Problem 44 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The compound that will not give iodoform on treatment with alkali and iodine is : (a) acetone (b) ethanol (c) diethyl ketone (d) isopropyl alcohol

Step-by-Step Solution

Answer

(c) diethyl ketone

Step 1: Identify the Iodoform Test Condition

The iodoform test is positive for compounds that have the structure R-CO-CH3 or R-CHOH-CH3, making them able to release a methyl ketone or a primary alcohol with an alpha methyl group upon oxidation.

Step 2: Analyze Each Compound

- Acetone ( R=C, CH3C(=O)CH3) is a methyl ketone and will give a positive iodoform test. - Ethanol ( CH3CH2OH) oxidizes to acetaldehyde (CH3CHO), which has the CH3-CO- group and will also give iodoform. - Diethyl ketone ( CH3CH2C(=O)CH2CH3) does not have a methyl group attached directly to the carbonyl carbon. - Isopropyl alcohol ( CH3CHOHCH3) gets oxidized to acetone (CH3C(=O)CH3) and will give iodoform.

Step 3: Select the Compound that Does Not Fulfill the Condition

Out of the options, diethyl ketone is the only compound that lacks a methyl group attached to the carbonyl carbon, which is necessary for the iodoform test.

Key Concepts

Methyl Ketone

Methyl ketones are a special type of carbonyl compound characterized by their distinct structure, which includes the carbonyl group (C=O) bonded directly to a methyl group (CH3). This unique arrangement is crucial for certain chemical reactions, such as the iodoform test. In this test, only compounds with a methyl ketone group will react positively, forming a yellow precipitate of iodoform. Understanding methyl ketones involves recognizing their general structure R-C(=O)-CH3. Here, 'R' represents any alkyl group which can vary in length and complexity.

• Methyl ketones are widely found in both natural and synthetic compounds.
• Acetone is a common example of a methyl ketone, easily identified by its chemical formula CH3C(=O)CH3.
• In the iodoform test, the presence of the CH3C(=O) group is essential for the reaction to occur.

Recognizing methyl ketones is important when predicting the results of the iodoform test, as they distinguish themselves by forming this distinctive yellow precipitation when interacting with iodine and alkali.

Oxidation of Alcohols

Alcohols are a diverse group that can undergo oxidation, leading to different types of products based on their structure. When alcohols with the structure R-CHOH-CH3 undergo oxidation, they can lead to the formation of carbonyl compounds, particularly methyl ketones or aldehydes, which are essential for the iodoform test. Oxidation is a chemical reaction where alcohols lose hydrogen atoms and gain oxygen atoms. Primary alcohols, with one alkyl group connected to the carbinol carbon (like ethanol), are normally oxidized to aldehydes, while secondary alcohols, such as isopropyl alcohol (CH3CHOHCH3), typically oxidize to ketones.

• Primary alcohols may progress from aldehydes to carboxylic acids upon further oxidation.
• Secondary alcohols stop at the ketone stage as further oxidation would break the molecule apart.

The likelihood of forming a methyl ketone increases if the side chain has an alpha methyl group. Thus, the iodoform test specifically identifies secondary alcohols that can be oxidized to methyl ketones, making it an important step in many synthetic and analytical procedures.

Carbonyl Compounds

Carbonyl compounds are a broad class of organic substances defined by the presence of a carbonyl group (C=O). This group imparts significant reactivity and characteristic properties to molecules like aldehydes and ketones. In the context of the iodoform test, the differentiation mainly centers around whether the carbonyl compound arises from a primary alcohol transformed into an aldehyde, or a secondary alcohol turned into a methyl ketone. Key attributes of carbonyl compounds include:

• The carbon atom in the carbonyl group is often electrophilic (electron-deficient), making it susceptible to nucleophilic attack.
• Aldehydes and ketones are distinguished by their respective structures, with ketones like methyl ketones having two alkyl groups attached to the carbonyl, versus just one in aldehydes.
• This electrophilic nature and structural variety enable carbonyl compounds to participate in diverse chemical reactions.

Understanding carbonyl compounds is crucial for identifying which types might succeed in the iodoform test, as they are intrinsic to reactions involving the oxidation of alcohols and the formation of unstable intermediates.