Question

Which among the following compounds has highest melting point?

• A. Phenol
• B. p-Nitrophenol
• C. p-Cresol
• D. o-Nitrophenol

Hint:

Remember this classic rule: Para-isomers generally exhibit much higher melting points than their ortho or meta counterparts because their high structural linearity and symmetry allow them to pack more tightly inside a solid crystal lattice.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
We need to compare the physical melting points of four related phenolic organic molecules and determine which one possesses the highest melting point based on its molecular arrangement and intermolecular forces.

Step 2: Key Formula or Approach:
The melting point of an organic compound depends on its structural symmetry and its capacity to establish a rigid crystal lattice via cohesive intermolecular forces, such as intermolecular hydrogen bonding.

Step 3: Detailed Explanation:
Let's contrast the hydrogen bonding modes of the two principal candidates, ortho-nitrophenol and para-nitrophenol: In

o-nitrophenol, the $-\text{OH}$ and $-\text{NO}_2$ functional groups are physically adjacent. This proximity allows the molecule to form strong

intramolecular hydrogen bonds (within the same molecule), creating a closed ring structure. Because the hydrogen bonding is self-contained, these molecules do not interact strongly with their neighbors, leading to a lower melting point and high steam volatility. In

p-nitrophenol, the $-\text{OH}$ and $-\text{NO}_2$ groups are located on opposite sides of the aromatic ring (para position). This spatial separation makes intramolecular bonding impossible. Instead, it forms an extensive, continuous network of strong

intermolecular hydrogen bonds linking adjacent molecules together in a highly organized, tight crystal lattice. Breaking this extensive network requires significant thermal energy, which results in a much higher melting point compared to its ortho counterpart, phenol, and cresols.

Step 4: Final Answer:
The compound with the highest melting point is p-nitrophenol, which corresponds to option (B).