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

Option (a) I and III are correct.

Step 1: Analyze Statement I

Statement I suggests acetophenone can be prepared by oxidizing 1-phenylethanol. Acetophenone is a ketone, and ketones can be formed by the oxidation of secondary alcohols like 1-phenylethanol. Therefore, Statement I is correct.

Step 2: Analyze Statement II

Statement II involves the reaction of benzaldehyde with methyl magnesium bromide. This reaction is a Grignard reaction, which typically forms alcohols, not ketones. Therefore, this method does not produce acetophenone. So, Statement II is incorrect.

Step 3: Analyze Statement III

Statement III indicates using a Friedel-Crafts acylation reaction of benzene with acetyl chloride. This is a classic method for synthesizing acetophenone, as the acyl group from acetyl chloride attaches to the benzene ring. Therefore, Statement III is correct.

Step 4: Analyze Statement IV

Statement IV involves the distillation of calcium benzoate. This process typically does not lead to acetophenone production and is not commonly used to synthesize acetophenone. Therefore, Statement IV is incorrect.

Step 5: Identify the Correct Pair

Based on the analysis, the correct statements are I and III. Checking the options, option (a) corresponds to these statements.

Key Concepts

Oxidation of 1-Phenylethanol

Oxidation of 1-Phenylethanol is a well-known chemical reaction used to transform secondary alcohols into ketones. In this case, 1-phenylethanol, which is a secondary alcohol, is oxidized to form acetophenone, a type of ketone. The process typically involves an oxidizing agent such as chromic acid, PCC (Pyridinium chlorochromate), or other similar reagents.
During the oxidation process, the hydroxyl group (-OH") on 1-phenylethanol loses a hydrogen atom and gains an oxygen atom. This transforms the alcohol into a ketone, which is characterized by the carbonyl group (C=O") attached to two adjacent carbon atoms. This is why ketones, including acetophenone, are derived from oxidizing secondary alcohols.
Importantly, this reaction is useful because it provides a straightforward route to synthesize acetophenone. By starting with a relatively accessible alcohol and applying an oxidizing agent, the desired ketone can be produced efficiently and with good yields.

Friedel-Crafts Acylation

Friedel-Crafts Acylation is a classic, widely used method for introducing acyl groups into aromatic compounds like benzene. This reaction specifically involves the addition of an acyl group from an acyl chloride (e.g., acetyl chloride) to an aromatic ring in the presence of a Lewis acid, like aluminum chloride (AlCl₃), acting as a catalyst.
This process enables the synthesis of acetophenone by reacting acetyl chloride with benzene. The aluminum chloride catalyst helps to generate an acylium ion from the acetyl chloride, which then reacts with the benzene ring, substituting a hydrogen atom on the ring with the acyl group. The result is an aromatic ketone, acetophenone.
This method is advantageous for synthesizing acetophenone due to its high specificity and relatively straightforward procedure. It reinforces the practical utility of Friedel-Crafts Acylation in organic synthesis, particularly in creating aromatic ketones like acetophenone.

Grignard Reaction

The Grignard Reaction is a fundamental tool in organic chemistry for forming carbon-carbon bonds. Despite its versatility, it's important to note that this reaction typically results in alcohol formation rather than ketones like acetophenone.
In a Grignard Reaction, an organomagnesium compound, such as methyl magnesium bromide, reacts with a carbonyl compound. While this reaction is excellent for creating alcohols by converting carbonyl groups into hydroxyl groups, it doesn't effectively produce ketones. Thus, when benzaldehyde reacts with methyl magnesium bromide, it forms an alcohol, not acetophenone.
While the Grignard Reaction is vital in organic synthesis, creating numerous alcohol derivatives, it's not the preferred method for producing acetophenone. This distinction is crucial for properly aligning synthesis methods with desired chemical outcomes.