Problem 108 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Test for identification of But-2-ene and benzene is (a) Tollen's Reagent test (b) \(1 \%\) Alkaline \(\mathrm{KMnO}_{2}\) (c) Iodoform test (d) \(\mathrm{Br}_{2}+\mathrm{H}_{2} \mathrm{O}\) test

Step-by-Step Solution

Answer

(b) Alkaline KMnO₄ test and (d) Br₂ + H₂O test can identify But-2-ene.

Step 1: Understand the Compounds

But-2-ene is an alkene, which is an unsaturated hydrocarbon with a double bond, whereas benzene is an aromatic compound that typically does not react with simple reagents due to its stability.

Step 2: Analyze Tollen's Reagent Test

Tollen's Reagent is typically used to identify aldehydes by forming a silver mirror. Neither but-2-ene nor benzene, being hydrocarbons, will react with Tollen's Reagent; this option can be eliminated.

Step 3: Consider Alkaline KMnO₄ Test

The alkaline KMnO₄ test is used to identify the presence of alkenes as it results in the formation of a diol when alkenes are oxidized. But-2-ene can react with KMnO₄, whereas benzene, being aromatic, will not react readily. This test can identify But-2-ene.

Step 4: Explore Iodoform Test

The iodoform test specifically identifies methyl ketones (or compounds that are oxidized to methyl ketones). Neither but-2-ene, nor benzene fits this criteria, eliminating this option for identification.

Step 5: Examine Br₂ + H₂O Test

The bromine water test is used to detect unsaturation in organic compounds. When added to but-2-ene, the solution will decolorize as the double bond reacts with bromine. Benzene, however, will not decolorize due to its stable aromatic nature, making this test useful in distinguishing but-2-ene from benzene.

Key Concepts

Alkene Identification

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond, which characterizes them as unsaturated compounds. The presence of this double bond makes them reactive, especially in addition reactions. Identifying alkenes involves tests that target this unsaturation. A common test is the bromine water test, where alkenes react with bromine, leading to a noticeable decolorization. This reaction occurs because the double bond allows bromine to add across it. To identify an alkene, you will look out for these key signs:

• The compound typically reacts with tests for unsaturation, like bromine water.
• Look for a rapid change in color in presence of a reagent.
• Remember that alkenes do not generally react with strong oxidizers like those used for aldehydes or other simple hydrocarbons tests.

Keeping these points in mind helps in accurately identifying a compound as an alkene.

Aromatic Compounds

Aromatic compounds, such as benzene, are characterized by their cyclic, planar structure with delocalized electrons within the ring. This unique arrangement imparts a special stability known as aromatic stability, which makes these compounds relatively unreactive compared to alkenes. As a result, they do not easily react with reagents like bromine water or Tollen's reagent. Aromatic compounds can be identified through:

• Their lack of reaction with reagents that test for unsaturation, due to the stable electron cloud.
• Distinct chemical reactions, such as electrophilic aromatic substitution, although these are not typical identification tests.
• Physical properties like a unique odor and burns with a sooty flame due to incomplete combustion.

Due to the stability of the aromatic ring, identification relies more on structural recognition rather than simple reactivity tests.

Tollen's Reagent

Tollen's reagent is a chemical test primarily used to identify aldehydes. It consists of a solution of silver nitrate in ammonia, which, when reacting with an aldehyde, results in the formation of a silver mirror on the test tube. This test is specific to aldehydes because they can be easily oxidized compared to other functional groups. Key features of Tollen's Reagent test include:

• Ineffectiveness with hydrocarbons like alkenes or aromatic compounds.
• Specific reaction with aldehydes only, not with ketones or alcohols.
• Distinct silver mirror formation, which is a clear positive result for aldehyde presence.

Given its specificity, Tollen's reagent is not suitable for testing alkenes or aromatic compounds, such as but-2-ene or benzene.

Bromine Water Test

The bromine water test is an essential method for identifying unsaturation in organic compounds, particularly alkenes. In this test, the orange-brown color of bromine water fades when it reacts with an alkene since the bromine adds across the carbon-carbon double bond, forming a dibromo compound. To effectively use the bromine water test:

• Observe the decolorization of bromine water, indicating the presence of an alkene.
• Note that aromatic compounds like benzene do not decolorize bromine water due to their stability, even though they contain double bonds in the ring.
• Understand that this test highlights the difference in reactivity between stable aromatic rings and reactive alkene double bonds.

Thus, using bromine water is particularly useful for distinguishing between alkenes and aromatic compounds in a mixture.