The order of melting point of ortho, para, meta nitrophenol is

Options

A. o gt m gt p
B. p gt m gt o
C. m gt p gt o
D. p gt o gt m

Explanation

To determine the order of melting points of ortho, para, and meta nitrophenol, we will analyze the structures and symmetry of each compound. Here's a step-by-step solution:

### Step 1: Draw the Structures
1. **Ortho-nitrophenol**: The -OH (hydroxyl) group and -NO2 (nitro) group are adjacent to each other on the benzene ring.
2. **Meta-nitrophenol**: The -OH group and -NO2 group are separated by one carbon atom on the benzene ring.
3. **Para-nitrophenol**: The -OH group and -NO2 group are opposite each other on the benzene ring.

### Step 2: Analyze Symmetry
- **Symmetry** plays a crucial role in determining the melting point of a compound. Higher symmetry generally leads to better packing in the crystal lattice, which results in a higher melting point.
- **Para-nitrophenol** has the highest symmetry because the substituents are opposite each other, allowing for a more efficient packing in the crystal lattice.
- **Meta-nitrophenol** has moderate symmetry as the substituents are separated by one carbon, leading to less efficient packing than para-nitrophenol.
- **Ortho-nitrophenol** has the lowest symmetry due to the proximity of the -OH and -NO2 groups, which can lead to steric hindrance and disrupt packing.

### Step 3: Order of Melting Points
Based on the analysis of symmetry:
- **Para-nitrophenol** has the highest melting point due to its high symmetry.
- **Meta-nitrophenol** has a lower melting point than para-nitrophenol but higher than ortho-nitrophenol.
- **Ortho-nitrophenol** has the lowest melting point due to its low symmetry and potential for hydrogen bonding which disrupts packing.

### Conclusion
The order of melting points is:
**Para-nitrophenol > Meta-nitrophenol > Ortho-nitrophenol**

### Melting Point Values (for reference)
- Para-nitrophenol: ~114°C
- Meta-nitrophenol: ~97°C
- Ortho-nitrophenol: ~44-45°C