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) 1 and 4 (d) 4 only

Step-by-Step Solution

Answer

Statement 2 is not true, as ketones do not form silver mirrors with Tollen's reagent. Option (b) is correct.

Step 1: Understand the Reaction with Iodine and Base

Acetophenone can react with iodine (I_2) in the presence of a base like sodium hydroxide (NaOH) to form iodoform (CHI_3), which is an indicator of a methyl ketone. Hence, statement 1 is true.

Step 2: Reaction with Tollen's Reagent Analysis

Tollen's reagent is used to test for aldehydes, which form a silver mirror upon reaction. Acetophenone is a ketone, not an aldehyde, so it does not give a silver mirror with Tollen's reagent. Therefore, statement 2 is not true.

Step 3: Oxidation with Alkaline KMnO4 Followed by Hydrolysis

When acetophenone is oxidized with alkaline potassium permanganate (KMnO_4), it undergoes cleavage and the resulting products can lead, upon hydrolysis, to benzoic acid. Therefore, statement 3 is true.

Step 4: Reaction to Form 2,4-Dinitrophenylhydrazone

2,4-Dinitrophenylhydrazine forms hydrazones with carbonyl compounds, including ketones like acetophenone. Hence, acetophenone can form 2,4-dinitrophenylhydrazone, making statement 4 true.

Key Concepts

Iodoform reaction

The iodoform reaction is a chemical test for the presence of methyl ketones or secondary alcohols oxidizable to methyl ketones. Specifically for acetophenone, a methyl ketone, it readily reacts with iodine (\(\mathrm{I}_2\)) in an alkaline medium, typically sodium hydroxide (\(\mathrm{NaOH}\)). This reaction results in the formation of iodoform (\(\mathrm{CHI}_3\)), a yellow crystalline solid with a distinct odor.
This reaction is valuable because it provides clear evidence of the methyl ketone group. Whenever you perform the iodoform test, consider whether the compound has the structural requirements to form iodoform. Acetophenone passes this test as it contains a methyl group adjacent to the carbonyl, essential for this chemical behavior.

Tollen's reagent

Tollen's reagent is a popular tool used to detect the presence of aldehydes. It is a mild oxidizing agent that involves ammoniacal silver nitrate. When an aldehyde reacts with Tollen's reagent, it is oxidized to form a carboxylic acid, and a silver mirror deposits as the silver is reduced.

• Key point: Acetophenone is a ketone and not an aldehyde.
• As a result, it does not give a positive test with Tollen's reagent and does not form a silver mirror.

This characteristic helps distinguish aldehydes from ketones in laboratory analysis. So, remember that while Tollen's test is quite informative, it’s selective for aldehydes and will not yield a silver mirror for substances like acetophenone.

Oxidation reactions

Oxidation reactions involve the loss of electrons or increase in oxidation state of a substance. Acetophenone undergoes an oxidation reaction in the presence of alkaline potassium permanganate (\(\mathrm{KMnO}_4\)). In this process, acetophenone cleaves, leading to the formation of benzoic acid upon hydrolysis.
These reactions are quite useful in organic chemistry to transform ketones and other carbonyl-containing compounds into acids and other derivatives. In your studies, it’s crucial to remember how specific oxidizing agents can influence different carbonyl compounds and the typical pathways such reactions might take.

Dinitrophenylhydrazine

Dinitrophenylhydrazine, abbreviated as DNPH, is used in an essential test for carbonyl groups, particularly useful for identifying ketones and aldehydes. Acetophenone, a ketone, reacts with DNPH to form a solid derivative known as 2,4-dinitrophenylhydrazone. This reaction turns the carbonyl compound into a deeply colored, solid derivative, facilitating identification.
This derivatization enhances the properties of these carbonyl compounds, making them more examinable by analytical techniques due to increased molecular weight and distinct color changes. In practical lab settings, DNPH serves as an effective method for characterizing unknown carbonyl compounds, as the derivatives' melting points are commonly documented.