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NEET Chemistry Notes Haloalkanes and Haloarenes – Polyhalogen Compounds

NEET Chemistry Notes Haloalkanes and Haloarenes – Polyhalogen Compounds

Polyhalogen Compounds

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.NEET Chemistry Notes Haloalkanes and Haloarenes - Polyhalogen Compounds 1

Iodoform (Triiodomethane; CHI3)

It is prepared by heating ethanol or acetone with sodium hydroxide and iodine or Na2C03 and I2 in water.
NEET Chemistry Notes Haloalkanes and Haloarenes - Polyhalogen Compounds 2
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  •  It is used as an antiseptic because it liberates iodine but due to obnoxious (unpleasant) smell, it has been replaced by other medicines.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.

DDT (Dichlorodiphenyltrichloromethane)
Preparation
NEET Chemistry Notes Haloalkanes and Haloarenes - Polyhalogen Compounds 3
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.

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