The Important Polyhalogen Compounds

Compounds with more than one halogen have major industrial, medical and environmental significance. NCERT highlights several.

Dichloromethane (CH2_2Cl2_2, methylene chloride): a widely used solvent — as a paint remover, a propellant in aerosols, and a process solvent in the pharmaceutical industry. It is harmful in high concentration (affects the central nervous system).

Trichloromethane (CHCl3_3, chloroform): a solvent for fats, alkaloids and iodine, and formerly used as an anaesthetic (now discontinued because it is harmful).

Important: chloroform is slowly oxidised by air and light to the poisonous gas phosgene (COCl2_2). To prevent this, it is stored in dark-coloured bottles filled to the brim (to exclude air) with a little ethanol added (which converts any phosgene to harmless ethyl carbonate).

More Polyhalogen Compounds

Triiodomethane (CHI3_3, iodoform): formerly used as an antiseptic — but its antiseptic action is due to the liberated free iodine, not the compound itself. Its objectionable smell has led to its replacement by modern antiseptics.

Tetrachloromethane (CCl4_4, carbon tetrachloride): used as a solvent and earlier in fire extinguishers (under the name "pyrene"). However, its vapour can react at high temperature to give phosgene, and it depletes the ozone layer, so its use is now restricted.

Freons (chlorofluorocarbons, CFCs): chlorofluorocarbon compounds of methane and ethane (e.g. freon-12, CCl2_2F2_2). They are very stable, unreactive, non-toxic and non-corrosive, so they were widely used as refrigerants and aerosol propellants.

Environmental problem: CFCs are the chief cause of ozone-layer depletion. In the stratosphere they release chlorine free radicals that catalytically destroy ozone. Their use is now being phased out under the Montreal Protocol.

Polyhalogen compounds chloroform, CCl4, freon and DDT with their uses

DDT and the Environmental Message

DDT (p,p'-dichlorodiphenyltrichloroethane): the first chlorinated organic insecticide, it was used widely against malaria-carrying mosquitoes and typhus-carrying lice after 1939 (Paul Muller won the Nobel Prize for discovering its insecticidal property).

Problems with DDT:

  • It is not biodegradable — it persists in the environment for a long time.
  • It bioaccumulates — building up in the fatty tissues of animals and concentrating up the food chain.
  • Many insects developed resistance to it.
  • It is toxic to fish and other wildlife.

For these reasons DDT use has been banned or severely restricted in many countries.

Key Point — uses to remember: DCM and chloroform are solvents (chloroform → phosgene on standing, stored with ethanol in dark bottles); iodoform was an antiseptic (acts via free iodine); CCl4_4 a solvent/fire extinguisher; freons (CFCs) deplete ozone; DDT is a persistent, bioaccumulating insecticide.

[NEET Important] Two environmental headlines: CFCs/freons → ozone-layer depletion (chlorine radicals), and DDT → persistent, non-biodegradable, bioaccumulating insecticide. These appear nearly every year.

Solved Examples

Example 1: Storage of chloroform

Why is chloroform stored in dark bottles filled to the brim, with a little ethanol added?

Solution: Chloroform is slowly oxidised by air and light to poisonous phosgene (COCl2_2). Dark bottles exclude light; filling to the brim excludes air; and the added ethanol converts any phosgene into harmless ethyl carbonate.

Example 2: Iodoform's antiseptic action

What is responsible for the antiseptic property of iodoform?

Solution: The free iodine liberated from iodoform (CHI3_3), not the compound itself, is responsible for its antiseptic action.

Example 3: Freons and ozone

Why are freons (CFCs) an environmental concern?

Solution: Freons are very stable and reach the stratosphere, where UV light releases chlorine free radicals that catalytically destroy ozone, causing ozone-layer depletion. This is why their use is being phased out.

Example 4: DDT's environmental problem

Give two reasons why DDT use has been banned in many countries.

Solution: (1) DDT is not biodegradable and persists in the environment; (2) it bioaccumulates in fatty tissues and concentrates up the food chain (and is toxic to fish/wildlife; insects also developed resistance).

Example 5: Uses of carbon tetrachloride

State two uses (or former uses) of carbon tetrachloride.

Solution: It is used as an industrial solvent (for oils, fats) and was earlier used in fire extinguishers ("pyrene"). Its use is now restricted because it depletes the ozone layer.

Example 6: Freon-12 formula and use

What is the formula and use of freon-12?

Solution: Freon-12 is CCl2_2F2_2 (dichlorodifluoromethane), used as a refrigerant and an aerosol propellant.

Example 7: Phosgene formation

Write the reaction by which chloroform forms phosgene.

Solution: Chloroform is oxidised by air/light: 2 CHCl3_3 + O2_2 → 2 COCl2_2 (phosgene) + 2 HCl. Phosgene is a poisonous gas, which is why chloroform must be stored carefully.

Example 8: Dichloromethane uses

State two uses of dichloromethane.

Solution: As a paint remover and as a solvent (e.g. in the pharmaceutical industry and as an aerosol propellant).

Example 9: Why CFCs were popular before their ban

Why were freons so widely used before their environmental impact was understood?

Solution: Freons are very stable, unreactive, non-toxic, non-corrosive and non-flammable, making them ideal as refrigerants and propellants — until it was discovered that their stability lets them reach the stratosphere and destroy ozone.

Example 10: First synthetic insecticide

Which compound was the first chlorinated organic insecticide, and what is its main drawback?

Solution: DDT (p,p'-dichlorodiphenyltrichloroethane). Its main drawback is that it is non-biodegradable and bioaccumulates, persisting in the environment and harming wildlife.