7.0 Physical Properties

  Alkyl Halides and Aryl Halides

• Alkyl halides are colourless when pure, however, when exposed to light, bromides and iodides may develop colour.

• Volatile organic compounds have a sweet smell.
  

• Alkyl halides upto $C_18$ are colourless liquids. Higher members are colourless solids. (Some exceptions are $CH_3F, CH_3Cl, CH_3Br, C_2H_5Cl$ which are gases are room temperature)
  

• They are insoluble in water as they are unable to form $H$-bonds with water molecules.
  

• They are highly soluble in organic solvents (alcohol, ether, benzene etc.)
  

• $R-Br$ and $R-I$ are heavier than water while $R-F$ and $R-Cl$ are lighter than water.
  

• Density of alkyl halides: $R-I > R-Br > R-Cl$. In general density of alkyl halides increases with increasing atomic mass of halogen atoms.
  
  Density of poly halogen derivatives of methane: $CCl_4 > CHCl_3 > CH_2Cl_2$
  

• Boiling Point: $R-I > R-Br > R-Cl > R-F$. As the size (and mass) of the halogen increases, the magnitude of van der Waals forces also increases.
  

• In isomeric alkyl halides, the boiling point decreases as the branching of the alkyl group increases. As there is an increase in branching, the molecule occupies a lesser surface area (It becomes spherical).
  

• Melting point of isomers depend on symmetry. Para-isomers have a higher melting point than ortho- and meta- isomers.