Q.Assertion: o-nitrophenol is less soluble in water than m- and p-isomers.
Reason: m- and p-nitrophenols exist as associated molecules.

Concept: Intramolecular vs. Intermolecular Hydrogen Bonding

The key is that o-nitrophenol forms an intramolecular hydrogen bond between the –OH and –NO₂ groups. This locks the molecule into a non-polar, chelate-like structure, reducing its ability to interact with water (a polar solvent). Hence, it is less soluble.

In contrast, m- and p-nitrophenols cannot form such an internal bond. Their –OH and –NO₂ groups are too far apart, so they instead form intermolecular hydrogen bonds with neighbouring molecules (i.e., they exist as "associated molecules" in the solid/liquid state). This association does not hinder — and actually still allows — hydrogen bonding with water, making them more soluble.

Steps:

1. o-Nitrophenol has an intramolecular H-bond → less polar, lower solubility in water.
2. m- and p-isomers lack intramolecular bonding → they form intermolecular H-bonds (associated molecules). …

Let’s start with the concept. Solubility in water depends on how well a molecule can form hydrogen bonds with water molecules. Phenol itself is moderately soluble because its –OH group can donate and accept hydrogen bonds with water. But when you add a nitro group (–NO₂) to the ring, the position of that group matters enormously.

In o-nitrophenol, the –OH and –NO₂ groups are right next to each other (ortho position). This allows the hydrogen of the –OH to form a strong intramolecular hydrogen bond with the oxygen of the –NO₂ group. That hydrogen is now “tied up” inside the molecule — it cannot easily reach out to water molecules. As a result, o-nitrophenol is less soluble in water.

In m- and p-nitrophenols, the –OH and –NO₂ groups are far apart. They cannot form an intramolecular hydrogen bond. Instead, the –OH group of one molecule can form intermolecular hydrogen bonds with the –OH or –NO₂ groups of neighbouring molecules. This leads to “associated molecules” — clusters held together by hydrogen bonds. But crucially, these –OH groups are still free to interact with water, so the solubility is higher than that of the ortho isomer.

Now let’s examine the given statements step by step.

1. Assertion: o-nitrophenol is less soluble in water than m- and p-isomers.

   This is correct. The intramolecular hydrogen bonding in o-nitrophenol reduces its ability to form hydrogen bonds with water, lowering its solubility. The meta and para isomers lack this internal bonding and thus dissolve better.

2. Reason: m- and p-nitrophenols exist as associated molecules.

   This is also correct. Because they form intermolecular hydrogen bonds, they exist as associated (dimeric or polymeric) structures in the solid or liquid state. This is a well-known fact — for example, p-nitrophenol has a much higher melting point than o-nitrophenol precisely because of this association.

3. Does the reason explain the assertion? …

Method: Intramolecular vs. Intermolecular Hydrogen Bonding Analysis

Step 1: Identify the key property difference

• o-nitrophenol has the –NO₂ and –OH groups adjacent (ortho position).
• m- and p-nitrophenols have these groups separated (meta and para positions).

Step 2: Analyse hydrogen bonding in each isomer

• o-nitrophenol: The –OH group forms a strong intramolecular hydrogen bond with the –NO₂ group. This "locks" the molecule, preventing it from forming hydrogen bonds with water.
• m- and p-nitrophenols: The –OH and –NO₂ groups are too far apart for intramolecular bonding. Instead, they form intermolecular hydrogen bonds with neighbouring molecules (and with water).

Step 3: Relate to solubility

• Intramolecular H-bonding (o-isomer) → molecule behaves as a non-polar entity → low solubility in water.
• Intermolecular H-bonding (m- and p-isomers) → molecules can interact with water via H-bonds → higher solubility.

Step 4: Evaluate the Assertion and Reason …

Common Mistakes & How to Avoid Them

Mistake 1: Confusing "Associated Molecules" with Solubility

The error: Students think that if molecules are "associated" (hydrogen-bonded to each other), they become more soluble in water. This is wrong.

Why it's wrong:

• Intramolecular H-bonding (in o-nitrophenol) → molecule curls up, cannot bond with water → less soluble
• Intermolecular H-bonding (in m- and p-nitrophenols) → molecules bond with each other, forming clusters — this does not increase water solubility

Correct understanding:

• Solubility in water depends on H-bonding with water, not between themselves.
• o-nitrophenol has intramolecular H-bonding → less available to bond with water → less soluble
• m- and p-nitrophenols have intermolecular H-bonding → still can bond with water → more soluble

✓ Avoid by: Remember: "Intra = inside, less water love; Inter = between, still water friendly"

Mistake 2: Misinterpreting the Reason Statement

The error: Students assume "associated molecules" means the same as "more soluble."

Why it's wrong:

• The Reason says: "m- and p-nitrophenols exist as associated molecules" — this is true (they form intermolecular H-bonds)
• But this does not explain why o-nitrophenol is less soluble

Correct logic:

• The real reason for lower solubility of o-nitrophenol is intramolecular H-bonding (chelation), which reduces its ability to form H-bonds with water
• The Reason describes a different phenomenon (intermolecular association in isomers), which is not the cause of the Assertion

✓ Avoid by: Always ask: "Does the Reason directly cause the Assertion?" If not, it's not the correct explanation.

Mistake 3: Thinking the Reason is False

The error: Some students think m- and p-nitrophenols do not form associated molecules.

Why it's wrong:

• m- and p-nitrophenols do form intermolecular H-bonds (higher melting points, lower volatility)
• This is a well-established fact

Correct fact:

• o-nitrophenol: intramolecular H-bonding → lower melting point, more volatile
• m- and p-nitrophenols: intermolecular H-bonding → higher melting points, less volatile

✓ Avoid by: Memorise the pattern: Ortho = intra (lower mp, more volatile, less water soluble); Meta/Para = inter (higher mp, less volatile, more water soluble)

Mistake 4: Choosing Option (A) — Both correct, Reason explains Assertion …