NEET Chemistry Notes Haloalkanes and Haloarenes – Polyhalogen Compounds
The polyhalogenated compounds (PHCs) are compounds with multiple substitutions of halogens.
Some of polyhalogen compounds are given below:
Dichloromethane(Methylene chloride; CH2Cl2)
- It is widely used as a solvent.
- It is used as a paint remover.
- It is used as a propellant in aerosols.
- It is used as a process solvent in the manufacture of drugs.
Trichloromethane (Chloroform; CHCl3)
- Chemically, chloroform is employed as a solvent for fats, alkaloids, iodine and other substances.
- In previous time, it was used as a general anaesthetic but now-a-days, it is replaced by less toxic and safe anaesthetic, ether.
- The major use of chloroform today is in the production of the freon refrigerant, R-22.
- Chloroform is stored in closed dark coloured bottles, filled completely so that air is kept out because it is oxidised by air in the presence of light.
Iodoform (Triiodomethane; CHI3)
It is prepared by heating ethanol or acetone with sodium hydroxide and iodine or Na2C03 and I2 in water.
This reaction is called iodoform reaction.
- It is yellow solid having melting point 119° C.
- It is insoluble in water but soluble in ethanol and ether.
- It is used as an antiseptic because it liberates iodine but due to obnoxious (unpleasant) smell, it has been replaced by other medicines.
- Its chemical properties are similar to CHCl3.
Tetrachloromethane (Carbon Tetrachloride; ccl4)
- It is produced in large quantities for use in the manufacture of refrigerants and propellants for aerosol cans.
- It is also used as leedstock in the synthesis of chlorofluorocarbons and other chemicals.
- It is widely used as an industrial solvent for oils, fats,
resins, lacquers, etc.
- It is used as ftre extinguisher but after using it to extinguish fire, the room should be ventilated to remove phosgene.
Freons (Chlorofluorocarbon; CC12F2)
- They are extremely stable, unreactive non-toxic, non-corrosive and easily liquefiable gases.
- It is manufactured from tetrachloromethane by Swarts reaction and used for aerosol propellants, refrigeration and air conditioning purposes.
- Freons deplete the protective ozone layer surrounding our planet, so their use has been banned in many countries.
It is a powerful insecticide but is not easily biodegradable. Therefore, its long term effects could be potentially dangerous and its use is banned in many countries.
IUPAC name of DDT is 2, 2-bis (4-chlorophenyl) -1, 1, 1-trichloroethane. It is non-biodegradable and slowly changes to another compound DDD by the loss of a molecule of HCl.