Question

o-Hydroxybenzaldehyde is a liquid at room temperature while p- Hydroxybenzaldehyde is a high melting solid because of:
A. H-bonding
B. ionisation energy
C. electron gain enthalpy
D. lattice enthalpy

Answer 1

As we know that there are molecules in liquid that are held by strong intermolecular interactions, which are weaker than the intramolecular interactions, which is found to hold the atoms together in polyatomic ions and molecules. It is found that higher the intermolecular forces, higher is the melting point.

Complete Solution :
- As we can see that intramolecular hydrogen bonding is present in o-Hydroxybenzaldehyde, and therefore it exists as a discrete molecule.
- Whereas, intermolecular hydrogen bonding is present in p-Hydroxybenzaldehyde and therefore it exists as associated molecules. It is found that a large amount of energy is needed to break these intermolecular hydrogen bonds.
- Therefore, p- Hydroxybenzaldehyde has a high melting point and boiling point compared to that of o-Hydroxybenzaldehyde. As, a result of this p- Hydroxybenzaldehyde is a high melting solid and o-Hydroxybenzaldehyde is a liquid at room temperature
- Hence, the correct option is (a), that is o-Hydroxybenzaldehyde is a liquid at room temperature while p- Hydroxybenzaldehyde is a high melting solid because of H-bonding.
So, the correct answer is “Option A”.

Note: - We should not get confused in the terms intermolecular hydrogen bonding and intramolecular hydrogen bonding. As in intermolecular hydrogen bonding, hydrogen bonding takes place between different molecules of the same or different compounds.
- Whereas, in intramolecular hydrogen bonding takes place within a molecule itself.