Question

Below are four topics. Their basic structure is the same, ie. benzene ring. The substance in (a) is benzene, but the other substances are derived from benzene where hydrogen has been replaced by another substance ie. Cl, Br or OH. Arrange materials in order of increasing boiling point and justify your order based on the connections that exist between the molecules which may be London forces, polar forces or hydrogen bonds.

a) C6H6

b) C6H5Cl

c) C6H5Br

d) C6H50H

Answer 1

Boiling points of each compound in increasing order:
benzene: 80^oC; chlorobenzene: 132^oC; bromobenzene: 156^oC; phenol: 181^oC
Benzene contains only weaker London forces and hence it requires the least amount of energy to make it boil.
Chlorobenzene contains London forces as well as polar forces because the C-Cl bond is a polar bond. Hence it has a higher boiling point than benzene. Bromobenzene as well contains similar forces as of chlorobenzene, however, Br is bigger in size and hence will have a larger surface area for interaction and that leads to higher energy requirement to boil the bromobenzene than the chlorobenzene. Thus bromobenzene has a higher boiling point than chlorobenzene and benzene.
Phenol contains polar C-O bond as well as there is a -OH bond which is more likely to have intermolecular hydrogen bonding. So in addition to London forces and polar forces, phenol exhibits strong hydrogen bonding as well. This leads to the highest boiling point of phenol as compared to the rest of other compounds in the given series.