Question

\(o\)-hydroxybenzaldehyde is a liquid while the \(p\)-isomer is a solid because:

• A. \(o\)-isomer shows intramolecular hydrogen bonding while \(p\)-isomer shows intermolecular hydrogen bonding.
• B. \(o\)-isomer shows intermolecular hydrogen bonding while \(p\)-isomer shows intramolecular hydrogen bonding.
• C. Both show intermolecular hydrogen bonding.
• D. Both show intramolecular hydrogen bonding.

Hint:

Intramolecular hydrogen bonding lowers intermolecular attraction and generally lowers melting and boiling points.

Answer 1

The Correct Option is A

Concept: Hydrogen bonding strongly affects physical properties such as boiling point and melting point.

Step 1: Understanding the ortho isomer. In \(o\)-hydroxybenzaldehyde, the \(-OH\) group and \(-CHO\) group are adjacent to each other. Therefore, hydrogen bonding occurs within the same molecule. This is called intramolecular hydrogen bonding. \[ \text{Intramolecular H-bonding} \] Because molecules are not strongly associated with each other, intermolecular attraction becomes weak. Hence, the compound has lower melting point and exists as a liquid.

Step 2: Understanding the para isomer. In \(p\)-hydroxybenzaldehyde, the two groups are far apart and cannot form intramolecular hydrogen bonding. Instead, hydrogen bonding occurs between neighboring molecules. \[ \text{Intermolecular H-bonding} \] This causes strong molecular association and increases the melting point. Therefore, the para isomer exists as a solid.

Step 3: Conclusion. Thus, option (1) is correct.