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

(b) 2 only

Step 1: Understand Acetophenone Properties

Acetophenone is a methyl ketone, having a structure \((C_6H_5COCH_3)\). It reacts with certain reagents due to its ketone group.

Step 2: Analyze Option 1: Iodoform Test

Acetophenone can react with iodine in the presence of sodium hydroxide to give iodoform because it contains a methyl ketone group, making it suitable for the iodoform test.

Step 3: Inspect Option 2: Tollen's Test

Tollen's reagent is used to test for aldehydes. Acetophenone is a ketone, so it does not form a silver mirror when treated with Tollen's reagent.

Step 4: Evaluate Option 3: Oxidation with Alkaline KMnO4

Acetophenone can be oxidized under strong conditions with alkaline \[KMnO_4\] to form benzoic acid after hydrolysis. The \[COCH_3\] group is oxidized to \[COOH\].

Step 5: Examine Option 4: DNPH Reaction

Acetophenone reacts with 2,4-dinitrophenylhydrazine (DNPH) leading to the formation of 2,4-dinitrophenylhydrazone, a test used to identify carbonyl compounds.

Step 6: Determine the False Statement(s)

From the above analysis, the only false statement is option 2. All other options accurately describe reactions involving acetophenone.

Key Concepts

Iodoform Reaction

When it comes to organic chemistry, one reaction that stands out due to its specificity is the iodoform reaction. This test is particularly useful for identifying methyl ketones, like acetophenone. A methyl ketone is a ketone with a methyl group - a handful of examples include acetone and acetophenone. The test involves reacting the compound with iodine (\(I_2\)) in the presence of a base such as sodium hydroxide (\(NaOH\)). If the test is positive, a pale yellow precipitate of iodoform - chemically named triiodomethane - is formed, which is easily recognizable by its yellow color and antiseptic smell.
Acetophenone holds a methyl ketone group, just like methyl ethyl ketone, and hence readily participates in this reaction to produce iodoform. It is also noteworthy that beyond confirming the presence of methyl ketones, the reaction sheds light on the entities possessing alcohol groups that could oxidize to methyl ketones.

Tollen's Reagent Test

The Tollen's reagent test, often discussed in organic chemistry courses, is a popular method for differentiating aldehydes from ketones. This test utilizes Tollen's reagent, a solution of silver nitrate (\(AgNO_3\)) in ammonia (\(NH_3\)), to detect the presence of aldehyde functional groups. On reaction with an aldehyde, Tollen's reagent gets reduced, depositing a shiny silver mirror on the inside surface of the test tube.
Acetophenone is a ketone, not an aldehyde, so this test is not suitable for it. Even though both aldehydes and ketones contain the carbonyl group, only the aldehydes are able to reduce Tollen's reagent because they have a hydrogen atom attached to the carbonyl carbon, which can be oxidized to a carboxylate ion. Knowing this difference helps quite a bit when differentiating compounds, as the absence of silver mirror formation indicates a negative test for ketones such as acetophenone.

Oxidation with KMnO4

Oxidation reactions are an integral part of organic chemistry, allowing researchers to transform compounds in meaningful ways. Acetophenone, with its compelling carbon backbone, can undergo oxidation reactions under specific conditions. Using alkaline potassium permanganate (\(KMnO_4\)) in a strong alkaline medium, acetophenone can oxidize to form benzoic acid, following a hydrolysis step. Potassium permanganate is a powerful oxidizing agent capable of attacking the methyl group adjacent to the carbonyl, eventually transforming it into a carboxylic acid group.
This reaction is particularly valuable because it allows chemists to create more complex molecules from simpler methyl ketones. It highlights how specific conditions can change the molecular structure, making previously impossible reactions feasible. Given the strong oxidative environment created by alkaline \(KMnO_4\), intermediates form that eventually yield benzoic acid, representing a classic transformation from a ketone to an aromatic carboxylic acid.

2,4-Dinitrophenylhydrazine Test

Assessing carbonyl compounds, including aldehydes and ketones, often involves the 2,4-dinitrophenylhydrazine (DNPH) test. This test checks for the presence of the carbonyl group, forming solid derivatives known as 2,4-dinitrophenylhydrazones. DNPH, when reacted with acetophenone, forms such a derivative, providing evidence of the presence of a carbonyl group.
For students learning organic chemistry, understanding this reaction provides a clear visualization of the transformation of carbonyl groups into more complex structures. The solid form of the derivative also offers practical advantages: it can be collected, purified, and analyzed further. This identification and analysis method becomes crucial in laboratories, particularly for confirming the presence of carbonyls in unknown compounds. This test, with its clear and impactful results, is widely appreciated for its role in structural determination and organic compound identification.