Problem 216 - A Complete Resource Book in Chemistry for JEE Main

Question

Ortho \(-\) Nitrophenol is less soluble in water than pand \(\mathrm{m}-\) Nitrophenols because (a) Melting point of o- Nitrophenol is lower than those of \(\mathrm{m}\) - and \(\mathrm{p}\) - isomers (b) o- Nitrophenol is more volatile in steam than those of \(\mathrm{m}\) - and \(\mathrm{p}\) - isomers (c) o- Nitrophenol shows Intramolecular H-bonding (d) o- Nitrophenol shows Intermolecular H-bonding

Step-by-Step Solution

Answer

(c) o- Nitrophenol shows Intramolecular H-bonding.

Step 1: Understanding the Solubility Concept

Solubility in water generally involves the ability of a compound to form interactions with water, such as hydrogen bonds. Compounds that can form inter-molecular hydrogen bonds with water are typically more soluble.

Step 2: Identify H-bonding in the o-nitrophenol

Ortho-nitrophenol can form intramolecular hydrogen bonds, where hydrogen forms a bond with another atom in the same molecule, rather than interacting with water, hence reducing its solubility.

Step 3: Comparing with m- and p-nitrophenol

Meta- and para-nitrophenol forms intermolecular hydrogen bonds with water, which increases their solubility compared to ortho-nitrophenol's intramolecular hydrogen bonding.

Step 4: Evaluating Given Options

Option (c), which states that ortho-nitrophenol shows intramolecular H-bonding, directly relates to the reduced solubility in water, while the other options do not have a direct impact on solubility.

Key Concepts

Intramolecular Hydrogen Bonding

Intramolecular hydrogen bonding occurs within a single molecule when a hydrogen atom interacts with an electronegative atom nearby. In the case of ortho-nitrophenol, the hydrogen atom near the hydroxyl group can form a bond with the oxygen atom in the nitro group within the same molecule.

This results in a stable, closed-loop structure that effectively reduces the molecule's ability to interact with water molecules. Unlike intermolecular hydrogen bonds, intramolecular bonds keep the interaction internal.

As a result, ortho-nitrophenol is less inclined to form the intermolecular attractions needed for solubility in water.

Intermolecular Hydrogen Bonding

Intermolecular hydrogen bonding is crucial for solubility in polar solvents like water. It involves hydrogen atoms in a molecule forming bonds with electronegative atoms, such as oxygen or nitrogen, in other molecules. This kind of hydrogen bonding promotes higher solubility because the molecules can mix thoroughly.

For meta- and para-nitrophenols, the lack of steric hindrance from the nitro group allows them to form these types of hydrogen bonds with water molecules.

• This means the molecules can dissolve more easily in water.
• They establish strong interactions between themselves and water, promoting solubility.

In contrast, the intramolecular bonding in ortho-nitrophenol restricts these interactions, making it less soluble.

Solubility of Isomers

The solubility of isomers, such as the ortho, meta, and para forms of nitrophenol, varies based on their molecular structure. This affects how they can interact with solvents. Ortho-nitrophenol has a unique arrangement due to the positioning of the nitro group next to the hydroxyl group, leading to intramolecular hydrogen bonding.

In contrast, the meta and para isomers facilitate more intermolecular interactions with solvents, such as water.

• Ortho-nitrophenol's intramolecular bonds prevent interactions with water, reducing solubility.
• Meta and para isomers are more soluble due to their capacity for intermolecular hydrogen bonding.

Understanding how isomer structures impact solubility helps predict and explain differences in chemical behavior.

Ortho Effect in Chemistry

The ortho effect in chemistry describes how the presence of a substituent group at the ortho position can alter a compound's properties, such as reactivity, acidity, and solubility. In ortho-nitrophenol, the proximity of the nitro and hydroxyl groups allows intramolecular hydrogen bonding.

This internal bonding decreases the molecule's ability to engage in intermolecular interactions necessary for solubility in solvents like water.

• The ortho effect results in specific properties unique to ortho isomers.
• It showcases the impact of structure on chemical behavior, particularly in terms of solubility.

Recognizing the ortho effect helps in the prediction of physical and chemical properties of compounds.