10. HALOGEN DERIVATIVES - part 03 - Physical properties

10. HALOGEN DERIVATIVES - part 03 - Physical properties

 Physical properties : 
  • Boiling point of alkyl halides is determined bypolarity of the C-X bond as well as the size of halogen atoms.
1 Nature of intermolecular forces:
  • Halogens (X = F, Cl, Br and I) are more electronegative than carbon.
  • Carbon atom that carries halogen develops apartial positive charge while the halogencarries a partial negative charge.
  • Thus carbon-halogen bond in alkyl halide is a polar covalent bond. Therefore alkyl halides are moderately polar compounds.
  • Size of the halogen atom increases from fluorine to iodine. Hence the C-X bond length
    increases. 
  • The C-X bond strength decreases with an increase in size of halogen. 
  • This is because as the size of p-orbital of halogen increases the p-orbital becomes more diffused  and the extent of overlap with orbital of carbon decreases.
     
2 Boiling point : 
  • Considerably higher than those of corresponding alkanes due to higher polarity and higher molecular mass. 
  • Within alkyl halides, for a given alkyl group, the boiling point increases with increasing atomic mass of halogen, because magnitude of van der Waals force increases with increase in size and mass of halogen.
  • Thus boiling point of alkyl halide decreases in the order :
    RI > RBr > RCl > RF

  • For the given halogen, boiling point rises with increasing carbon number.
  • For isomeric alkyl halides, boiling point decrease with increased branching as surface area decreases on branching and van der Waals forces decrease.
3 Solubility : 
  • Insoluble in water. 
  • It is due to inability of alkyl halides to form hydrogen bonds with water. 
  • Attraction between alkyl halide molecules is stronger than attraction between alkyl halide and water. Alkyl halides are soluble in non-polar organic solvents.
  • Aryl halides are also insoluble in water but soluble in organic solvents. 
  • If aryl halides are not modified by presence of any other functional group, they show properties similar to corresponding alkyl halides. 
  • The isomeric dihalobenzenes have nearly thesame boiling points, but melting points of these isomers show variation. 
  • Melting point of para isomer is quite highcompared to that of ortho or meta isomer. This is because of its symmetrical structure which can easily pack closely in the crystal lattice. 
  • As a result intermolecular forces of attraction are stronger and therefore greater energy is required to overcome its lattice energy.

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