Question

A compound of molecular formula \(\left(\mathrm{C}_{8} \mathrm{H}_{6} \mathrm{O}_{4}\right)\) is a benzene dicarboxylic acid which can form anhydride on heating. The correct statement regarding this compound is (A) Most acidic among all isomeric dicarboxylic acid (B) Less acidic than \(\mathrm{CH}_{3} \mathrm{COOH}\) (C) Less acidic than \(\mathrm{PhCOOH}\) (D) None of these

Short answer

The correct statement regarding phthalic acid is: (A) Most acidic among all isomeric dicarboxylic acids.

Step-by-step solution

Identify the benzene dicarboxylic acid that forms an anhydride

The given molecular formula is \(\mathrm{C}_{8} \mathrm{H}_{6} \mathrm{O}_{4}\), which indicates a benzene dicarboxylic acid. There are three possible isomers of benzene dicarboxylic acid: ortho-phthalic acid, meta-phthalic acid, and para-phthalic acid(or terephthalic acid). Among these three isomers, only phthalic acid (ortho-phthalic acid) can form an anhydride on heating. So, the compound we'll analyze in this exercise is phthalic acid.

Compare the acidity of phthalic acid with the acidic compounds in statements

We will now compare the acidity of phthalic acid with the acidic compounds in the given statements. (A) Most acidic among all isomeric dicarboxylic acid: Ortho-phthalic acid (phthalic acid) is more acidic than its isomers meta- and para-phthalic acids due to the intramolecular hydrogen bonding. (B) Less acidic than \(\mathrm{CH}_{3} \mathrm{COOH}\) (acetic acid): Phthalic acid is more acidic than acetic acid because of the strong electron-withdrawing effect of the two carboxyl groups attached to the benzene ring. These groups destabilize the anion formed after losing H+ ion, thus making phthalic acid more acidic. (C) Less acidic than \(\mathrm{PhCOOH}\) (benzoic acid): Phthalic acid is also more acidic than benzoic acid, as it has two carboxyl groups, resulting in a stronger resonance effect. This resonance effect destabilizes the anion formed on losing H+ ion, making phthalic acid more acidic.

Conclusion

Based on our analysis and comparison, we can conclude that the correct statement regarding phthalic acid is: (A) Most acidic among all isomeric dicarboxylic acids.

Key concepts

Phthalic Acid

Phthalic acid, also known as ortho-phthalic acid, is an aromatic compound with the molecular formula \(\mathrm{C}_{8}\mathrm{H}_{6}\mathrm{O}_{4}\). It is a benzene dicarboxylic acid, meaning it contains a benzene ring with two carboxylic acid groups \(-COOH\) attached to it. The structure of phthalic acid allows for unique chemical properties, such as its ability to form anhydrides upon heating.

This occurs because the ortho (1,2) positioning of the carboxylic groups enables the molecule to easily lose a water molecule (\(H_2O\)) resulting in the formation of phthalic anhydride. Phthalic acid is used in the synthesis of various chemicals, including dyes, perfumes, and polyesters, due to its reactivity and structural properties.

Acidity Comparison

When comparing the acidity of phthalic acid to other compounds, it's essential to consider the nature of the substituents attached to the benzene ring. The two carboxyl groups in phthalic acid have a strong electron-withdrawing effect, which plays a critical role in its acidity. Here's how it compares:

• Phthalic acid is more acidic than acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)). The presence of additional carboxyl groups in phthalic acid increases its electron-withdrawing capability, thus better stabilizing the carboxylate anion formed by losing \(H^+\). This shifts the equilibrium towards a greater degree of ionization compared to acetic acid.
• Compared to benzoic acid (\(\mathrm{PhCOOH}\)), phthalic acid is also more acidic. The presence of the second carboxyl group in phthalic acid allows for a more effective resonance stabilization of the anion formed, thereby enhancing its acidic strength.
• Within the isomers of benzene dicarboxylic acids, ortho-phthalic acid emerges as the most acidic. This is due to the extra stability provided by intramolecular interactions compared to its meta and para counterparts.

Intramolecular Hydrogen Bonding

Intramolecular hydrogen bonding is a significant factor influencing the acidity of phthalic acid. In ortho-phthalic acid, the proximity of the carboxylic groups allows them to interact via hydrogen bonds. Here's why this is important:

• Hydrogen bonding within the molecule helps stabilize the conjugate base formed after the loss of a proton. This stabilization increases the acidity as it favors the release of \(H^+\).
• Such intramolecular interactions are absent in meta and para isomers, making ortho-phthalic acid uniquely more acidic than its isomeric counterparts.
• Intramolecular hydrogen bonding also assists in maintaining an ideal geometry that facilitates better anion formation, making it a strong acid among benzene dicarboxylic acids.

Understanding these internal interactions helps explain why phthalic acid exhibits such high acidity in comparison to its isomers and other carboxylic acids.