Question

The compound/s which will show significant intermolecular H-bonding is/are : 

 

• A. (a), (b) and (c)
• B. (a) and (b) only
• C. (b) only
• D. (c) only

Hint:

When assessing hydrogen bonding, check for three things: 1. Presence of H-bond donors (H on N, O, F). 2. Presence of H-bond acceptors (lone pairs on N, O, F). 3. Check for intramolecular H-bonding (usually in ortho-substituted phenols) and steric hindrance (bulky groups near the H-bonding site), as both can prevent intermolecular H-bonding.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We need to determine which of the three given compounds can form significant intermolecular hydrogen bonds. For this, a molecule needs an H atom attached to a highly electronegative atom (N, O, F) and a lone pair on another N, O, or F atom, without significant steric hindrance or preference for intramolecular H-bonding.
Step 2: Detailed Explanation:
- Compound (a): o-Nitrophenol. This molecule has a hydroxyl group (–OH) and a nitro group (–NO\(_2\)) in ortho positions (adjacent to each other). The hydrogen of the –OH group forms a strong intramolecular hydrogen bond with one of the oxygen atoms of the neighboring nitro group. This internal bonding significantly reduces the availability of the –OH group to bond with other molecules. Therefore, it exhibits very weak intermolecular H-bonding.
- Compound (b): N-(4-hydroxyphenyl)acetamide (Paracetamol). This molecule has multiple sites for H-bonding: the phenolic –OH group and the amide –NH– group can both act as H-bond donors, while the phenolic oxygen and the carbonyl oxygen (C=O) can act as H-bond acceptors. The functional groups are far apart (para), so intramolecular H-bonding is not possible. Thus, these groups are fully available to form strong intermolecular hydrogen bonds with neighboring molecules.
- Compound (c): 2,6-di-tert-butyl-4-methylphenol (BHT). This molecule has a hydroxyl (–OH) group, which is capable of H-bonding. However, the –OH group is surrounded by two extremely bulky tert-butyl groups. This crowding, known as steric hindrance, physically blocks other molecules from approaching the –OH group. Consequently, intermolecular hydrogen bonding is effectively prevented.
Step 3: Final Answer:
Based on the analysis, only compound (b) can form significant intermolecular hydrogen bonds.