Why does ethanal have a lower boiling point than either ethanol or ethanoic acid?
Put simply, there’s no hydrogen bonding between the molecules. In ethanol and ethanoic acid, an -OH group is present. The oxygen pulls electron density off the hydrogen, making it particularly positively charged. The positive-ish hydrogen can then be weakly bonded to by other molecules’ oxygen atoms. These are pretty weak interactions (i.e. not as good as a covalent bond, less than 10% as strong in fact) but they still mean you need to put more heat into the compound to break the hydrogen bonds and boil the liquid. Ethanal has an oxygen, but it isn’t attached to a hydrogen, so the polarisation of a hydrogen atom doesn’t occur. The only bonding interaction between ethanal molecules are the relatively weak van der Waals’ forces and dipole-dipole interactions between the C=O groups in each molecule. Hence there is less intermolecular bonding to overcome in ethanal to separate all the molecules from each other and make it boil.
