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 methyl vinyl ether, option (d).

Step 1: Analyze Reaction with 2,4-Dinitrophenylhydrazine

2,4-Dinitrophenylhydrazine reacts with carbonyl groups (aldehydes and ketones) to form a yellow precipitate. Since the compound does not give a precipitate, it likely does not contain a carbonyl group.

Step 2: Analyze Reaction with Metallic Sodium

Metallic sodium reacts with alcohols to produce hydrogen gas. Since there is no reaction, the compound likely does not contain an alcohol group either.

Step 3: Evaluate Option (a): Propanal (CH3-CH2-CHO)

Propanal ( C_3H_6O) is an aldehyde. It should react with 2,4-Dinitrophenylhydrazine, forming a precipitate, but this does not occur; hence, it is unlikely to be the compound.

Step 4: Evaluate Option (b): Allyl alcohol (CH2=CH-CH2OH)

Allyl alcohol is an alcohol and will react with metallic sodium. However, since the compound does not react with sodium, this can be ruled out as well.

Step 5: Evaluate Option (c): Acetone (CH3-CO-CH3)

Acetone is a ketone and should react with 2,4-Dinitrophenylhydrazine to give a yellow precipitate. But since the compound doesn't give this precipitate, it is likely not acetone.

Step 6: Evaluate Option (d): Methyl vinyl ether (CH2=CH-O-CH3)

Methyl vinyl ether lacks both functional groups—carbonyl and hydroxyl—required for the reactions with 2,4-Dinitrophenylhydrazine and metallic sodium. Therefore, it fits the criteria of not reacting in these tests.

Step 7: Conclusion: Identify the Compound

Given that methyl vinyl ether neither has a carbonyl nor an alcohol group, it aligns with both observed characteristics: no precipitation with 2,4-Dinitrophenylhydrazine and no reaction with sodium.

Key Concepts

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. They provide insight into how a molecule interacts with other substances. Recognizing these groups helps chemists predict the properties and reactivity of organic compounds.

In organic chemistry, knowing the functional groups of a compound can lead to understanding its behavior in reactions. For example:

• Alcohols: Contain a hydroxyl group (-OH). They typically react with sodium metal to produce hydrogen gas, indicating the presence of an alcohol.
• Aldehydes and Ketones: Contain a carbonyl group (C=O). These groups often participate in reactions with specific reagents like 2,4-dinitrophenylhydrazine, forming distinct products.
• Ethers: Characterized by an oxygen atom connecting two alkyl or aryl groups. These usually do not participate in the reactions that involve carbonyl compounds or alcohols.

Understanding these differences is crucial for identifying unknown compounds in experiments, as seen in the example problem where lack of specific reactions helped deduce the compound's identity.

2,4-Dinitrophenylhydrazine Test

The 2,4-Dinitrophenylhydrazine (DNPH) test is a common method used to detect the presence of aldehydes and ketones. This test is valuable because it results in a visible change—formation of a yellow or orange precipitate—when a carbonyl group is present.

• Mechanism: When DNPH is added to a solution containing an aldehyde or ketone, it reacts with the carbonyl group to form a hydrazone derivative. This derivative is usually a brightly colored precipitate.
• Specificity: DNPH does not react with esters, ethers, or other functional groups. Only compounds with a true carbonyl group will produce a precipitate.

Identifying the lack of precipitation in a DNPH test, as seen in the problem, indicates that the compound in question does not have a carbonyl function. Hence, molecules such as methyl vinyl ether, which lack such groups, pass this test unreacted.

Alcohols Reaction with Sodium

The reaction between alcohols and sodium metal is a classic chemical test used to confirm the presence of the hydroxyl group. This simple test takes advantage of the reactivity of sodium with alcohols to liberate hydrogen gas.

• Reaction Principle: Alcohols react with metallic sodium to form a sodium alkoxide and hydrogen gas, which is observed as bubbles in the reaction mixture.
• Equation: The general equation for this reaction is:
  \[ \text{R-OH} + \text{Na} \rightarrow \text{R-ONa} + \frac{1}{2} \text{H}_2 \]
• Indicators: The evolution of gas is a clear indication of the presence of an alcohol group in the molecule.

In the exercise, the absence of a reaction with sodium indicates the compound doesn’t contain an alcohol group. Methyl vinyl ether fits this criterion perfectly, as it lacks a hydroxyl group, confirming its identity when compared with the other options.