Assign reasons for the following:
(i) Boiling points of n-alkanes increase regularly with the increase in the number of carbon atoms.
(ii) Branched-chain alkanes have lesser boiling points than the straight chain alkanes.
(i) Alkanes are non-polar molecules and hence are held together by weak Vander Waal’s forces of attraction amongst their molecules. These forces act on the surface of molecules and their magnitude increases with the increase in surface area of the molecules. Thus, with the increase in the number of carbon atoms, the magnitude of Vander Waals forces increases and with that boiling points also increase.
(ii) This is because the branching of the chain makes the molecule more compact and bring the various atoms closer. As a result, the molecular size decreases. This decreases the surface area and hence the magnitude of Vander Waal’s forces (inter-particle forces) and lead to the decrease in boiling point. The boiling points of isomeric alkanes are given below:
