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Chapter 27: Problem 136

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) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{O}-\mathrm{CH}_{3}\)

Short Answer

Expert verified
The compound is methyl vinyl ether, option (d).

Step by step solution

01

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.
02

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.
03

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.
04

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.
05

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.
06

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

Methyl vinyl ether lacks both functional groups鈥攃arbonyl and hydroxyl鈥攔equired for the reactions with 2,4-Dinitrophenylhydrazine and metallic sodium. Therefore, it fits the criteria of not reacting in these tests.
07

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.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

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鈥攆ormation of a yellow or orange precipitate鈥攚hen 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鈥檛 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.

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Most popular questions from this chapter

A compound of the formula \(\mathrm{C}_{4} \mathrm{H}_{10} \mathrm{O}\) reacts with sodium and undergoes oxidation to give a carbonyl compound which does not reduce Tollen's reagent, the original compound is (a) sec-butyl alcohol (b) \(n\)-butyl alcohol (c) isobutyl alcohol (d) diethyl ether

An unknown compound dissolves in sulphuric acid, but does not decolourize bromine water and does not react with sodium. Which of the following classes of molecules behave in this manner? (a) phenol (b) alkene (c) alcohol (d) ether

The reaction of \(\mathrm{CH}_{3} \mathrm{MgBr}\) with acetone and hydrolysis of the resulting product gives (a) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHOH}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\)

Which of the following is the most suitable method for removing the traces of water from ethanol? (a) Heating with sodium metal (b) Hassing dry HCl gas through it (c) Distilling it (d) Reacting with \(\mathrm{Mg}\)

Out of the following the number of compounds which can be used to remove water from alcohol is \(\mathrm{CaO}, \mathrm{Na}, \mathrm{Mg}, \mathrm{CaCl}_{2},\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{O}\right)_{2} \mathrm{Mg}\), conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}, \mathrm{PCl}_{5}\)
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