Polyhalogen Compounds

Full Notes

Polyhalogen compounds are organic molecules containing two or more halogen atoms.

Dichloromethane (Methylene Chloride)

Formula: CH₂Cl₂

Dichloromethane is a volatile, colourless liquid with a sweet smell. It is moderately polar and widely used as a solvent.

Properties & Uses:

• Used in paint removers, aerosol propellants, metal cleaning, and degreasing agents.
• Employed in pharmaceuticals and photographic film processing.
• Boiling point: approx. 313 K.

Health Effects:

• Prolonged exposure can affect the central nervous system.
• May cause dizziness, nausea, and skin/eye irritation.
• Vapours are harmful when inhaled; recommended to use in well-ventilated areas.

Trichloromethane (Chloroform)

Formula: CHCl₃

Chloroform is a colourless liquid with a distinct sweet smell. Historically used as an anaesthetic, it has largely been replaced due to safety concerns.

Preparation: Prepared by chlorination of methane (CH₄) or ethanol in presence of bleaching powder.

Uses:

• Used as a solvent for fats, alkaloids, iodine, etc.
• Employed in lab-scale organic synthesis.
• Intermediate in the production of freon (CF₂Cl₂).

Health & Environmental Concerns:

• Chloroform gets oxidised in air to phosgene (COCl₂), a highly toxic compound.
• Stored in dark coloured bottles to prevent light-induced decomposition.
• Causes liver damage, dizziness, and can be carcinogenic in long exposure.

Triiodomethane (Iodoform)

Formula: CHI₃

Iodoform is a yellow crystalline solid with a distinct antiseptic smell.

Uses:

• Mild antiseptic, especially in dressing wounds.
• Antibacterial properties due to slow liberation of iodine.

Preparation: Iodoform is obtained by iodination of ethanol or acetone in the presence of alkali.

Properties:

• Melting point: 423 K.
• Characteristic yellow colour and odour used as a test for methyl ketones.

Tetrachloromethane (Carbon Tetrachloride)

Formula: CCl₄

Carbon tetrachloride is a clear, heavy liquid, non-flammable, used historically in fire extinguishers and cleaning agents.

Uses:

• Formerly used in dry cleaning, fire suppression, and refrigerants.
• Now limited due to ozone depletion potential.

Health Hazards:

• Highly toxic to the liver and kidneys.
• Inhalation can lead to neurological damage and even coma.
• Exposure limits are strictly regulated.

Environmental Concerns:

• Contributes to ozone layer depletion.
• Persistent and accumulates in the environment.

Freons

General Formula: CCl₂F₂ (Freon-12 is the most common)

Freons are chlorofluorocarbons (CFCs) used extensively in refrigeration and aerosol systems.

Properties:

• Non-toxic, non-flammable, odourless gases or low-boiling liquids.
• Very stable under atmospheric conditions.

Uses:

• Refrigerants in air conditioners, refrigerators, and freezers.
• Propellants in aerosol sprays.

Environmental Impact:

• Freons are major ozone-depleting substances.
• Their stability allows them to reach the stratosphere, where they release chlorine atoms that destroy ozone molecules.
• Use is being phased out under the Montreal Protocol.

p,p′-Dichlorodiphenyltrichloroethane (DDT)

DDT is a powerful insecticide, introduced during World War II, known for controlling malaria and typhus.

Structure: Contains two chlorinated phenyl rings and a trichloroethane backbone.

Uses:

• Used to control mosquitoes, lice, and other vectors.
• Effective in agricultural pest control (though largely banned now).

Toxicity & Issues:

• Persistent in soil and water; accumulates in food chains (biomagnification).
• Causes reproductive issues, nervous system damage, and is toxic to birds (egg-shell thinning).
• Banned in many countries due to environmental persistence and bioaccumulation.

Summary Table