Question

Do aldehydes, ketones, amines, alcohols, or carboxylic acids have higher BP when they are of similar molecular weights? Arrange from lowest BP to highest BP and explain your reasoning.

Answer 1

The boiling points of aldehydes, ketones, amines, alcohols and carboxylic acids are much higher than the boiling points of alkanes of comparable molecular masses. The boiling points of these compounds vary in the order

Acid > Alcohol > Amine > Aldehyde > Ketone > Alkane.

The above order of boiling points can be easily explained on the basis of the intermolecular forces present in the molecules. The stronger are the intermolecular forces present in an organic compound, the stronger are the bonds in the molecule and thus, the higher is the boiling point.

The major intermolecular forces present in organic molecules are dispersion forces, dipole-dipole forces and hydrogen bonding forces. The strength of these forces vary in the order

hydrogen bonding > dipole-dipole forces > dispersion forces.

The major intermolecular forces in alkanes are dispersion forces. Dispersion forces depend on the molecular mass of the compound and hence, increases with increasing molecular weights.

The major forces present in aldehydes and ketones are dipole-dipole forces. Aldehydes can show some hydrogen bonding forces due to the presence of a hydrogen atom attached to a highly electron-withdrawing C=O group.

The major forces in alcohols and amines are hydrogen bonding forces. Since O atom is more electronegative than N atom, hence, the hydrogen bonding in alcohol is stronger than in amines.

Carboxylic acids show the strongest intermolecular forces and the intermolecular forces present in acids are a combination of both dipole-dipole forces and hydrogen bonding forces and thus, carboxylic acids have the highest boiling points.