Problem 7 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following will not give iodoform test? (a) ethanol (b) ethanal (c) pentan-3-one (d) pentan-2-one

Step-by-Step Solution

Answer

Pentan-3-one will not give the iodoform test.

Step 1: Understand the Iodoform Test

The iodoform test is used to identify the presence of a methyl ketone (a ketone with the structure \(CH_3CO-\)). Ethanol and certain alcohols that can be oxidized to these structures can also give a positive test. It's important to recognize that only structures with a \(CH_3C=O\) group or ethanol (which can be oxidized to ethanal) will give a positive result.

Step 2: Analyze Ethanol

Ethanol (\(CH_3CH_2OH\)) can be oxidized to ethanal (acetaldehyde, \(CH_3CHO\)) which contains the \(CH_3C=O-\) group. Therefore, ethanol will give a positive iodoform test as it transforms into ethanal.

Step 3: Analyze Ethanal

Ethanal (\(CH_3CHO\)) directly contains the \(CH_3C=O-\) group necessary to give a positive iodoform test. Hence, ethanal will give a positive reaction.

Step 4: Analyze Pentan-3-one

Pentan-3-one (\(CH_3CH_2COCH_2CH_3\)) does not have the \(CH_3C=O-\) group. It has a methylene group \(CH_2-\) next to the carbonyl, which means it will not give a positive iodoform test.

Step 5: Analyze Pentan-2-one

Pentan-2-one (\(CH_3COCH_2CH_2CH_3\)) contains the \(CH_3C=O-\) group, making it eligible to give a positive iodoform test. Therefore, pentan-2-one will react positively.

Key Concepts

Methyl Ketone

A methyl ketone is a specific kind of ketone characterized by the presence of a methyl group ( CH_3- ) directly bonded to the carbonyl group ( C=O ). The general structure of a methyl ketone can be represented as CH_3CO- R, where R can be any alkyl group. This specific structural feature is crucial for the iodoform test, a qualitative test used in organic chemistry to identify methyl ketones and some secondary alcohols that can be oxidized to methyl ketones. When a compound contains this distinct CH_3C=O- group, it can react with iodine in the presence of a base to form iodoform (CHI_3), which is a yellow precipitate. This reaction is the basis of the iodoform test. Key points to remember about methyl ketones include:

• They possess the specific CH_3C=O- structural unit.
• They are one of the few organic compounds that can produce a positive iodoform test result.

Ethanol

Ethanol ( CH_3CH_2OH ) is a simple alcohol that can be oxidized to form ethanal ( CH_3CHO ), also known as acetaldehyde. This transformation is vital in the context of the iodoform test because ethanal contains the necessary CH_3C=O- group to yield a positive test result. When ethanol is oxidized, it undergoes a process where the hydroxyl group ( -OH ) is converted into a carbonyl group ( C=O ), creating ethanal. In the presence of iodine and a base, ethanal then reacts to form the yellow precipitate of iodoform. Why ethanol is significant:

• Can be oxidized to acetaldehyde, which gives a positive iodoform test.
• Provides a clear example of how alcohols can be linked to this test through oxidation.

Pentan-3-one

Pentan-3-one ( CH_3CH_2COCH_2CH_3 ) is a ketone but not a methyl ketone because it lacks the necessary CH_3C=O- group required for a positive iodoform test. Instead, it has the carbonyl group located with methylene groups ( CH_2- ) on both sides. This positioning of the carbonyl means that pentan-3-one cannot produce the iodoform reaction. It serves as an example of a ketone that does not qualify for the test due to its structural arrangement. Important insights on pentan-3-one:

• Lacks the methyl ketone structure ( CH_3C=O- ).
• Does not form iodoform, hence gives a negative iodoform test.

Pentan-2-one

Pentan-2-one ( CH_3COCH_2CH_2CH_3 ) is a clear example of a methyl ketone because it contains the CH_3C=O- group. This essential structural feature allows it to successfully undergo an iodoform test, producing the yellow precipitate known as iodoform, confirming the presence of a methyl ketone group within the compound. Its ability to give a positive iodoform test lies in the methyl group attached to the carbonyl carbon, which reacts with iodine and a base to yield iodoform. Understanding pentan-2-one's role:

• Contains the necessary methyl ketone group ( CH_3C=O- ).
• Produces a positive result in an iodoform test.