Hi,

 

Look,

 

Hydrogen bonding occur only in those compounds where a hydrogen is attached to a highly electronegative group [ eg: O, N, F ]. As carbon is not that electronegative, hydrogen attached to carbon do not take part in H-bonding.

Dispersion forces / vanderwaal's forces occur between any kind of molecules. So, if two molecules are present, They would always have dispersion forces. If we say, that molecules have hydrogen bonding, it means that h bonding is more than dispersion forces. So, we don't have to consider dispersion forces.

 

Hope you have got it !!!!