Problem 160 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following statements; Acetophenone can be prepared by I. oxidation of 1 -phenylethanol II. reaction of benzaldehyde with methyl magnesium bromide III. Friedal Crafts reaction of benzene with acetyl chloride IV. Distillation of calcium benzoate Which of the above statements are correct? (a) I and III (b) III and IV (c) II and III

Step-by-Step Solution

Answer

Option (a) I and III are correct.

Step 1: Understand Acetophenone Formation

Acetophenone is an aromatic ketone, specifically the simplest one. It is formed by introducing a ketone group adjacent to a benzene ring. This is typically achieved through methods that introduce or transform functional groups into a ketone on the benzene ring.

Step 2: Analyze Statement I

Statement I suggests the oxidation of 1-phenylethanol. Oxidation of a secondary alcohol like 1-phenylethanol typically leads to the formation of a ketone. Thus, oxidation of 1-phenylethanol can indeed form acetophenone.

Step 3: Evaluate Statement II

Statement II involves the reaction of benzaldehyde with methyl magnesium bromide, a Grignard reagent. This reaction typically yields an alcohol, specifically a secondary alcohol, not acetophenone. Hence, this process does not form acetophenone.

Step 4: Evaluate Statement III

Statement III mentions the Friedel-Crafts acylation reaction of benzene with acetyl chloride. In this reaction, benzene reacts with acetyl chloride in the presence of a Lewis acid catalyst, such as AlCl3, to form acetophenone. This method is a classic way to produce acetophenone.

Step 5: Consider Statement IV

Statement IV refers to the distillation of calcium benzoate. However, this typically leads to the formation of benzoic acid upon decomposition, not acetophenone. Therefore, it does not produce acetophenone.

Step 6: Conclude Based on Evaluations

Based on the evaluations: Statement I leads to acetophenone by oxidation of 1-phenylethanol, and Statement III produces acetophenone via Friedel-Crafts acylation. The correct choice that includes valid methods for producing acetophenone is (a) I and III.

Key Concepts

Friedel-Crafts Acylation

The Friedel-Crafts acylation is an important organic chemical reaction used to introduce an acyl group into an aromatic compound. When applying Friedel-Crafts acylation to produce acetophenone, you start with benzene and react it with acetyl chloride.
The beauty of this reaction lies in its ability to directly attach the acyl group to the benzene ring, resulting in a ketone.

• First, benzene acts as the aromatic substrate.
• Acetyl chloride is the acylating agent.
• You need a catalyst, usually a Lewis acid like aluminum chloride (AlCl₃), to promote the reaction.

This reaction involves the formation of an acylium ion by the catalyst from acetyl chloride, which then attacks the aromatic ring. This electrophilic aromatic substitution leads to the formation of acetophenone and a proton in the product stream. Utilizing Friedel-Crafts acylation is a staple method for synthesizing ketones like acetophenone from benzene.

Oxidation Reactions

Oxidation reactions are a broad class of chemical reactions involving the transfer of electrons. In the context of synthesizing acetophenone, oxidation transforms 1-phenylethanol into acetophenone.
This transformation is achieved because oxidizing a secondary alcohol like 1-phenylethanol generally leads to the formation of a ketone.

• The alcohol group (\(-\text{OH}\)) on 1-phenylethanol is converted to a carbonyl group (\(\text{C=O}\)).
• You often use oxidizing agents like chromium trioxide (\(\text{CrO}_3\)) or pyridinium chlorochromate (PCC) for this purpose.

The oxidation of 1-phenylethanol to acetophenone is a classic example of upgrading a simple alcohol to a more useful ketone product, which will have different reactivity and synthesis applications.

Grignard Reagents

Grignard reagents are a class of organometallic compounds crucial in the formation of carbon-carbon bonds. They are typically formed by reacting an alkyl, vinyl, or aryl halide with magnesium in anhydrous ether, imparting the compound significant nucleophilic character.
In the reaction scenario of producing acetophenone, however, using a Grignard reagent like methyl magnesium bromide with benzaldehyde does not yield acetophenone. Instead, this process results in the formation of a secondary alcohol.

• Grignard reagents add to carbonyl groups, generating alcohols after protonation.
• The reaction with benzaldehyde ultimately cannot create the desired ketone without further steps.

In this context, understanding the limitation of Grignard reagents in directly forming acetophenone emphasizes the need for considering reaction specificity and sequence in synthesis strategies.

Calcium Benzoate Decomposition

Calcium benzoate decomposition is another chemical process often examined in organic synthesis, but it does not lead to the formation of acetophenone.
Instead, this decomposition primarily results in forming benzoic acid, not a ketone.

• Calcium benzoate is subjected to decomposition through processes like distillation.
• The expected primary product of calcium benzoate decomposition is benzoic acid.

Understanding this helps in distinguishing between potential and actual synthetic routes for specific compounds. While it may seem plausible to consider this decomposition might produce acetophenone, the chemistry involved confirms it does not contribute to acetophenone synthesis.