Question

Which of the following has a higher boiling point?
(A) Butanol
(B) Butanal
(C) Both of them have similar boiling point
(D) Cannot say anything

Answer 1

Hint: A hydrogen bond is formed when a hydrogen atom forms bond with an electronegative oxygen, nitrogen or fluorine atom. Due to the presence of intermolecular hydrogen bonding, the boiling point increases.

Complete answer:
Write the structural formulae of butanol and butanal as shown below:
\[ {\text{Butanal: C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{CHO }}
\\
{\text{Butanol: C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{OH }} \]
Butanal is an aldehyde. It contains a carbon-oxygen double bond. Butanol is an alcohol. It contains hydroxyl groups. Hydroxyl groups contain oxygen-hydrogen single bonds.
The boiling points of butanol and butanal vary widely. But they have similar water solubilities.
In butanal, due to absence of hydroxyl group, formation of intermolecular hydrogen bonding is not possible. So molecular association is not possible.
In butanol, due to the presence of hydroxyl group, there is formation of intermolecular hydrogen bonding. This results in molecular association. You need a large amount of energy to break these associations.
Hence, the boiling point of butanol is higher than the boiling point of butanal.

Hence, the correct option is the option (A) Butanal.

Additional Information: Both butanol and butanal can form hydrogen bonds with water, as they contain electronegative oxygen atoms. Hence, both have almost similar solubility in water.

Note: In the formation of intermolecular hydrogen bonds, oxygen atom of one butanol molecule forms a hydrogen bond with hydrogen atom of hydroxyl group of other butanol molecules. Similarly, oxygen atom of second butanol molecule forms a hydrogen bond with hydrogen atom of hydroxyl group of third butanol molecule.