Problem 100 - A Complete Resource Book in Chemistry for JEE Main

Question

Ethene and ethyne can be distinguished by (a) \(\mathrm{Br}_{2}\) water (b) \(\mathrm{KMnO}_{4}\) solution (c) cuprous chloride solution (d) any of the above

Step-by-Step Solution

Answer

Use cuprous chloride solution (c) to distinguish between ethene and ethyne.

Step 1: Understanding Ethene and Ethyne

Ethene (C_2H_4) is an alkene with a double bond, while ethyne (C_2H_2) is an alkyne with a triple bond. These structural differences affect how they react with certain chemical reagents.

Step 2: Reaction with Br_2 Water

Bromine water reacts with compounds having double or triple bonds. Ethene decolorizes bromine water due to the addition reaction across the double bond, while ethyne reacts by breaking the triple bond, also decolorizing it. This test is not effective for distinguishing between them, as both ethene and ethyne will react with bromine water.

Step 3: Reaction with KMnO_4 Solution

Cold dilute alkaline KMnO_4 solution, also known as Baeyer's reagent, is used to test for unsaturation. This reagent will react with both ethene and ethyne, changing color from purple to brown. However, it primarily oxidizes ethene to diols and ethyne reacts to form diones, but such reactions don't effectively distinguish one from the other through a simple visual color change.

Step 4: Reaction with Cuprous Chloride Solution

Cuprous chloride solution ( Cu_2Cl_2 in NH_3 ) can react specifically with alkynes, such as ethyne, to form a precipitate known as a copper acetylide complex. Ethene does not form this complex with cuprous chloride, making this reagent suitable for distinguishing between the two.

Step 5: Conclusion

The only reagent that can effectively distinguish between ethene and ethyne through a unique reaction is cuprous chloride solution. Ethyne forms a precipitate while ethene does not, allowing for differentiation.

Key Concepts

Hydrocarbon reactions

Hydrocarbon reactions are chemical reactions that involve compounds made up of hydrogen and carbon. In hydrocarbons, you'll find molecules like alkenes (ethene) and alkynes (ethyne), which behave differently in reactions due to their structure.

• Alkenes have at least one carbon-to-carbon double bond, whereas alkynes have at least one carbon-to-carbon triple bond.
• These structural differences significantly influence their chemical reactivity, particularly in addition reactions.
• Understanding how these reactions work allows chemists to identify these compounds in mixtures.

Reacting them with different chemicals can help us determine their identity and structure based on observable changes like color changes or precipitate formation. Each type of hydrocarbon reacts distinctively based on its characteristic bonds, providing a basis for differentiation.

Bromine water test

The Bromine water test is a classic method used to detect unsaturation in hydrocarbons like alkenes and alkynes. Both ethene (an alkene) and ethyne (an alkyne) will react with bromine water.

• When bromine, a reddish-brown liquid, encounters these unsaturated compounds, it undergoes an addition reaction.
• In this reaction, bromine connects across the multiple bonds, causing the solution to lose its color. This color change from reddish-brown to colorless indicates the presence of a double or triple bond.

Although both ethene and ethyne decolorize bromine water, this test won't help distinguish between them as both react similarly. However, it is a useful test to confirm the presence of unsaturation in a given hydrocarbon.

KMnO4 reaction

The KMnO4 reaction is another method used to confirm the presence of unsaturation within hydrocarbons. Often referred to as Baeyer's test, it involves cold, dilute alkaline KMnO4 .

• This reagent starts with a purple color and changes to brown when it reacts with unsaturated hydrocarbons like alkenes and alkynes.
• In this reaction, ethene primarily oxidizes to form diols, while ethyne can oxidize to form diketones.

Despite the distinct chemical transformations, the basic color change from purple to brown that accompanies oxidation is similar for both compounds, making it inadequate for distinguishing ethene from ethyne through mere observation.

Cuprous chloride test

The Cuprous chloride test is a specific method that can differentiate alkynes, like ethyne, from alkenes, such as ethene, due to its unique reaction pathway.

• Cuprous chloride ( Cu_2Cl_2 solution in ammonia) serves as the reagent.
• This test is based on the capability of alkynes to form a copper acetylide complex which results in the formation of a precipitate when ethyne reacts with this reagent.
• Ethene, on the other hand, will not react and will not form a precipitate with cuprous chloride, remaining clear.

This particular response makes the cuprous chloride test a powerful tool for distinguishing between ethyne and ethene, as it capitalizes on the distinct property of alkynes.