Polyhalogen Compounds
Polyhalogen Compounds are carbon compounds containing more than one halogen atom. They are widely used in industry and agriculture but also pose significant environmental and health concerns.
1. Dichloromethane (Methylene Chloride)
Formula: $CH_2Cl_2$
- Uses: Widely used as a solvent, paint remover, and propellant in aerosols.
- Health Impact: Direct contact harms the human central nervous system. Exposure causes dizziness, nausea, and numbness.
2. Trichloromethane (Chloroform)
Formula: $CHCl_3$
Historically used as a general anaesthetic but replaced due to toxicity.
Formation of Phosgene (Poison)
Chloroform slowly oxidizes in the presence of air and light to form an extremely poisonous gas called Phosgene (Carbonyl Chloride).
$$ 2CHCl_3 + O_2 \xrightarrow{\text{light}} \underbrace{2COCl_2}_{\text{Phosgene}} + 2HCl $$
Storage: It is stored in closed dark-colored bottles completely filled to the brim (to exclude air). 1% Ethanol is added to retard oxidation and convert any phosgene formed into harmless diethyl carbonate.
3. Triiodomethane (Iodoform)
Formula: $CHI_3$
- Properties: A yellow crystalline solid with a distinct, objectionable smell.
- Uses: Used as an antiseptic. Its antiseptic action is due to the liberation of free Iodine ($I_2$) upon decomposition, not iodoform itself.
4. Tetrachloromethane (Carbon Tetrachloride)
Formula: $CCl_4$
- Uses: Manufacture of refrigerants, propellants, and formerly as a fire extinguisher (under the name Pyrene) and cleaning fluid.
- Health: It causes liver cancer and ozone depletion. Exposure leads to dizziness and nerve damage.
5. Freons (CFCs)
Chlorofluorocarbons (CFCs) of methane and ethane are collectively known as Freons.
- Freon-12 ($CCl_2F_2$): The most common Freon. Produced by the Swarts reaction ($CCl_4 + SbF_3$).
- Properties: Stable, unreactive, non-toxic, non-corrosive, and easily liquefiable. Used in refrigeration and air conditioning.
- Environmental Impact: In the stratosphere, UV radiation breaks down CFCs releasing Chlorine radicals ($\dot{Cl}$). These radicals catalyze the breakdown of Ozone ($O_3$) to Oxygen ($O_2$), depleting the ozone layer.
6. DDT (p,p'-Dichlorodiphenyltrichloroethane)
DDT was the first chlorinated organic insecticide. It is prepared by heating chlorobenzene with chloral ($CCl_3CHO$) in the presence of concentrated sulfuric acid.
Biological Magnification
DDT is highly stable and fat-soluble. It is not metabolized easily by animals and accumulates in fatty tissues. Its concentration increases up the food chain (Biomagnification), causing toxicity in higher animals (thinning of eggshells in birds). It is banned in many countries.
Polyhalogen Quiz
Test your knowledge on these important compounds. 10 MCQs with explanations.
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