The above order is based on the fact that most of the reactions of halides involve the cleavage of C-X bond. Larger the bond dissociation energy of the C-X bond, smaller is the reactivity. The bond dissociation energy decreases with the increase in the size of halogen atom, i.e.,
C-Cl > C-Br > C-I. Hence, the reactivity increases from alkyl chlorides to bromides and from bromides to iodides. In the halo derivatives, the halogen atom is attached to the carbon atom. As the halogen atom is more electronegative than carbon, the bond between carbon and halogen is polar in character (supported by high dipole of the order of 2 - 2.2 D).
Nature of C-X Bond
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The general formula of Haloalkane (CnH2n+1 - X) or Haloarene is 'RX' where R - is an alkyl or aryl group and X is a halogen atom (F, Cl, Br or I). The halogen derived hydrocarbons are a highly reactive class of compounds. The reactivity is due to the presence of polar carbon-halogen bond in these molecules. For a given alkyl or aryl group, the order of reactivity is: Chlorides > Bromides > Iodides.
When more than one hydrogen atoms of the alkane molecule have been replaced by halogen atoms they are known as 'polyhalogen derivatives of alkanes or polyhaloalkanes'. These compounds show the effects of two or more C-X bonds. When one or more atoms of the alkyl side chain of a benzene derivative are replaced by the halogen atoms, side chain substituted halo compounds are obtained. These compounds are not regarded as true aryl halides because halogen atom is not directly attached to the benzene ring. These are called side chain substituted alkyl halides or aralkyl halides. For example,
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