Question

Using which of the following reagents one can perform a simple test that can be uscd to differentiate between \(\mathrm{C}_{6} \mathrm{II}_{5} \mathrm{C} \equiv \mathrm{CII}\) and \(\mathrm{C}_{6} \mathrm{I}_{5} \mathrm{CII}=\mathrm{CH}_{2} ?\) (1) \(\mathrm{NaOIL} / \mathrm{II}_{2} \mathrm{O}\) (2) \(\mathrm{Br}_{2} / \mathrm{CCl}_{4}\) (3) \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{OII}\) (4) \(\mathrm{CrO}_{3} / \mathrm{II}_{2} \mathrm{SO}_{4}\)

Short answer

Use \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{OH}\) to differentiate. It reacts with \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C} \equiv \mathrm{CH}\), forming a white precipitate.

Step-by-step solution

- Understand the compounds

First, recognize that \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C} \equiv \mathrm{CH}\) is a terminal alkyne while \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH} = \mathrm{CH}_{2}\) is an alkene.

- Analyze the reagents

Consider the reactivity of the given reagents with the compounds: 1. \(\mathrm{NaOH} / \mathrm{H}_{2} \mathrm{O}\) 2. \(\mathrm{Br}_{2} / \mathrm{CCl}_{4}\) 3. \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{OH}\) 4. \(\mathrm{CrO}_{3} / \mathrm{H}_{2} \mathrm{SO}_{4}\)

- Select the differentiating reagent

Among these, \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{OH}\) is Tollens' reagent, which specifically reacts with terminal alkynes to form a precipitate of silver acetylide.

- Perform the test

When \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C} \equiv \mathrm{CH}\) (the terminal alkyne) is treated with \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{OH}\), it forms a white precipitate. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH} = \mathrm{CH}_{2}\) (the alkene) does not form a precipitate with this reagent.

Key concepts

terminal_alkyne_test

Terminal alkynes are a unique group of hydrocarbons that possess a triple bond between two carbon atoms. In this particular exercise, the importance of terminal alkynes is highlighted because of their ability to react with specific reagents in tests. The terminal alkyne, \(\text{C}_6\text{H}_5 \text{C} \equiv \text{CH}\), possesses an acidic hydrogen atom at the end of the triple bond. This unique feature allows it to participate in a specific reaction with Tollens' reagent.
When terminal alkynes are treated with \(\text{Ag(NH}_3)_2\text{OH}\), a white precipitate of silver acetylide forms, distinguishing them from alkenes like \(\text{C}_6\text{H}_5 \text{CH}= \text{CH}_2\), which do not react similarly. This simple test, referred to as the terminal alkyne test, is a useful tool to differentiate between these two types of hydrocarbons.

Tollens_reagent

Tollens' reagent is primarily known for its use in detecting aldehydes but also plays a crucial role in organic chemistry to identify terminal alkynes. This reagent consists of an aqueous solution of silver nitrate \( \text{AgNO}_3 \) mixed with ammonia \( \text{NH}_3 \). When mixed, they form \( \text{Ag(NH}_3)_2^+ \), the active component of Tollens' reagent.
The reagent's interaction with terminal alkynes produces a silver acetylide precipitate \( \text{AgC} \equiv \text{CAg} \). This reaction is very specific, making Tollens' reagent an excellent choice for distinguishing terminal alkynes from other hydrocarbons like alkenes.
The practical application of Tollens' reagent lies in its simplicity and specificity, making it a go-to reagent for both laboratory and educational settings where the quick identification of terminal alkynes is necessary.

organic_chemistry

Organic chemistry is a branch of chemistry focused on the study of carbon-containing compounds. These compounds form the basis of living organisms and numerous synthetic materials. In organic chemistry, the functionality and reactivity of molecules are key to understanding their behavior in different reactions.
Hydrocarbons, a major class of organic compounds, include alkanes, alkenes, and alkynes. Differentiating these hydrocarbons is essential. Techniques such as the terminal alkyne test provide an effective way to do this.
Grasping the fundamentals of organic chemistry involves not only recognizing different types of bonds and functional groups but also understanding how these properties influence a molecule's reactivity. Practical exercises, like the one described here, contribute significantly to a comprehensive understanding of these principles by allowing students to apply theoretical knowledge in real-world scenarios.