The boiling points of organic compounds depend on the strength of the intermolecular forces in them. These forces are :
(a) van der Waals forces and
(b) dipole-dipole interactions
These forces are dependent on the
(i) molecular mass and
(ii) surface area of the molecules
(i) As the molecular mass of the compound increases, the boiling point also increases. Therefore the correct order is :
chloromethane < bromomethane < dibromomethane < bromoform
(ii) Amongst molecules with same mass, it is the size of the molecule that determines the boiling point. Branched compounds are more compact and therefore have less surface area as compared to their straight chain counterparts and therefore lower boiling point. The order of boiling point is :
iso-propyl chloride < 1-chloropropane < 1-chlorobutane