Problem 56 - A Complete Resource Book in Chemistry for JEE Main

Question

$$ \begin{aligned} &\text { [ } \frac{\mathrm{CH}_{1} \mathrm{COCl}}{\mathrm{AlCl}_{3}} \mathrm{~A} \frac{\mathrm{Zn} / \mathrm{Hg}}{\mathrm{HCl}} \mathrm{B}\\\ &\text { Here B is } \end{aligned} $$ (a) Phenyl ethane (b) Phenyl propane (c) Phenyl methane (d) Methyl phenyl ketone

Step-by-Step Solution

Answer

The compound \( B \) is phenyl ethane, option (a).

Step 1: Identify the Starting Material

The starting material in the exercise is acetyl chloride \( \mathrm{CH_3COCl} \), which is used in the presence of a catalyst \( \mathrm{AlCl_3} \). This suggests a Friedel-Crafts Acylation reaction.

Step 2: Perform Friedel-Crafts Acylation

During Friedel-Crafts Acylation, the acetyl group attaches to the aromatic ring, in this case, producing acetophenone (methyl phenyl ketone, \( \mathrm{C_6H_5COCH_3} \)).

Step 3: Perform Clemmensen Reduction

The product of the Friedel-Crafts Acylation, \( \mathrm{C_6H_5COCH_3} \), is then treated with \( \mathrm{Zn} / \mathrm{Hg} \) in \( \mathrm{HCl} \). This is known as Clemmensen Reduction, which reduces the carbonyl group (ketone) to a methylene group, converting acetophenone into ethylbenzene (phenyl ethane, \( \mathrm{C_6H_5CH_2CH_3} \)).

Step 4: Identify Compound B

After Clemmensen Reduction, the compound \( \mathrm{B} \) is confirmed as phenyl ethane (ethylbenzene). This matches option (a) in the question.

Key Concepts

Clemmensen Reduction

The Clemmensen Reduction is a widely used chemical reaction that involves the transformation of a carbonyl group, typically found in ketones or aldehydes, into a methylene (H_2) group. This transformation is achieved by using a combination of zinc amalgam (Zn/Hg) and hydrochloric acid (HCl).

Heret's how it works in simple terms:

• The reaction starts with a ketone or aldehyde.
• Zinc amalgam and hydrochloric acid are added to the mixture.
• The carbonyl group (C=O bond) is reduced to a methylene group (CH_2 bond).

This reduction is particularly useful in organic chemistry for converting acetophenone to ethylbenzene. By removing the oxygen from the carbonyl group, you create a simpler molecule, making it a key step in many synthetic processes.

Acetophenone

Acetophenone, known chemically as methyl phenyl ketone (C_6H_5COCH_3), is an important intermediate in organic chemistry. It's primarily recognized for its sweet, floral aroma, often likened to that of orange blossoms.

Key properties of acetophenone include:

• It's a colorless liquid at room temperature.
• It has a boiling point of approximately 202°C (395°F).
• It's used in fragrances, flavorings, and in the synthesis of other chemicals.

In a Friedel-Crafts Acylation reaction, acetophenone is often synthesized by introducing an acetyl group to benzene using acetyl chloride and aluminum chloride as a catalyst. This compound is particularly notable as it serves as a precursor to many chemical reactions, including the Clemmensen Reduction, where it becomes ethylbenzene.

Acetyl Chloride

Acetyl chloride (CH_3COCl) is a reactive acyl chloride derived from acetic acid. In organic synthesis, it is widely used as an acetylating agent, because it transfers its acetyl group (C(O)CH_3) to other compounds. This reactivity makes it an essential reactant in Friedel-Crafts Acylation.

Here are key features of acetyl chloride:

• It is a colorless liquid with a pungent smell.
• It reacts vigorously with water, releasing hydrochloric acid fumes.
• It's often used to introduce acetyl groups into alcohols and amines.

In Friedel-Crafts Acylation, acetyl chloride reacts with an aromatic ring such as benzene in the presence of a catalyst like aluminum chloride (AlCl_3) to form acetophenone. This step is crucial for constructing more complex organic compounds from simpler precursors.

Ethylbenzene

Ethylbenzene (C_6H_5CH_2CH_3) is an aromatic hydrocarbon that serves as a solvent and a precursor to styrene, the monomer for polystyrene, a common plastic. It is often formed as a result of the Clemmensen Reduction of acetophenone.

Important aspects of ethylbenzene include:

• Its a clear, colorless liquid with a sweet, gasoline-like odor.
• It has a boiling point of 136°C (277°F).
• It's used extensively in the petrochemical industry.

By reducing acetophenone to ethylbenzene, the Clemmensen Reduction contributes to producing this versatile and industrially significant compound. Ethylbenzene exemplifies how modifying simple chemical structures can lead to widely used commercial products.