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 (d) I and IV

Step-by-Step Solution

Answer

I and III are correct; option (a) is the answer.

Step 1: Understand the Requirements

We need to determine which statements describe valid preparations for acetophenone. Focus on identifying the correct chemical reactions and processes involved.

Step 2: Analyze Statement I

Statement I suggests that acetophenone can be produced by the oxidation of 1-phenylethanol. This is possible because oxidizing 1-phenylethanol (a secondary alcohol) forms acetophenone, a ketone.

Step 3: Analyze Statement II

Statement II involves reacting benzaldehyde with methyl magnesium bromide (a Grignard reagent). This would not yield acetophenone, as the product would be a secondary alcohol, phenylethanol, upon hydrolysis.

Step 4: Analyze Statement III

Statement III states that acetophenone can be synthesized through Friedel-Crafts acylation of benzene using acetyl chloride. This is a standard method for producing acetophenone.

Step 5: Analyze Statement IV

Statement IV suggests obtaining acetophenone by distilling calcium benzoate. However, this reaction is not a feasible method for producing acetophenone directly.

Step 6: Determine Correct Statements

From our analysis, Statements I and III correctly describe methods for preparing acetophenone: the oxidation of 1-phenylethanol and Friedel-Crafts reaction of benzene with acetyl chloride. Check options for the matching pair.

Key Concepts

Oxidation of 1-phenylethanol

Oxidation is a crucial organic chemistry process in which a molecule loses electrons, often involving the gain of oxygen or loss of hydrogen. In the context of preparing acetophenone, oxidation is used to transform 1-phenylethanol, which is a secondary alcohol, into acetophenone, a ketone. The process can be initiated using oxidizing agents such as chromic acid (H₂CrO₄), pyridinium chlorochromate (PCC), or potassium dichromate (K₂Cr₂O₇).
When 1-phenylethanol undergoes oxidation, the hydroxyl group (-OH) is converted to a carbonyl group (C=O), resulting in the formation of acetophenone. The electron-rich environment of the secondary alcohol makes this conversion possible, as the oxidizing agents facilitate the removal of hydrogen atoms, leading to the formation of the ketone.
Key points about this process include:

• 1-phenylethanol's structure allows it to be easily oxidized compared to primary alcohols, which often form different products.
• Understanding the nature of oxidizing agents is important, as different agents may provide slightly different yields and reaction conditions.

This method is a straightforward approach to synthesizing acetophenone in a laboratory setting due to its efficiency and predictability.

Friedel-Crafts Acylation

The Friedel-Crafts Acylation process is an essential reaction for introducing acyl groups into aromatic compounds, and it’s a widely used method for the synthesis of acetophenone. This technique involves the use of benzene and an acyl chloride—in this case, acetyl chloride—along with an aluminum chloride (AlCl₃) catalyst.
In this reaction, the acyl group from the acetyl chloride replaces one of the hydrogen atoms on the benzene ring, resulting in the formation of acetophenone. The catalyst, aluminum chloride, plays a critical role as it facilitates the generation of the acylium ion (RCO⁺), a reactive intermediate that attacks the electron-rich benzene ring.
Some important aspects of the Friedel-Crafts Acylation include:

• It leads to the formation of a carbon-carbon bond, making it a valuable tool for building complex molecular structures.
• The reaction is particularly useful in avoiding polyacylation, because the product is generally less reactive than the starting material.

The reaction conditions typically require an inert environment to prevent unwanted side reactions, ensuring that acetophenone is the primary product.

Grignard Reagent Reactions

Grignard reagents are organomagnesium compounds famously used in creating carbon-carbon bonds. While extremely versatile, Grignard reagents like methyl magnesium bromide were mentioned in the original exercise in relation to preparing acetophenone. However, this type of reaction is not suitable for direct acetophenone synthesis from benzaldehyde. Benzaldehyde, when reacted with methyl magnesium bromide, forms an intermediate that, upon hydrolysis, produces phenylethanol—not acetophenone. This transformation occurs because the Grignard reagent adds a methyl group to the carbonyl carbon of benzaldehyde, generating a secondary alcohol after reaction workup.
Key details about Grignard Reactions include:

• The versatility of Grignard reagents makes them useful in various synthetic pathways, although not suitable in every context.
• They are sensitive to moisture and require strict conditions to prevent degradation.

While Grignard reagents are indispensable in organic synthesis for forming alcohols and expanding carbon skeletons, their usage needs careful consideration to achieve desired outcomes like acetophenone.