This article provides you the revision notes on CBSE Class 12 Chemistry: Chapter- Haloalkanes and Haloarenes, to give you a quick glance of the chapter. This article is a continuation of the revision notes on Class 12 Chemistry, Chapter- Haloalkanes and Haloarenes, Part-I.
In Part-I you got acquainted with the haloalkane and haloarenes, their preparation, their physical and chemical properties, stereoisomerism, etc. In Part-II, you will get to know various important polyhalogeno compounds along with their preparation, properties and uses. These quick notes are prepared strictly according to the latest CBSE syllabus for Class 12th Chemistry.
The main topics covered in this part are:
o Polyhalogen compounds
• Dichloromethane, CH2Cl2
• Trichloromethane, CHCl3
• Triiodomethane, CHI3
• Tetrachloromethane, CCl4
• Benzene hexachloride (BHC)
• Dichloro diphenyl trichloroethane (DDT)
The key notes of the chapter are as follows:
Carbon compounds containing more than one halogen atom are known as polyhalogen compounds. For example: CFCl3, CH2Cl2 etc.
Some important polyhalogen compounds are described below:
1. Dichloromethane, CH2Cl2
Dichloromethane is prepared by the direct chlorination of CH4.
• It is a colourless, sweet-smelling, volatile liquid
• It has a low boiling point and low inflammability.
• It is insoluble in water
• It is widely used as a paint remover
• It is used as a solvent and cleaning agent in chemical manufacture, textiles, electronics, metals and plastic industries
• It is also used as a process solvent in the manufacture of drugs
2. Trichloromethane, CHCl3 (Chloroform)
CHCl3 is manufactured by chlorination of methane followed by separation by fractional distillation.
• Chloroform is a sweet - smelling liquid
• Chloroform is slowly oxidised by air in the presence of light to form poisonous phosgene gas
To avoid this oxidation chiorofrom is always stored in dark coloured bottles filled to the brim to exclude any air. Further bottles are also filled with small amount of ethyl alcohol so as to destroy traces of phosgene if formed, to harmless diethyl carbonate.
Some important chemical reactions of chloroform are:
• Chloroform is employed as a solvent for fats, alkaloids, iodine and other substances.
• It mainly used in the production of the freon refrigerant R-22.
3. Triiodomethane, CHI3 (Iodoform)
Any compound containing CH3CO− or CH3CH(OH)− group, when heated with iodine and aquous NaOH gives yellow precipitate of iodoform.The reaction is known as iodoforin reaction.
Iodoform can also be prepared conveniently by using bleaching powder, CaOCl2.
Iodoform was used earlier as an antiseptic due to the liberation of free iodine but due to its objectionable smell, it has been replaced by other formulations containing iodine.
4. Tetrachloromethane, CCl4 (Carbon tetrachloride)
CCl4 is prepared by reacting carbon disulphide (CS2) with Cl2 in the presence of AlCl3.
• Iodoform is a colourless, non-inflammable, poisonous liquid.
• When heated with steam at 773 K, it undergoes oxidation to form phosgene, COCl2 gas.
• Carbon tetrachloride is used as a solvent for oils, fats, resins
• It is used as a refrigerant and a dry cleaning agent
• It is used as a fire extinguisher under the name pyrene
Note: When carbon tetrachloride is released into the air, it causes the atmospheric temperature to rise and depletion of ozone layer. Depletion of the ozone layer is believed to increase human exposure to ultraviolet rays, leading to increased skin cancer, eye diseases and disorders, and possible disruption of the immune system.
Freons are the chlorofluoro compounds of methane and ethane.
Dichloro difluoro methane or Freon 12 (CCl2F2) is one of the most common freons in and is manufactured from tetrachloromethane by Swarts reaction
• Freons are extremely stable, unreactive, non-toxic, non-corrosive and easily liquefiable gases
• They lead to the depletion of ozone layer surrounding our planet
• Freons are used in aerosol propellants, refrigeration and air conditioning purposes
6. Benzene hexachloride (BHC)
It is also famous by its trade name, gammaxene or lindane
It is prepared by reacting benzene with chlorine in the presence of sunlight.
BHC is mainly sused as a pesticide in agriculture.
7. Dichloro diphenyl trichloroethane (DDT)
It is manufactured by the condensation of chloral with chlorobenzene in the presence of H2SO4.
• DDT is a white powder insoluble in water.
• It is a highly stable fat-soluble compound.
• It is mainly used as an insecticide.
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