Problem 136 - A Complete Resource Book in Chemistry for JEE Main

Question

An organic compound \(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}\) does not give a precipitate with 2, 4-dinitrophenylhydrazine and does not react with metallic sodium. It could be (a) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CHO}\) (b) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH}\) (c) \(\mathrm{CH}_{3}-\mathrm{CO}-\mathrm{CH}_{3}\) (d) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{O}-\mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

The compound is \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{O}-\mathrm{CH}_{3} \) (option d).

Step 1: Analyze Reaction with 2, 4-Dinitrophenylhydrazine

2, 4-Dinitrophenylhydrazine (2, 4-DNP) is used to test for carbonyl groups, which are present in aldehydes and ketones, to form a yellow/orange precipitate. The compound does not react with 2, 4-DNP, indicating it is neither an aldehyde nor a ketone. Therefore, options (a) \( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CHO} \) and (c) \( \mathrm{CH}_{3}-\mathrm{CO}-\mathrm{CH}_{3} \) can be eliminated since they contain carbonyl groups.

Step 2: Analyze Reaction with Metallic Sodium

Metallic sodium reacts with alcohols, producing hydrogen gas. The given compound does not react with sodium, indicating it is not an alcohol. Thus, option (b) \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH} \) is eliminated since it is an alcohol.

Step 3: Identify the Remaining Compound

The only remaining option is (d) \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{O}-\mathrm{CH}_{3} \). This compound is an ether, which typically does not react with 2, 4-DNP or metallic sodium.

Key Concepts

Carbonyl Group Testing

In organic chemistry, identifying functional groups is crucial to understanding the properties of a compound. Carbonyl groups, which include aldehydes and ketones, can be detected using a reagent called 2,4-dinitrophenylhydrazine (2,4-DNP). When 2,4-DNP is mixed with a compound containing a carbonyl group, it forms a yellow or orange precipitate. This reaction helps to confirm the presence of carbonyl groups because the new, larger molecule formed is less soluble.
The exercise involved compound identification by using 2,4-DNP to test for carbonyl groups. If no precipitate was formed, this indicated the absence of both aldehydes and ketones in the sample. In the provided solution, options (a) and (c) were eliminated due to the presence of carbonyl groups, which are not compatible with the experiment's result of a non-reacting substance with 2,4-DNP.

Reactions with Metallic Sodium

The test involving metallic sodium is a classic method to determine the presence of certain hydrogen atoms that can be released as hydrogen gas. Specifically, this test is used to detect alcohols, as they have hydroxyl groups (-OH) that react with sodium to liberate hydrogen gas. This reaction is quite visible due to the effervescence or bubbling that occurs.

In this exercise, the compound did not react with metallic sodium, indicating that it was not an alcohol. The alcohol option (b) was ruled out based on this understanding. Alcohols react strongly with sodium, so the absence of visible reaction further indicated that the compound lacked a hydroxyl group. This helped narrow down the possibilities to find the correct compound.

Ether Identification

Ethers are a class of organic compounds characterized by an oxygen atom connected to two alkyl or aryl groups. Their structural formula, such as (R-O-R'), denotes this oxygen linkage. Ethers typically do not react with 2,4-DNP or metallic sodium, which makes them distinguishable from aldehydes, ketones, and alcohols.

In the given exercise, the compound that conformed to the test results was an ether, shown by option (d). Since the compound did not give a precipitate with 2,4-DNP, nor did it react with metallic sodium, it could be identified as an ether. Ethers are generally stable and unreactive with agents used for detecting aldehydes, ketones, and alcohols, making them unique in this series of tests. This reinforces the importance of using tests selectively to discern complex organic structures.