Which of the following molecules shows intramolecular hydrogen bonding?

Options

A. p-Nitrophenol
B. Ethanol
C. o-Nitrophenol
D. Methanamine

Answer

To determine which of the given molecules shows intramolecular hydrogen bonding, we need to analyze the structures of the molecules provided in the options. Intramolecular hydrogen bonding occurs within a single molecule, while intermolecular hydrogen bonding occurs between different molecules.

### Step-by-Step Solution:

1. **Understanding Intramolecular Hydrogen Bonding**:
- Intramolecular hydrogen bonding occurs when a hydrogen atom covalently bonded to an electronegative atom (like oxygen, nitrogen, or fluorine) interacts with another electronegative atom within the same molecule.

2. **Analyzing the Options**:
- We need to look at the structures of the given molecules and identify if any of them can form intramolecular hydrogen bonds.

3. **Option 1: Para-nitrophenol**:
- Structure: The hydroxyl (–OH) group and the nitro (–NO2) group are on the para position of the benzene ring.
- Analysis: The distance between the –OH and –NO2 groups is too far for intramolecular hydrogen bonding to occur. Therefore, this molecule does not show intramolecular hydrogen bonding.

4. **Option 2: Ethanol**:
- Structure: Ethanol has a structure of CH3-CH2OH.
- Analysis: In ethanol, the –OH group can form hydrogen bonds with other ethanol molecules (intermolecular), but there is no intramolecular hydrogen bonding as there is no other electronegative atom within the same molecule to bond with.

5. **Option 3: Ortho-nitrophenol**:
- Structure: In ortho-nitrophenol, the –OH group is adjacent to the –NO2 group on the benzene ring.
- Analysis: The hydrogen atom of the –OH group can form a hydrogen bond with the electronegative oxygen of the nitro group, which is present in the same molecule. This leads to the formation of intramolecular hydrogen bonding.

6. **Option 4: Methanamine**:
- Structure: Methanamine has the structure CH3-NH2.
- Analysis: In methanamine, the nitrogen can form hydrogen bonds with the hydrogen atoms of other methanamine molecules (intermolecular), but there is no intramolecular hydrogen bonding since there is no other electronegative atom within the same molecule.

### Conclusion:
Based on the analysis, the molecule that shows intramolecular hydrogen bonding is **ortho-nitrophenol**.