Problem 230 - A Complete Resource Book in Chemistry for JEE Main

Question

A liquid was mixed with ethanol and a drop of concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) was added a compound with a fruity smell was formed. The liquid was: \(\quad\) (a) HCHO (b) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{COOH}\) (d) \(\mathrm{CH}_{3} \mathrm{OH}\) Sodium

Step-by-Step Solution

Answer

The liquid is \(\mathrm{CH}_3 \mathrm{COOH}\) (acetic acid).

Step 1: Understanding the Reaction

The given reaction involves the formation of a compound with a fruity smell when a liquid is mixed with ethanol and concentrated sulfuric acid, \( \mathrm{H}_2\mathrm{SO}_4 \). Typically, this scenario is part of esterification, where an acid reacts with an alcohol (ethanol, in this case) in the presence of an acid catalyst to form an ester.

Step 2: Identify the Candidate

For esterification, we need a carboxylic acid to react with the alcohol. Reviewing the options, (a) \(\mathrm{HCHO}\) is formaldehyde, (b) \(\mathrm{CH}_3 \mathrm{COCH}_3\) is acetone (a ketone), (c) \(\mathrm{CH}_3 \mathrm{COOH}\) is acetic acid (a carboxylic acid), and (d) \(\mathrm{CH}_3 \mathrm{OH}\) is methanol. Out of these, \(\mathrm{CH}_3 \mathrm{COOH}\) is a carboxylic acid.

Step 3: Conclusion of Reaction Outcome

Acetic acid (\(\mathrm{CH}_3 \mathrm{COOH}\)) reacts with ethanol in acidic conditions (using \(\mathrm{H}_2\mathrm{SO}_4\) as a catalyst) to form ethyl acetate, a compound with a fruity smell. This confirms the identity of the liquid as acetic acid.

Key Concepts

Carboxylic Acid

Carboxylic acids are organic compounds characterized by the presence of a carboxyl group \(\( -COOH \)\). This group consists of a carbonyl \(\( C=O \)\) and a hydroxyl group \(\( -OH \)\), giving it acidic properties. Carboxylic acids are crucial in various chemical reactions, especially in esterification processes.
In the context of esterification, carboxylic acids react with alcohols to form esters and water. A classic example is acetic acid \(\( \mathrm{CH_3COOH} \)\), which readily reacts with ethanol to produce ethyl acetate, imparting a fruity aroma.

• The acidic property of carboxylic acids allows them to donate a hydrogen ion \(\( \mathrm{H^+} \)\), which is important in their ability to engage in chemical reactions with bases and alcohols.
• Carboxylic acids are named based on the longest carbon chain containing the carboxyl group, and typically end in "-oic acid."

Understanding these basics helps in recognizing how carboxylic acids like acetic acid play a fundamental role in producing compounds with desirable scents.

Ethyl Acetate

Ethyl acetate, a widely recognized ester, is known for its pleasant, fruity aroma. It is formed through an esterification reaction involving ethanol and acetic acid.
Esterification occurs when the carboxyl group of acetic acid reacts with the hydroxyl group of ethanol. This process is facilitated by heating and the presence of an acid catalyst like sulfuric acid.
During the reaction:

• The hydrogen atom from the alcohol's hydroxyl group and the hydroxyl group from the carboxylic acid are replaced, forming water as a byproduct.
• The remaining portions of the alcohol and acid fuse together to form the ester, ethyl acetate \(\( \mathrm{CH_3COOC_2H_5} \)\).

This reaction is reversible; however, ethyl acetate can be naturally encountered in fruits and is used industrially as a solvent in paints and coatings. By understanding its production and characteristics, students can better grasp its significance in both natural and synthetic contexts.

Sulfuric Acid as Catalyst

Sulfuric acid, a strong acid with the formula \(\( \mathrm{H_2SO_4} \)\), is often used as a catalyst in chemical reactions, including esterification. A catalyst is a substance that speeds up a chemical reaction without being consumed in the process.
In esterification:

• Sulfuric acid serves to donate a proton \(\( \mathrm{H^+} \)\), momentarily enhancing the reactivity of the carboxylic acid.
• This protonation makes the hydroxyl group on the carboxylic acid a better leaving group, facilitating the formation of the ester linkage.
• The catalyst helps drive the reaction to completion by removing water, a byproduct of esterification, encouraging the forward reaction.

Using \(\( \mathrm{H_2SO_4} \)\) as a catalyst in esterification exemplifies its critical role in enhancing reaction rates and efficiency, allowing for the smooth transformation of reactants into desired products.