Question

Why are the boiling points of aldehydes and ketones lower than the boiling points of carboxylic acids?

Answer 1

Amongst aldehydes and ketones, ketones have higher boiling point. This is due to the presence of two electron donating alkyl groups around the C=OC=O group which makes them more polar.

For example: the boiling point of CH3−CHOCHX3−CHO is 322 K and dipole moment is 2.52 D.

Boiling point of CH3−CO−CH3CHX3−CO−CHX3is 329 K and dipole moment is 2.88D

Dipole moment of CH3−CO−CH3CHX3−CO−CHX3 is greater than of CH3−CHOCHX3−CHO. This because there are two electron donating CH3CHX3 groups around C=OC=O bond whereas there is only one CH3CHX3group around C=OC=O in CH3CHOCHX3CHO.

As dipole moment is greater so it is more polar and hence has higher boiling point.

Answer 2

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Both alcohols and acids involve intermolecular hydrogen bonding. But no intermolecular H - bonding occur in aldehydes and ketones.

Therefore, attractive forces in aldehydes and ketones are lesser/weaker than acids and alcohols. Hence, lesser energy is required to break the attractive forces in aldehydes and ketones and thus they have low boiling points(low boiling point = less energy to break the attractive forces).