves as a source of chlorine which first oxidizes ethanol acetaldehyte, which is then further chlorinated to chloral. Chloral reacts with Ca(OH)₂, given by CaOCl₂, to give chloroform and calcium formate.