Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

Benzene reacts with \(\mathrm{CH}_{3} \mathrm{COCl}\) in the presence of anhydrous \(\mathrm{AlCl}_{3}\) to give (a) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{3}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}_{2} \mathrm{Cl}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{3}\)

Step-by-Step Solution

Answer

The reaction yields acetophenone, CC_6 79H_5COCH_3, which is option (d).

Step 1: Identify the Reaction Type

The reaction described involves benzene (C_6 79H_6) and acetyl chloride (CH_3COCl) in the presence of CAlCl_3. This indicates a Friedel-Crafts acylation reaction, a common method for introducing an acyl group into an aromatic ring.

Step 2: Understand the Mechanism

In a Friedel-Crafts acylation, the CAlCl_3 functions as a Lewis acid catalyst. It coordinates with the chlorine atom of acetyl chloride, producing a highly reactive acylium ion (CH_3CO^+). This ion acts as an electrophile in the reaction.

Step 3: Reaction with Benzene

The acylium ion (CCH_3CO^+) attacks the electron-rich benzene ring, forming a resonance-stabilized CC_6 79H_5COCH_3 intermediate. This forms the basis for integration into the benzene structure, resulting in the substitution reaction.

Step 4: Restore Aromaticity

The intermediate then loses a proton (H^+) to restore the aromaticity of the benzene ring. This finalizes the substitution, yielding the product CC_6 79H_5COCH_3.

Step 5: Identify the Product

The resulting structure is an acetophenone molecule, which fits the chemical formula CC_6 79H_5COCH_3. This corresponds to option (d) given in the question.

Key Concepts

Benzene Reactions

Benzene is a unique and essential component in organic chemistry, frequently involved in a variety of reactions due to its stable aromatic structure. A notable feature of benzene is its ability to undergo substitution reactions while maintaining its aromaticity. Friedel-Crafts reactions, including both alkylation and acylation, are classic examples of how benzene can be transformed.
Friedel-Crafts acylation, in particular, allows benzene to form new carbon-carbon bonds by incorporating an acyl group such as an acylium ion. In this type of reaction, benzene reacts with an acyl chloride, like acetyl chloride, in the presence of a catalyst like anhydrous AlCl₃. This reaction results in the introduction of a carbonyl group attached directly to the aromatic ring.
Some key points about benzene reactions are:

• Benzene's reactions often involve catalysts to increase reactivity.
• Despite substitution, benzene retains its strong aromatic stability.
• Friedel-Crafts acylation is significant for forming ketone derivatives when benzene reacts with acyl chlorides.

Understanding benzene reactions is crucial for exploring the chemistry of aromatic compounds and their applications in various fields.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a fundamental type of reaction that aromatic compounds like benzene undergo. It involves the replacement of a hydrogen atom on the aromatic ring with an electrophile, which is a species that accepts electrons.
The general steps for EAS include formation of a highly reactive electrophile, its attack on the aromatic ring, and finally restoration of aromaticity.

• The electrophile attacks the ring, forming a sigma complex that temporarily interrupts the ring's aromatic stabilization.
• The sigma complex is resonance-stabilized, allowing it to exist long enough for subsequent reactions.
• Finally, the loss of a proton (H⁺) from the sigma complex restores the aromaticity of the ring, and the substitution is complete.

In the context of a Friedel-Crafts acylation, the acylium ion serves as the electrophile. Mastering the intricacies of EAS enables chemists to precisely modify aromatic compounds, which is crucial in the synthesis of various important chemical intermediates.

Acylium Ion

The acylium ion is a vital intermediate in the Friedel-Crafts acylation process. It is formed when an acyl chloride, like acetyl chloride (CH₃COCl), reacts with a Lewis acid catalyst such as AlCl₃. The reaction occurs as the chlorine atom is captured by the catalyst, leaving behind a positively charged species known as the acylium ion (CH₃CO⁺).
This positively charged ion is an excellent electrophile due to its deficiency of electrons, making it highly reactive towards electron-rich compounds such as benzene. Its role is pivotal in transferring the acyl group to the aromatic ring.

• The acylium ion is more stable than other potential electrophiles due to resonance between the carbon and oxygen atoms.
• It is responsible for the introduction of a carbonyl group into the benzene ring during Friedel-Crafts acylation.
• Unlike other electrophiles, the acylium ion does not lead to carbocation rearrangements, making the process more predictable and straightforward.

Recognizing the importance of the acylium ion helps in understanding how electrophilic substitution reactions involving carbonyl compounds proceed efficiently.