Problem 65 - A Complete Resource Book in Chemistry for JEE Main

Question

Out of these acids, the strongest acid is (a) o-nitrobenzoic acid (b) \(\mathrm{p}\) - nitrobenzoic acid (c) \(\mathrm{m}-\) nitrobenzoic acid (d) benzoic acid

Step-by-Step Solution

Answer

The strongest acid is o-nitrobenzoic acid.

Step 1: Understand Acid Strength

The strength of an acid is determined by its ability to dissociate and release protons (H+ ions). When a substituent is present on the aromatic ring, it can affect the acid's strength through electron-withdrawing or electron-donating effects.

Step 2: Identify the Effect of Substituents

Nitro groups (-NO2) are strong electron-withdrawing groups. Their presence on the benzene ring decreases electron density, stabilizing the carboxylate ion formed after the acid dissociates, thereby increasing the acid's strength.

Step 3: Analyze the Position of Nitro Groups

The position of the nitro group relative to the carboxylic acid group affects its electron-withdrawing power. The ortho (o-) position allows for resonance interactions and more significant electron-withdrawing effects than the meta (m-) or para (p-) positions.

Step 4: Compare the Options and Determine the Strongest Acid

o-nitrobenzoic acid has the nitro group in the ortho position, providing the strongest electron-withdrawing effect compared to m-nitrobenzoic, p-nitrobenzoic, and benzoic acid with no nitro group. Therefore, o-nitrobenzoic acid is the strongest acid among the options.

Key Concepts

Electron-Withdrawing Groups

Electron-withdrawing groups are functional groups that pull or "withdraw" electron density away from the rest of the molecule. They achieve this due to their electronegativity or the presence of electronegative atoms. In the context of acids, electron-withdrawing groups play a crucial role in determining acid strength.

• These groups stabilize the negative charge on the conjugate base, which forms after the acid loses a proton (H⁺).
• This stabilization makes it easier for the acid to release a proton, thereby increasing acid strength.
• An example of a strong electron-withdrawing group is the nitro group (-NO₂), known for its ability to significantly stabilize conjugate bases.

In summary, electron-withdrawing groups improve an acid's ability to dissociate by stabilizing the resulting negative charge.

Substituent Effects

Substituents are groups of atoms that are attached to the main structure of a molecule, influencing its chemical properties. In aromatic compounds, such as benzene rings, substituents can modify the electron distribution within the molecule:

• Substituents can either donate or withdraw electron density. This affects the acidity by altering the stability of the conjugate base.
• Electron-withdrawing substituents enhance acidic properties, as they stabilize charged species by dispersing the negative charge.
• Conversely, electron-donating groups reduce acid strength, as they destabilize the conjugate base by increasing electron density in areas that prefer to maintain a negative charge.

Substituent effects are crucial when predicting the behavior of aromatic acids and can dramatically alter their acid strength depending on the nature of the substituent and its position on the ring.

Resonance Interactions

Resonance interactions refer to the delocalization of electrons within a molecule, typically involving pi bonds and lone pairs. This concept is particularly important in organic chemistry for stabilizing structures:

• In acidic molecules, resonance can delocalize and stabilize the negative charge of the conjugate base across the molecule.
• The position of substituents, such as nitro groups in benzoic acids, can influence resonance. For example, ortho and para substituents facilitate resonance with the carboxylate group after deprotonation, enhancing stability.
• The meta position, however, generally provides less resonance stabilization compared to ortho and para, resulting in weaker acid strength.

Understanding resonance interactions can help predict the relative acidity of compounds by revealing how electronic structures and substituent positions interact to stabilize or destabilize the molecule's charged states.