Question

The boiling point of p-nitrophenol is higher than that of o-nitrophenol because

• A. $NO_2$ group atp-position behave in a different way from that at o-position
• B. intramolecular hydrogen bonding exists in p-nitrophenol
• C. there is intermolecular hydrogen bonding in p-nitrophenol
• D. p-nitrophenol has a higher molecular weight than o-nitrophenol

Answer 1

The Correct Option is C

To understand why the boiling point of p-nitrophenol is higher than that of o-nitrophenol, we need to examine the types of hydrogen bonding present in each compound and how they affect the boiling point.

Step-by-step Analysis:

1. Molecular Structure and Hydrogen Bonding

The nitrophenol molecules differ in the position of the nitro group (NO_2) on the benzene ring. In p-nitrophenol, the nitro group is positioned para (opposite) to the hydroxyl (OH) group, while in o-nitrophenol, it is ortho (adjacent) to the hydroxyl group.

• o-Nitrophenol: Due to the close proximity of the functional groups, o-nitrophenol experiences intramolecular hydrogen bonding. This means that hydrogen bonds form within the same molecule, limiting the likelihood of intermolecular hydrogen bonding between different molecules.
• p-Nitrophenol: In contrast, p-nitrophenol allows for intermolecular hydrogen bonding. The spatial separation of the functional groups in the para position facilitates the formation of hydrogen bonds between different molecules, thereby creating a network of bonded molecules.

2. Impact on Boiling Point

The boiling point of a substance is impacted by the strength and type of intermolecular forces present. When strong intermolecular forces like hydrogen bonds are present, more energy (in the form of heat) is needed to separate the molecules and convert the liquid into gas, resulting in a higher boiling point.

Hence, due to the presence of intermolecular hydrogen bonding in p-nitrophenol, it has a higher boiling point when compared to o-nitrophenol, which primarily exhibits intramolecular hydrogen bonding.

Conclusion: The correct answer is that p-nitrophenol exhibits intermolecular hydrogen bonding, leading to its higher boiling point compared to o-nitrophenol.

The correct answer can be summarized as:

There is intermolecular hydrogen bonding in p-nitrophenol.