Compound of Carbon Containing Halogen: Distinction between Haloalkanes and Haloarenes (For CBSE, ICSE, IAS, NET, NRA 2022)

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Stability of Carbocation

Reactions with Metals

  • Haloalkanes and haloarenes react with a variety of metals (zinc, magnesium and lithium) .
  • The compound obtained have a metal atom directly bonded to a carbon atom.
  • Such compound in which a metal atom is directly bonded to a carbon atom are known as organometallic compounds.
  • The organometallic compounds of magnesium with alkyl and aryl halides are known as Grignard reagents.

Alkyl halides react with metallic sodium in presence of dry ether to form symmetrical higher alkanes This reaction is called Wurtz reaction.

When haloarenes react with alkyl halides in presence of sodium and dry ether, the alkyl derivatives of benzene are formed. This reaction is called Wurtz-Fitting reaction.

Reactions with Metals

When haloarenes are treated with sodium, diaryls are produced. This reaction is called Fitting reaction.

Reactions with Metals

Reduction

  • Haloalkanes can be reduced to the corresponding alkanes.
  • For example, bromoethane can be reduced to ethane by using metal catalyst such as nickel or palladium or platinum or by using hydroiodic acid (HI) in presence of red phosphorus.
Stability of Reduction

Distinction between Haloalkanes and Haloarenes

  • Haloalkane and haloarenes can be distinguished by silver nitrate .
  • Haloalkanes react with to give white precipitate of while haloarenes do not react.

Some Useful Polyhalogeno Compounds

Chloroform

Preparation from Ethanol

Chloroform is prepared in the laboratory by treating ethanol or propanone with chlorine gas in the presence of an alkali:

Preparation from Ethanol
  • Chloroform is a colorless sweet smelling liquid (b. p. 334K) .
  • It is oxidized by air in the presence of light to a poisonous gas, phosgene.
  • Phosgene is carbonyl chloride, .
  • Therefore, Chloroform is stored in dark colored bottles to protect it from light.
  • A small amount of ethanol is added to chloroform to convert toxic phosgene, if formed, into a nontoxic compound, ethyl carbonate.

  • Chloroform is used in isocyanide test for the detection of primary amines.
  • In this test a mixture of amine and chloroform is heated with alcoholic NaOH. A foul smelling isocyanide is obtained. This test is also known as carbylamines test.

Iodoform

  • Iodoform is a pale yellow solid with a distinct smell.
  • Its IUPAC name is triiodo methane.

Preparation

Iodoform is prepared by heating ethanol or acetone with iodine in the presence of alkali.

Preparation of Chloroform
  • Yellow crystals of iodoform can be recognized by the characteristic smell.
  • Formation of iodoform is used to test compounds containing and group. This test is known as iodoform test .
  • Iodoform is used as an antiseptic.

Dichlorodiphenyltrichloroethane (DDT)

It is available in several different forms: powder, aerosols, granules, etc.

Dichlorodiphenyltrichloroethane

Uses

  • It is used mainly to control mosquito-borne malaria.
  • It is used as an agricultural insecticide.
  • The use of DDT has been banned in many countries because being non-biodegradable, it accumulates in environment.
  • It is toxic to other living organisms such as: mammals, birds, fishes, etc.

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