Explain, why `o`-hydroxybenzaldehyde is a liquid at room temperature while `p`-hydroxybenzaldehyde is a high melting solid?

Options

A. H-bonding
B. ionisation energy
C. electron gain enthalpy
D. lattice enthalpy.

Explanation

To explain why o-hydroxybenzaldehyde is a liquid at room temperature while p-hydroxybenzaldehyde is a high melting solid, we can follow these steps:

### Step 1: Understand the Structures
- **o-Hydroxybenzaldehyde** has the hydroxyl (-OH) group and the aldehyde (-CHO) group on adjacent carbon atoms in the benzene ring (ortho position).
- **p-Hydroxybenzaldehyde** has the hydroxyl (-OH) group and the aldehyde (-CHO) group on opposite sides of the benzene ring (para position).

### Step 2: Analyze Hydrogen Bonding
- In o-hydroxybenzaldehyde, an **intramolecular hydrogen bond** can form between the hydroxyl group and the hydrogen of the aldehyde group within the same molecule. This bond is relatively weak.
- In p-hydroxybenzaldehyde, there is no possibility of forming such an intramolecular bond. Instead, it can form **intermolecular hydrogen bonds** with other p-hydroxybenzaldehyde molecules. These intermolecular bonds are stronger than the intramolecular bonds in o-hydroxybenzaldehyde.

### Step 3: Determine Physical States
- The presence of **intramolecular hydrogen bonding** in o-hydroxybenzaldehyde leads to a structure that is less stable and allows it to exist as a liquid at room temperature.
- The **intermolecular hydrogen bonding** in p-hydroxybenzaldehyde leads to a more stable structure, resulting in a higher melting point and making it a solid at room temperature.

### Step 4: Conclusion
- Therefore, the key difference in the physical states of o-hydroxybenzaldehyde and p-hydroxybenzaldehyde is due to the type of hydrogen bonding present: weak intramolecular hydrogen bonding in o-hydroxybenzaldehyde versus strong intermolecular hydrogen bonding in p-hydroxybenzaldehyde.