Question

Intramolecular hydrogen bonding is found in

• A. HF
• B. CH\(_3\)OH
• C. o-Nitrophenol
• D. o-Cresol

Hint:

Intramolecular H-bonding occurs when donor and acceptor groups are in close proximity on the same molecule (e.g., o-nitrophenol).

Answer 1

The Correct Option is C

Intramolecular hydrogen bonding occurs when a hydrogen atom forms a bond with an electronegative atom within the same molecule.
In o-nitrophenol, the \(-OH\) and \(-NO_2\) groups are present in ortho positions, allowing the formation of a hydrogen bond within the molecule.
Other options:
- HF → forms intermolecular H-bonding
- CH\(_3\)OH → also shows intermolecular H-bonding
- o-Cresol → primarily forms intermolecular H-bonds

Answer 2

Step 1: Concept of Intramolecular Hydrogen Bonding
Intramolecular hydrogen bonding is a type of hydrogen bonding that occurs within the same molecule. It typically happens when a hydrogen atom is covalently bonded to a highly electronegative atom like oxygen or nitrogen, and there is another electronegative atom nearby within the same molecule that can act as a hydrogen bond acceptor. This type of bonding is favored when the donor and acceptor groups are positioned close to each other, such as in the ortho positions of a benzene ring. Intramolecular hydrogen bonding stabilizes the molecule internally and can affect its physical properties like solubility and boiling point.

Step 2: Structure of o-Nitrophenol
o-Nitrophenol is an aromatic compound where the hydroxyl group (–OH) and the nitro group (–NO₂) are attached to adjacent carbon atoms on a benzene ring, known as the ortho position. This proximity allows the hydrogen atom of the –OH group to form a hydrogen bond with one of the oxygen atoms of the nitro group. This hydrogen bonding happens within the same molecule, hence called intramolecular hydrogen bonding. It causes the molecule to assume a specific conformation that minimizes its energy by forming a five- or six-membered ring involving the hydrogen bond.

Step 3: Comparison with Other Isomers
In contrast to o-Nitrophenol, m-Nitrophenol (meta) and p-Nitrophenol (para) have the –OH and –NO₂ groups farther apart on the benzene ring. In these isomers, the distance between the two functional groups is too great to allow intramolecular hydrogen bonding. Instead, they often participate in intermolecular hydrogen bonding, where hydrogen bonds are formed between different molecules. This difference significantly affects their physical properties; for example, o-Nitrophenol tends to be less soluble in water but more volatile compared to its isomers because intramolecular hydrogen bonding reduces the molecule’s ability to interact with water molecules.

Step 4: Final Conclusion
Therefore, among the nitrophenol isomers, only o-Nitrophenol exhibits intramolecular hydrogen bonding due to the close spatial arrangement of the –OH and –NO₂ groups. This internal hydrogen bonding makes o-Nitrophenol distinct in terms of boiling point, solubility, and spectroscopic properties. It is a classic example used to illustrate the concept of intramolecular hydrogen bonding in organic chemistry.