Problem 28 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The compound that does NOT liberate \(\mathrm{CO}_{2}\), on treatment with aqueous sodium bicarbonate solution, is (a) Benzoic acid (b) Benzenesulphonic acid (c) Salicylic acid (d) Carbolic acid (Phenol)

Step-by-Step Solution

Answer

(d) Carbolic acid (Phenol) does not liberate CO2.

Step 1: Understand the Reaction

When a compound reacts with aqueous sodium bicarbonate ( ext{NaHCO}_3), it generally produces carbon dioxide ( ext{CO}_2) if it contains an acidic hydrogen strong enough to displace the carbonic acid produced. This happens with carboxylic acids and some other acids, but not with weaker acids such as phenol.

Step 2: Identify the Acids and Their Strength

Among the given options, benzoic acid, benzenesulfonic acid, and salicylic acid are all known to be strong enough acids to react with sodium bicarbonate to produce ext{CO}_2.

Step 3: Check Phenol's Reaction with NaHCO3

Phenol, being a weak acid, does not liberate ext{CO}_2 when treated with aqueous sodium bicarbonate. Its acidity is not sufficient to release ext{CO}_2 from ext{NaHCO}_3.

Step 4: Confirm the Answer

Based on the exploration of each compound's tendency to react with sodium bicarbonate, phenol (also known as carbolic acid), which is (d), does not react to produce ext{CO}_2.

Key Concepts

Acid Strength

When we talk about acid strength, we are discussing how easily an acid can donate its hydrogen ion. The stronger the acid, the more readily it gives up this proton. Here are some key factors influencing acid strength:

• Electronegativity: More electronegative atoms hold onto their electrons more tightly, which can increase acidity.
• Size of the atom: Larger atoms can better surround and stabilize the negative charge, increasing the acid's strength.
• Resonance stabilization: If the conjugate base is stabilized by resonance, the parent acid is stronger.
• Inductive effect: Atoms or groups that pull electron density away from the acidic proton can increase acidity.

These principles explain why sulfuric acid or hydrochloric acid are strong compared to a weaker one like acetic acid. With carboxylic acids like benzoic acid, the carboxylate ion can stabilize the negative charge, making these acids strong enough to react with sodium bicarbonate.

Phenol

Phenol is an aromatic compound containing a hydroxyl group directly attached to a benzene ring. It's also known as carbolic acid. Despite its name, phenol is a weak acid. Here's why:

• The hydroxyl group's proton ( I64 ) in phenol can be released; however, the negative charge is not as well-stabilized as in carboxylic acids.
• The phenoxide ion formed is stabilized somewhat through resonance, but not sufficiently to make phenol as acidic as carboxylic acids.
• Phenol's aromatic ring can donate electron density into the hydroxyl group, further reducing its ability to lose the hydrogen ion.

For this reason, when phenol is treated with sodium bicarbonate, it does not produce carbon dioxide. Its acidity is not enough to effectively react with bicarbonate ion to release carbon CO a