Problem 88 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Statement-1: \(p\)-Hydroxybenzoic acid has a lower boiling point than \(o\)-hydroxybenzoic acid. Statement-2 : o-Hydroxybenzoic acid has intramolecular hydrogen bonding.

Step-by-Step Solution

Answer

Statement-1 is correct because \( p \)-Hydroxybenzoic acid has stronger intermolecular hydrogen bonds. Statement-2 is also correct; it explains \( o \)-Hydroxybenzoic acid's lower boiling point.

Step 1: Understanding the Compounds

First, let's understand the compounds involved: \( p \)-Hydroxybenzoic acid and \( o \)-Hydroxybenzoic acid. \( p \) means para, which is a position on the benzene ring opposite to the functional group, while \( o \) means ortho, which is adjacent to the functional group on the ring. Both compounds have a hydroxyl group (-OH) on the benzene ring and a carboxylic acid group (-COOH).

Step 2: Analyzing Boiling Points

The boiling point of a compound is influenced by the type and strength of intermolecular forces present. \( o \)-Hydroxybenzoic acid participates in intramolecular hydrogen bonding, which decreases the availability of these groups for intermolecular bonding, whereas \( p \)-Hydroxybenzoic acid cannot form intramolecular hydrogen bonds, leading to stronger intermolecular hydrogen bonding.

Step 3: Role of Intramolecular Hydrogen Bonding

In \( o \)-Hydroxybenzoic acid, the proximity of the hydroxyl group to the carboxyl group leads to the formation of intramolecular hydrogen bonds. This interaction locks the polar groups within the same molecule, thus reducing their ability to interact with other molecules (less intermolecular hydrogen bonding).

Step 4: Impact on Boiling Points

As \( o \)-Hydroxybenzoic acid has weaker intermolecular forces due to intramolecular hydrogen bonding, it has a lower boiling point relative to \( p \)-Hydroxybenzoic acid, which forms strong intermolecular hydrogen bonds that require more energy to break.

Key Concepts

Intramolecular Hydrogen Bonding

Intramolecular hydrogen bonding occurs within a single molecule. When molecules have groups that can form hydrogen bonds close to each other, they can effectively create bonds that stabilize the internal structure. In the case of \( o \)-hydroxybenzoic acid, the hydroxyl (-OH) group sits right next to the carboxylic acid (-COOH) group on the benzene ring. This adjacency facilitates the formation of a hydrogen bond between these two groups within the same molecule.
Because this bond is formed intramolecularly, it reduces the ability of these groups to form hydrogen bonds with other molecules, which impacts the compound's physical properties like boiling point. This internal bonding decreases the molecule's interaction with other molecules, making it less 'sticky' to its neighbors and generally lowering boiling points compared to other isomers without such bonding.

Intermolecular Forces

Intermolecular forces are pivotal in determining the boiling points of substances. They are the forces of attraction between molecules, not within a single molecule, like intramolecular forces. The strength of these forces varies, which directly affects how much energy is needed to separate the molecules during a phase change like boiling.
The two hydroxybenzoic acids show differing boiling points due to these forces. In \( o \)-hydroxybenzoic acid, intramolecular hydrogen bonds dominate, leaving fewer groups available for intermolecular bonding. Contrastingly, in \( p \)-hydroxybenzoic acid, no such internal bonding occurs, allowing it stronger intermolecular hydrogen bonds, as its \(-OH\) and \(-COOH\) groups can bond more efficiently with those of neighboring molecules.
While intramolecular hydrogen bonding stabilizes the molecule internally, it weakens its ability to bond with other molecules, significantly lowering the boiling point when compared to \( p \)-hydroxybenzoic acid.

Ortho and Para Isomers

Ortho and para refer to the positions of substituents on the benzene ring in dibenzene derivatives. "Ortho" (\(o\)) indicates that the groups are adjacent to each other on the ring, while "para" (\(p\)) denotes that they are opposite each other, sitting on carbons that are separated by other carbons.
In aromatic compounds, such configurations greatly influence their chemical properties and behavior. For example, in \(o\)-hydroxybenzoic acid, the proximity of the hydroxyl and carboxyl groups allows for intramolecular hydrogen bonding. This is not possible in \(p\)-hydroxybenzoic acid where these groups are too far apart to interact intramolecularly.
This positioning explains why \(o\)-hydroxybenzoic acid experiences unique internal stabilization. Conversely, the absence of such interactions in the "para" isomers allows for stronger interactions with neighboring molecules, affecting their boiling points. These differences highlight how the relative position of functional groups can impact molecular behavior drastically.