Question

In which of the following, intramolecular hydrogen bonding is present?

• A. Resorcinol
• B. Catechol
• C. Quinol
• D. o-Cresol

Hint:

For intramolecular H-bonding to occur in substituted benzene rings, the two participating groups must be in the ortho (1,2) position to each other. Groups in meta (1,3) or para (1,4) positions are too far apart. Common examples include catechol, o-nitrophenol, and salicylaldehyde.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Hydrogen bonding can be of two types:
- Intermolecular hydrogen bonding: Occurs between two or more different molecules.
- Intramolecular hydrogen bonding: Occurs within a single molecule, typically when a hydrogen atom bonded to a highly electronegative atom (like O, N, F) can interact with another electronegative atom within the same molecule. This usually requires the formation of a stable five or six-membered ring.
Step 2: Analyzing the Structures:
- (A) Resorcinol (Benzene-1,3-diol): The two -OH groups are on carbons 1 and 3 of the benzene ring. They are too far apart to form an intramolecular hydrogen bond. They will form intermolecular hydrogen bonds with other resorcinol molecules.
- (B) Catechol (Benzene-1,2-diol): The two -OH groups are on adjacent carbon atoms (positions 1 and 2). The hydrogen atom of one -OH group can form a hydrogen bond with the oxygen atom of the adjacent -OH group. This forms a stable six-membered ring structure, facilitating intramolecular hydrogen bonding.
- (C) Quinol (Hydroquinone, Benzene-1,4-diol): The two -OH groups are on opposite sides of the benzene ring (positions 1 and 4). They are very far apart and cannot form an intramolecular hydrogen bond. They will form extensive intermolecular hydrogen bonds.
- (D) o-Cresol (2-Methylphenol): This molecule has one -OH group and one -CH₃ group. While the -OH group can form intermolecular hydrogen bonds, there is no second electronegative atom in a suitable position for intramolecular hydrogen bonding.
Step 3: Final Answer:
Among the given options, only catechol has two hydroxyl groups in close enough proximity (ortho positions) to allow for the formation of an intramolecular hydrogen bond. Therefore, option (B) is correct.