Question

When ${\mathrm{C}}_2{\mathrm{II}}_2,{\mathrm{CH}}_4$ and ${\mathrm{C}}_2{\mathrm{H}}_4$ pass through a test tube which contains ammonical ${\mathrm{Cu}}_2{\mathrm{Cl}}_2$, find out which gas comes out unaffected from the test tube. (1) ${\mathbf{C}}_2{\mathrm{H}}_2$ and ${\mathrm{CH}}_4$ (2) ${\mathrm{C}}_2{\mathrm{H}}_2$ and ${\mathrm{C}}_2{\mathrm{H}}_4$ (3) ${\mathbf{C}}_2{\mathrm{H}}_2$ (4) ${\mathrm{C}}_2{\mathrm{H}}_4$ and ${\mathrm{CH}}_4$

Short answer

The gases that come out unaffected are ${\text{CH}}_4$ and ${\text{C}}_2{\text{H}}_4$. The correct answer is option (4).

Step-by-step solution

Understanding the Reagent

Ammoniacal ${\text{Cu}}_2{\text{Cl}}_2$ is used to detect terminal alkynes. Terminal alkynes like ${\text{C}}_2{\text{H}}_2$ will react with ${\text{Cu}}_2{\text{Cl}}_2$ to form a precipitate.

Analyzing the Given Compounds

The given compounds are ${\text{C}}_2{\text{H}}_2$, ${\text{CH}}_4$, and ${\text{C}}_2{\text{H}}_4$.

Reaction with Ammoniacal \text{Cu}_2\text{Cl}_2

Among these, ${\text{C}}_2{\text{H}}_2$ is a terminal alkyne and will react with ${\text{Cu}}_2{\text{Cl}}_2$. ${\text{CH}}_4$ (methane) is an alkane and ${\text{C}}_2{\text{H}}_4$ (ethylene) is an alkene; neither will react with ${\text{Cu}}_2{\text{Cl}}_2$.

Determine the Unaffected Gases

Since ${\text{C}}_2{\text{H}}_2$ reacts with ammoniacal ${\text{Cu}}_2{\text{Cl}}_2$, the gases that come out unaffected are ${\text{CH}}_4$ and ${\text{C}}_2{\text{H}}_4$.

Key concepts

alkynes detection

In organic chemistry, detecting different types of hydrocarbons accurately is crucial. Terminal alkynes are a type of hydrocarbon that have triple bonds at the end of their carbon chain, such as ${\text{C}}_2{\text{H}}_2$ (acetylene). When trying to detect terminal alkynes in a mixture, ammoniacal ${\text{Cu}}_2{\text{Cl}}_2$ (Cuprous chloride in ammonia) is often used. This reagent reacts specifically with terminal alkynes to form a copper acetylide precipitate. This unique reaction helps in identifying the presence of terminal alkynes and differentiates them from other hydrocarbons.

organic chemistry reagents

Reagents play a vital role in organic chemistry as they help in facilitating various reactions. Ammoniacal ${\text{Cu}}_2{\text{Cl}}_2$ is a specialized reagent used to detect terminal alkynes like acetylene (${\text{C}}_2{\text{H}}_2$). When acetylene passes through a solution of ammoniacal ${\text{Cu}}_2{\text{Cl}}_2$, it reacts to form a red precipitate of copper(I) acetylide. This precipitate formation indicates the presence of terminal alkynes. On the other hand, reagents like $\text{NaOH}$ are used for deprotonation in organic reactions, while others like ${\text{KMnO}}_4$ are employed for oxidation reactions.

chemical reactions in test tubes

Carrying out chemical reactions in test tubes is a fundamental practice in chemistry labs. It allows for the observation of reactions and the identification of substances based on their reactivity. For instance, when passing a mixture of ${\text{C}}_2{\text{H}}_2$, ${\text{CH}}_4$, and ${\text{C}}_2{\text{H}}_4$ through a test tube containing ammoniacal ${\text{Cu}}_2{\text{Cl}}_2$, only the ${\text{C}}_2{\text{H}}_2$ will react, forming a precipitate. This visual transformation helps in detecting the presence of specific compounds. Such test-tube reactions are used extensively in educational settings to teach students about chemical properties and reactivity patterns.