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

Intramolecular H-bonding occurs in species 2 (salicylate ion) and 4 (o-nitrophenol). Option (c) is correct.

Step 1: Understanding Intramolecular Hydrogen Bonding

Intramolecular hydrogen bonding occurs when a hydrogen atom is shared between two electronegative atoms within a single molecule. Typically, this happens when a hydrogen atom is adjacent to groups like -OH or -NH2 in a molecule, facilitating hydrogen bond formation with another electronegative group, like -COOH or -NO2 that is close by.

Step 2: Analyzing Each Species

We'll examine each given species to determine the possibility of intramolecular hydrogen bonding: 1. **Acetate ion** - This ion ( ext{CH}_3 ext{COO}^- ext{)} does not have a structure to support intramolecular hydrogen bonding. 2. **Salicylate ion** - This ion ( ext{C}_7 ext{H}_5 ext{O}_3^- ext{)} has a similarly related structure to salicylic acid, which allows for intramolecular hydrogen bonding between the OH group and the carboxylate group. 3. **Propanoic acid** - ext{C}_2 ext{H}_5 ext{COOH} does not have any groups suitable for forming intramolecular hydrogen bonds. 4. **o-nitrophenol** - The presence of an ext{OH} group adjacent to the ext{NO}_2 group allows for intramolecular hydrogen bonding.

Step 3: Choosing the Correct Option

Based on the above analysis, intramolecular hydrogen bonding occurs in species 2 (salicylate ion) and 4 (o-nitrophenol). Therefore, the correct answer is option (c) 2,4.

Key Concepts

Chemical Bonding

Chemical bonds are the strong forces that hold atoms together in molecules. There are several types of chemical bonds, but the major ones include covalent, ionic, and hydrogen bonds.

• Covalent Bonds: These are formed when two atoms share one or more pairs of electrons. This bond is common in organic compounds.
• Ionic Bonds: This type occurs when one atom donates one or more electrons to another, leading to attraction between oppositely charged ions.
• Hydrogen Bonds: A special type of dipole-dipole interaction that arises between an electronegative atom and hydrogen. In our context, we're exploring intramolecular hydrogen bonding, which happens within a single molecule.

Intramolecular hydrogen bonding involves a hydrogen atom that is shared between two electronegative atoms. These bonds are often seen in organic molecules with functional groups like -OH or -NH2 close to another electronegative atom or group, such as -COOH or -NO2. This bond is not as strong as covalent bonds but significantly influences the molecule's properties and stability.

Molecular Structure Analysis

Analyzing the structure of molecules helps to understand their chemical behavior and the possibility of forming different types of bonds. Intramolecular hydrogen bonding can be emblematic of specific molecular structures.

• Functional Groups: These are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. For instance, the -OH group is a key player in forming hydrogen bonds.
• Spatial Arrangement: The geometry of the molecule plays a crucial role. In cases such as salicylate ion or o-nitrophenol, the proximity of the -OH group to another electronegative group within the same molecule allows for this specific type of hydrogen bonding.

To assess if intramolecular hydrogen bonding is possible, one should examine if the molecule’s structure allows hydrogen to interact closely with another electronegative atom. This property can affect solubility, boiling points, and other physical properties of the compound.

Organic Chemistry

Organic chemistry, the study of carbon-containing compounds, often deals with complex structures that allow for the occurrence of unique bonding scenarios. Intramolecular hydrogen bonds are especially notable in organic compounds.

• Importance of Hydrogen Bonds: Although weaker than covalent bonds, hydrogen bonds have a profound effect on a molecule's three-dimensional shape and reactivity. In organic compounds, intramolecular hydrogen bonds can stabilize the molecule's structure.
• Common Compounds: Organic compounds like salicylates and nitrophenols often exhibit intramolecular hydrogen bonding due to their functional groups and structures. This results in unique behavior and characteristics that are highly relevant in fields such as pharmaceuticals.

Organic chemistry takes advantage of these interactions to synthesize new materials and understand the complex behavior of active compounds. Knowing how a molecule might use intramolecular hydrogen bonding is crucial for predicting its physical and chemical properties.