Problem 104 - A Complete Resource Book in Chemistry for JEE Main

Question

In which pair, both the compounds show iodoform test? (a) Acetone and acetophenone (b) Acetophenone and benzophenone (c) Acetone and benzophenone (d) Ethanol and acetone

Step-by-Step Solution

Answer

(d) Ethanol and acetone.

Step 1: Understand the Iodoform Test

The iodoform test is a chemical reaction that identifies methyl ketones and alcohols with the structure \( ext{CH}_3 ext{CH(OH)R} \). Compounds like acetone and ethanol can give a positive iodoform test.

Step 2: Analyze Each Compound

Check if each compound fits the structural conditions for a positive iodoform test. Acetone has the structure \( ext{CH}_3 ext{COCH}_3 \) and ethanol has \( ext{CH}_3 ext{CH}_2 ext{OH} \), both of which will result in a positive test. Acetophenone and benzophenone do not have the required methyl group adjacent to the carbonyl group, and benzophenone does not even have a methyl group.

Step 3: Identify the Correct Pair

Look through each of the answer choices to find a pair where both compounds fit the criteria for a positive iodoform test. After analyzing, we find that both acetone and ethanol fit the criteria for a positive reaction.

Step 4: Confirm the Solution

As identified, (d) Ethanol and acetone is the correct pair. Both compounds meet the structural requirements for the iodoform test.

Key Concepts

Methyl Ketones

Methyl ketones are a type of organic compound characterized by the presence of a carbonyl group (C=O) adjacent to a methyl group (CH₃). This structural feature is crucial for certain chemical reactions, precisely the iodoform test. In this test, the methyl ketone reacts with iodine in the presence of a base to produce a yellow precipitate called iodoform (CHI₃). This reaction is used to identify compounds containing the methyl ketone group because the yellow solid is easily recognizable. Some common methyl ketones include acetone and acetophenone. However, it is important to note that not all ketones are methyl ketones; the key factor is the methyl group on the carbon adjacent to the carbonyl.

Acetone

Acetone, also known as propanone, is the simplest and smallest ketone, with the molecular formula (CH₃)₂CO . Acetone is highly volatile and flammable, commonly used as a solvent in nail polish remover and in various industrial applications. It is also an important reagent in organic chemistry laboratories. In the iodoform test, acetone reacts to form a yellow precipitate of iodoform, indicating the presence of the methyl ketone group. This reaction helps to confirm its identity in a mixture of compounds. Acetone's structure includes two methyl groups attached to the carbonyl carbon, making it a classic example of a methyl ketone.

Ethanol

Ethanol is a simple alcohol with the formula CH₃CH₂OH . Known for its use in alcoholic beverages, ethanol is also used as a solvent, fuel, and in medical applications as an antiseptic. Ethanol can give a positive iodoform test due to its conversion to acetaldehyde in the presence of an oxidizing agent.

• Ethanol oxidizes to acetaldehyde ( CH₃CHO ), which can undergo further reaction to form iodoform.
• This conversion allows ethanol to indirectly show a positive iodoform test result, despite not being a methyl ketone itself.

This test is therefore not only for methyl ketones but also for alcohols that can be oxidized to them.

Organic Chemistry

Organic chemistry is the branch of chemistry that deals with the study of carbon-containing compounds. It encompasses a broad array of molecules, pathways, and reactions crucial to the development of pharmaceuticals, plastics, fuels, and more. Understanding the behavior of different functional groups, like ketones and alcohols, is central to this field.

Functional Groups

In organic chemistry, functional groups determine the characteristic reactions of molecules. For example, methyl ketones contain a carbonyl group next to a methyl group, while alcohols like ethanol contain a hydroxyl group.

Importance of Reaction Mechanisms

Reactions like the iodoform test highlight how specific structural features lead to distinct chemical behaviors. By studying these reactions, students can predict outcomes and identify unknown compounds in a laboratory setting. This type of problem-solving is a core skill in organic chemistry.