Unit – I B

Halogen Derivatives of Alkanes

  • Saturated aliphatic hydrocarbons are called alkanes. Their general formula is CnH2n+2.
  • When one or more hydrogen atoms of saturated aliphatic hydrocarbons or aromatic hydrocarbons are replaced by corresponding number of halogen atoms, (Cl, Br, I), then the new compounds obtained are called, halogen derivatives of alkanes or of arenes.
  • Examples of halogen derivatives of alkanes:
    a) CH3Cl (Methyl chloride) (Chloromethane),
    b) C2H5Br (Ethyl bromide) (Bromethane).
  • Note: In haloalkanes halogen atom is attached to sp3 hybridised carbon atom.

Halogen Derivatives of Arenes

  • Aromatic hydrocarbons are called arenes.
  • Examples of halogen derivatives of arenes:
  • Note: In haloarenes halogen atom is attached to sp2 hybridised carbon atom.

Classification Halogen Derivatives of Alkanes on the Basis of Number of Halogen Atoms: 

  • Depending upon number of halogen atoms present in the molecule, halogen derivatives of alkanes are classified as monohalogen derivatives of alkanes and polyhalogen derivatives of alkanes.
  • Polyhalogen derivatives of alkanes are further classified as dihalogen, trihalogen, tetrahalogen derivatives of alkanes and so on.

1) Monohalogen derivatives of alkanes :

  • When one hydrogen atom of alkane is replaced by one halogen atom, then the compound obtained is called as monohalogen derivative of alkanes. They are commonly called as alkyl halides or haloalkenes. Their general formula is CnH2n+1X
  • e.g. CH3Cl (Methyl chloride), C2H5Br (Ethyl bromide)

2) Dihalogen derivatives of alkanes :

  • When two hydrogen atoms of alkane are replaced by two halogen atoms, then the compound obtained is called as dihalogen derivative of alkanes. Their general formula is CnH2nX2
  • e.g. C2H4Cl2 (Dichloroethane).

3) Trihalogen derivatives of alkanes :

  • When three hydrogen atoms of alkane is replaced by three halogen atoms, then the compound obtained is called as trihalogen derivative of alkanes. Their general formula is CnH2n-1X3
  • e.g. CHCl3 (Chloroform)

4) Tetrahalogen derivatives of alkanes :

  • When four hydrogen atoms of alkane is replaced by four halogen atoms, then the compound obtained is called as tetrahalogen derivative of alkanes. Their general formula is CnH2n-2X4
  • e.g. CCl4 (Carbon tetrachloride)

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