Problem 135 - A Complete Resource Book in Chemistry for JEE Main

Question

Which is not true about acetophenone? (1) Reacts with \(\mathrm{I}_{2} / \mathrm{NaOH}\) to form iodoform (2) Reacts with Tollen's reagent to form silver mirror (3) On oxidation with alkaline \(\mathrm{KMnO}_{4}\) followed by hydrolysis gives benzoic acid (4) Reacts to form 2,4 -dinitrophenyl-hydrazone (a) 2 and 4 (b) 2 only (c) \(\mathrm{l}\) and 4 (d) 4 only

Step-by-Step Solution

Answer

Statement 2 is not true; option (b) 2 only is correct.

Step 1: Understand the properties of acetophenone

Acetophenone is a ketone with the chemical formula: \( ext{C}_6 ext{H}_5 ext{C}( ext{O}) ext{CH}_3 \). Ketones generally do not react with Tollen's reagent (which is used to detect aldehydes), as ketones are typically not oxidized by this reagent. This characteristic is key in determining the veracity of the statements.

Step 2: Analyze iodoform reaction

The iodoform test is used to detect methyl ketones. Acetophenone is a methyl ketone, as it has a \( ext{CH}_3 ext{C}( ext{O}) ext{R} \) structure where R is a phenyl group. Therefore, acetophenone will react with \( ext{I}_2/ ext{NaOH} \) to form iodoform.

Step 3: Analyze reaction with Tollen's reagent

Since acetophenone is a ketone, it does not react with Tollen's reagent, which is specific for aldehydes. The reaction with Tollen's reagent forming a silver mirror would not occur with acetophenone.

Step 4: Analyze oxidation with alkaline KMnO4

Alkaline \( ext{KMnO}_4 \) can oxidize acetophenone to form benzoic acid. The phenyl group is preserved, and the methyl group is oxidized to a carboxylic acid.

Step 5: Analyze formation of 2,4-dinitrophenyl-hydrazone

2,4-Dinitrophenylhydrazine is a test used to detect aldehydes and ketones by forming a precipitate known as 2,4-dinitrophenylhydrazone. Since acetophenone is a ketone, it will form 2,4-dinitrophenyl-hydrazone when reacted with 2,4-DNP.

Step 6: Conclusion

From the analysis of each statement, we conclude that acetophenone does not react with Tollen's reagent (statement 2 is false). Acetophenone does form 2,4-dinitrophenylhydrazone (statement 4 is true).

Key Concepts

Iodoform reaction

The iodoform reaction is a chemical test that helps identify the presence of methyl ketones or ethanol. It involves the reaction between a compound and iodine (\(\mathrm{I}_2\)) in the presence of a base such as sodium hydroxide (\(\mathrm{NaOH}\)). This reaction results in the formation of a yellow precipitate known as iodoform.
In the case of acetophenone, a methyl ketone, the chemical structure is \(\mathrm{C}_6\mathrm{H}_5\mathrm{C}(\mathrm{O})\mathrm{CH}_3\). Here, the methyl group attached to the carbonyl group reacts with iodine under alkaline conditions to form iodoform.
The steps involved in this process are:

• Iodination: The methyl group gets iodinated, which forms triiodomethane (iodoform).
• Base-assisted cleavage: The base assists in the cleavage of the iodoform from the rest of the carbon skeleton.

This reaction is particularly useful for distinguishing between different types of ketones, as not all ketones will react in this manner. Acetophenone, having the methyl group directly attached to the carbonyl, readily undergoes the iodoform reaction.

Tollen's reagent

Tollen's reagent is primarily used to detect the presence of aldehydes. The reagent consists of a solution of silver nitrate (\(\mathrm{AgNO}_3\)) in ammonia (\(\mathrm{NH}_3\)). When an aldehyde is present, Tollen's reagent is reduced, leading to the deposition of metallic silver, often forming a distinct silver mirror on the inside of the reaction vessel.
However, ketones typically do not react with Tollen's reagent. Acetophenone, being a ketone, does not generate a silver mirror when treated with Tollen's reagent. The reactivity difference is linked to the general stability of ketones against mild oxidizing agents, such as the ammoniacal silver solution.

• Aldehydes are easily oxidized to carboxylic acids, thus reducing silver ions to metallic silver.
• Ketones generally do not undergo such oxidation reactions as they lack the necessary hydrogen atom adjacent to the carbonyl group.

This differentiation is essential in organic chemistry for identifying and confirming carbonyl-containing compounds.

Oxidation with KMnO4

Potassium permanganate (\(\text{KMnO}_4\)) is a powerful oxidizing agent often used in organic chemistry to convert aliphatic chains into carboxylic acids or other oxygenated products. When acetophenone is treated with alkaline \(\text{KMnO}_4\), the methyl group is oxidized to form a carboxylic acid.
The phenyl ring in acetophenone remains largely unaffected during this oxidation process, while the adjacent methyl group is transformed into the carboxylic acid group, leading to the formation of benzoic acid (\(\text{C}_6\text{H}_5\text{COOH}\)).
This reaction includes several steps:

• Permanganate provides an oxygen atom that joins with the methyl group.
• The carbon-hydrogen bonds in the methyl group are broken and replaced with oxidized forms.

Such reactions with permanganate showcase its utility in converting methyl and methylene groups adjacent to aromatic systems into useful functional groups like carboxyl acids.

2,4-Dinitrophenylhydrazone

2,4-Dinitrophenylhydrazine (2,4-DNP) is commonly used in qualitative analysis to identify carbonyl groups in aldehydes and ketones. The reagent reacts with these groups to produce a colored precipitate known as a 2,4-dinitrophenylhydrazone.
When acetophenone, which includes a carbonyl group, is treated with 2,4-DNP, it forms a yellow to orange precipitate. This precipitate is the 2,4-dinitrophenylhydrazone derivative of acetophenone.
The mechanism of this reaction includes:

• Nucleophilic attack: The nitrogen from the 2,4-DNP attacks the electrophilic carbon in the carbonyl group of acetophenone.
• Hydrazone formation: Water is eliminated, and a stable azo compound is formed.

This test is widely used because the resulting precipitates are typically highly colored, making it easy to confirm the presence of carbonyl groups visually. It's a simple yet powerful tool for characterizing and identifying organic molecules in both educational and professional laboratory settings.