Problem 26 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Write down the reactions involved in the preparation of the following, using the reagents indicated against it in parenthesis. Ethylbenzene from benzene \(\left[\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}, \mathrm{PCl}_{5}\right.\), anhydrous \(\left.\mathrm{AlCl}_{3}\right]\).

Step-by-Step Solution

Answer

Ethanol is converted to ethyl chloride, then benzene reacts with it to form ethylbenzene using Friedel-Crafts alkylation.

Step 1: Chlorination of Ethanol

Start with ethanol \[\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH} + \mathrm{PCl}_{5} \xrightarrow{} \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl} + \mathrm{POCl}_{3} + \mathrm{HCl}.\]In the presence of phosphorus pentachloride (\(\mathrm{PCl}_{5}\)), ethanol (\(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\)) is converted into ethyl chloride (\(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\)).

Step 2: Friedel-Crafts Alkylation

Perform a Friedel-Crafts alkylation reaction using benzene and ethyl chloride.\[\mathrm{C}_{6} \mathrm{H}_{6} + \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl} \xrightarrow{\text{anhydrous}\ \mathrm{AlCl}_{3}} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C}_{2} \mathrm{H}_{5} + \mathrm{HCl}.\]In the presence of anhydrous aluminum chloride (\(\mathrm{AlCl}_{3}\)), benzene (\(\mathrm{C}_{6} \mathrm{H}_{6}\)) reacts with ethyl chloride to form ethylbenzene (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C}_{2} \mathrm{H}_{5}\)).

Key Concepts

Chlorination

Chlorination is a chemical reaction where a chlorine atom is introduced into a compound. In the context of organic chemistry, chlorination is often used to convert alcohols into chlorides, which can then be used for further reactions. Chlorination is an important step in the synthesis of ethylbenzene from ethanol. The process begins with ethanol (\( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH} \)) reacting with phosphorus pentachloride (\( \mathrm{PCl}_{5} \)).

This reaction replaces the hydroxyl group (\(-OH\)) in ethanol with a chloride ion, resulting in the formation of ethyl chloride (\( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl} \)). The reaction can be represented by the equation:

• \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH} + \mathrm{PCl}_{5} \rightarrow \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl} + \mathrm{POCl}_{3} + \mathrm{HCl} \)

The phosphorus-oxygen product is phosphorous oxychloride (\( \mathrm{POCl}_{3} \)), which forms as an additional product along with hydrogen chloride (\( \mathrm{HCl} \)). Chlorination is crucial since it transforms ethanol into a more reactive ethyl chloride, facilitating further reactions, like Friedel-Crafts alkylation, to produce ethylbenzene.

Friedel-Crafts Alkylation

Friedel-Crafts Alkylation is an essential reaction in organic chemistry used to introduce alkyl groups into aromatic rings. This reaction is particularly significant when synthesizing alkylbenzenes, such as ethylbenzene. It involves an electrophilic aromatic substitution mechanism where an alkyl group is transferred to an aromatic compound, in this case, benzene (\( \mathrm{C}_{6} \mathrm{H}_{6} \)).

The Friedel-Crafts alkylation of benzene with ethyl chloride (\( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl} \)) is catalyzed by anhydrous aluminum chloride (\( \mathrm{AlCl}_{3} \)). The reaction proceeds as follows:

• \( \mathrm{C}_{6} \mathrm{H}_{6} + \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl} \xrightarrow{\text{anhydrous}\ \mathrm{AlCl}_{3}} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C}_{2} \mathrm{H}_{5} + \mathrm{HCl} \)

Anhydrous \( \mathrm{AlCl}_{3} \) serves as a Lewis acid catalyst, meaning it can accept an electron pair, thus generating a highly reactive intermediate which facilitates the reaction. This method is powerful, enabling the transformation of simple alkanes into more complex aromatic compounds by attaching an alkyl group to the aromatic ring, resulting in ethylbenzene (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C}_{2} \mathrm{H}_{5} \)). Friedel-Crafts reactions are a cornerstone in the functionalization of aromatic systems, extending the versatility of benzene derivatives.

Benzene Derivatives

Benzene derivatives are a broad class of compounds that arise from modifications to the benzene ring structure. These derivatives retain benzene's characteristic aromatic properties while introducing functionalities that allow these compounds to be used in various chemical applications. Understanding benzene derivatives is essential for comprehending many organic synthesis pathways and applications.

The benzene ring (\( \mathrm{C}_{6} \mathrm{H}_{6} \)) is known for its stability and resonance structures, allowing it to engage in reactions like electrophilic aromatic substitution, as seen in the Friedel-Crafts Alkylation. This stability arises from the delocalized electrons in the ring structure, which also contributes to its ability to stabilize various substituents. Different substituents and groups can replace hydrogen atoms on the benzene ring, forming many derivatives with diverse properties.

Some common benzene derivatives include:

• Toluene (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3} \)) - used as an industrial solvent.
• Phenol (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH} \)) - plays a role in synthesizing plastics and other chemicals.
• Aniline (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \)) - utilized in the production of dyes.

Studying benzene derivatives expands our understanding of organic chemistry by demonstrating how the introduction of different functional groups can modify chemical behavior and utility.