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Haloarenes

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Haloarenes

Haloarenes are the chemical compounds containing arenes, where one or more hydrogen atoms bonded to an aromatic ring are replaced with halogens. The nature of the C-X bond depends on both the nature of carbon in the aromatic ring and the halogen attached. Halogens are generally denoted by “X”.

Salient Points on the Nature of C-X Bond in Haloarenes are:

  • The C-X bond in haloarenes is polarized, as halogens are more electronegative than carbon. Due to the high electronegativity of halogen, it attracts the electron cloud more towards itself and thus gains a slight negative charge, on the other hand, carbon obtains a slight positive charge.

  • As halogens need only one electron to achieve their nearest noble gas configuration, only one sigma bond is formed between one carbon and one halogen atom.

  • Due to the increase in atomic size from fluorine to astatine, the C-X bond length in haloarenes increases from fluorine to astatine, and bond dissociation strength decreases.

  • Dipole moment depends on the difference in electronegativity of carbon and halogens and as we know that the electronegativity of halogens decreases down the group, the dipole moment also decreases. There is an exception to C-Cl and C-F dipole moments. Though the electronegativity of Cl is less than F, the dipole moment of a C-Cl bond is more than C-F.


    Physical properties


  • 1. Melting and boiling points

    The boiling points of monohalo benzene which are all liquids follow the order
    Iodo > Bromo > Chloro
    The boiling points of isomeric dihalobenzene are nearly the same
    The melting point of para isomer is generally higher than the melting points of ortho and meta isomers. The higher melting point of p-isomer is due to its symmetry which leads to more close packing of its molecules in the crystal lattice and consequently strong intermolecular attractive force which requires more energy for melting
    p –Dihalo benzene > o- Dichloro benzene > m-Dichloro benzene



    2. Solubility
    Haloarenes are insoluble in water because they cannot form hydrogen bonds with water, but are soluble in organic solvents



    3. Density
    Haloarenes are all heavier than water and their densities follow the order.
    Iodo benzene > Bromo benzene > Chloro benzene



    Uses of Haloalkanes and Haloarenes

    • These are used as hydrophobic solvents in chemistry.

    • These are used in organic synthesis.

    • These are used in industry.

    • Chloroquine is very useful in the treatment of malaria.

    • DDT is used as an insecticide.