Problem 90 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following species, intramolecular H-bonding occurs? 1\. acetate ion 2\. salicylate ion 3\. propanoic acid 4\. o-nitrophenol (a) \(1,3,4\) (b) \(1,2,4\) (c) 2,4 (d) 4 only

Step-by-Step Solution

Answer

(c) 2,4

Step 1: Understanding Intramolecular Hydrogen Bonding

Intramolecular hydrogen bonding occurs when a hydrogen atom is bonded to a highly electronegative atom (such as oxygen, nitrogen, or fluorine) within the same molecule, and this hydrogen atom forms a hydrogen bond with another electronegative atom within the same molecule. This often occurs in cyclic structures where the necessary donor and acceptor atoms are positioned close enough to form a stable interaction.

Step 2: Analyzing Acetate Ion

The acetate ion, ext{CH}_3 ext{COO}^-, lacks sufficient proximity between hydrogen and another electronegative atom within the ion to form intramolecular hydrogen bonds. This is because the acetate ion has two oxygen atoms bonded to a central carbon without an internal structure that allows for hydrogen bonding between them.

Step 3: Analyzing Salicylate Ion

The salicylate ion can engage in intramolecular hydrogen bonding because it is derived from salicylic acid, where the hydroxyl group (OH) is ortho to the carboxylate group (COO^-). The proximity allows the hydrogen from the OH group to form a hydrogen bond with the carboxylate oxygen.

Step 4: Analyzing Propanoic Acid

Propanoic acid ( ext{CH}_3 ext{CH}_2 ext{COOH}) does not form intramolecular hydrogen bonds as the carboxyl group present at the end of the carbon chain cannot create a cyclic structure necessary for intramolecular bonding as it lacks an additional electronegative atom within bond-forming range.

Step 5: Analyzing o-Nitrophenol

In o-nitrophenol, the nitro group (NO_2) is ortho to the hydroxyl group on the benzene ring. This positions the oxygen of the nitro group close enough to the hydroxyl hydrogen, allowing the formation of an intramolecular hydrogen bond, thus making it a perfect candidate for intramolecular hydrogen bonding.

Step 6: Conclusion of Analysis

From the analysis, we can determine that intramolecular hydrogen bonding occurs in the salicylate ion and o-nitrophenol. These are the molecules which inherently have the structural arrangement required for intramolecular hydrogen bonding. Hence, the correct answer is option (c) 2,4.

Key Concepts

Salicylate Ion

The salicylate ion is an interesting example of how intramolecular hydrogen bonding can impact molecular chemistry. Derived from salicylic acid, the salicylate ion comprises a benzene ring with a carboxylate group ( COO^−) and a hydroxyl group ( OH) positioned ortho (adjacent) to each other. This proximity allows the hydroxyl hydrogen to form a hydrogen bond with the oxygen in the carboxylate group within the same molecule.

Here are the key points about salicylate ion:

• The structure promotes the formation of a six-membered ring, stabilizing the intramolecular hydrogen bonding.
• This bonding affects the ion's acidity and reactivity, making the salicylate ion unique in its chemical behavior.
• Such bonds can significantly lower the energy, influencing solubility and other physical properties of the molecule.

Understanding salicylate ion helps us appreciate how structural arrangement can impact molecular interactions and properties, illustrating the power of intramolecular hydrogen bonding.

o-Nitrophenol

o-Nitrophenol exhibits a notable example of intramolecular hydrogen bonding due to the strategic position of its functional groups. In this molecule, a nitro group ( NO_2) is located ortho to a hydroxyl group ( OH) on a benzene ring. This proximity creates an ideal setting for the oxygen of the nitro group to form a hydrogen bond with the hydrogen of the hydroxyl group within the same molecule.

Main characteristics of o-nitrophenol:

• The intramolecular hydrogen bond formation results in a stable six-membered ring structure within the molecule.
• This type of bonding lowers the molecule's overall energy, contributing to its stability.
• It can also influence the melting and boiling points, generally increasing them relative to molecules that cannot form such bonds.

By studying o-nitrophenol, chemists gain insights into how subtle changes in molecular structure can profoundly affect chemical properties and behaviors.

Chemistry JEE Main

Chemistry JEE Main is an essential examination for students aspiring to pursue engineering in India. Intramolecular hydrogen bonding is one of the critical topics covered in the chemistry section of this exam. Understanding this concept is vital for problem-solving and applying theoretical knowledge to practical scenarios.

Key reasons why intramolecular hydrogen bonding is crucial for JEE Chemistry:

• It helps in understanding the stability and reactivity of organic and inorganic compounds.
• Questions often require students to identify molecules capable of such bonding, which involves interpreting molecular structures and functional group placements.
• Mastery of this topic enables students to predict molecular behavior, such as solubility and acidity, critical for solving more complex chemistry problems.

Chemistry JEE Main emphasizes concepts that bridge the gap between theoretical chemistry and real-world applications, and intramolecular hydrogen bonding stands out as a fundamental concept in this domain.