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Thursday, 23 February 2017

Haloalkanes and Haloarenes - II


Physical properties of haloalkanes:

Solubility
  • Although haloalkanes are polar in nature, yet they are practically very slightly soluble in water.
  • In order for a haloalkane to dissolve in water, energy is required to overcome the attractions between the haloalkane molecules and break the hydrogen bonds between water molecules.
  • However Haloalkanes are not able to form hydrogen bonds with water and therefore, less energy is released when new attractions are set up between the haloalkane and the water molecules because these are not as strong as the original hydrogen bonds in water molecules.
  • As a result, solubility of haloalkanes in water is low.
Density
  • Simple fluoro and chloroalkanes are lighter than water while bromides and polychloro devrivatives are heavier than water.
  • With the increase in number of carbon atoms, the densities go on increasing. With the increase in number of halogen atoms, the densities go on increasing. The densities increase in the order: Fluoride < chloride < bromide < iodide
  • The density also increases with increasing number and atomic mass of the halogen.
Boiling Points
  • Molecules of organic halogen compounds are generally polar.
  • Due to the polarity as well as higher molecular mass as compared to the parent hydrocarbon, the intermolecular forces of attraction (dipole – dipole and van der Waals) between the molecules are stronger in halogen derivatives of alkanes.
  • As a result melting and boiling points of chlorides, bromides and iodides are considerably higher than those of the parent hydrocarbon of comparable molecular mass.
  • For the same alkyl group the boiling points of alkyl chlorides, bromides and iodides follow the order RI > RBr > RCl > RF where R is an alkyl group. This is because with the increase in the size of the halogen, the magnitude of van der Waals force increase.
  • In general, the boiling points of chloro, bromo and iodo compounds increase with increase in the number of halogen atoms.
  • For the same halogen atom, the boiling points of haloalkanes increase with increase in the size of alkyl groups.
  • For isomeric alkyl halides, the boiling points decrease with branching. This is because branching of the chain makes the molecule more compact and, therefore, decrease the surface area. Due to decrease in surface area, the magnitude of van der Waals forces of attraction decreases and consequently, the boiling points of the branched chain compound is less than those of the straight chain compounds.
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