Problem 9 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Identify the reagent from the following list which can easily distinguish between 1-butyne and 2-butyne [2002S] (a) bromine, \(\mathrm{CCl}_{4}\) (b) \(\mathrm{H}_{2}\), Lindlar catalyst (c) dilute \(\mathrm{H}_{2} \mathrm{SO}_{4}, \mathrm{HgSO}_{4}\) (d) ammonical \(\mathrm{Cu}_{2} \mathrm{Cl}_{2}\) solution

Step-by-Step Solution

Answer

Ammonical Cu₂Cl₂ solution distinguishes between 1-butyne and 2-butyne.

Step 1: Understanding the structures

1-butyne is a terminal alkyne with the triple bond between the first and second carbon atoms, CH₃C≡CHCH₃. 2-butyne is an internal alkyne with the triple bond between the second and third carbon atoms, CH₃C≡CCH₃.

Step 2: Reagent Analysis

Examine each reagent to see which one reacts differently with terminal and internal alkynes: (a) Bromine in CCl₄ provides a color change that reacts with both 1-butyne and 2-butyne in similar ways. (b) H₂ with Lindlar catalyst hydrogenates both to cis-alkenes, offering no distinction. (c) Dilute H₂SO₄ with HgSO₄ gives a ketone through 'Markovnikov' hydration to both. (d) Ammonical Cu₂Cl₂ forms a copper-acetylide complex with terminal alkynes like 1-butyne, but does not react with internal alkynes like 2-butyne.

Step 3: Deciding the correct reagent

Since ammonical Cu₂Cl₂ solution can form a precipitate with 1-butyne (a terminal alkyne) but will not react with 2-butyne (an internal alkyne), it can easily distinguish between the two.

Key Concepts

Alkynes

Alkynes are a fascinating group of hydrocarbons characterized by their carbon-carbon triple bonds. These compounds are akin to alkanes and alkenes, yet they carry a higher degree of unsaturation. This triple bond consists of one sigma bond and two pi bonds, providing unique reactivity in organic chemistry.
Terminal and internal alkynes showcase distinct differences due to the position of the triple bond:

• Terminal alkynes: These have the triple bond at the end of the carbon chain, making the hydrogen atom attached to the terminal carbon quite acidic.
• Internal alkynes: The triple bond resides between two carbon atoms within the chain, leading to less acidic hydrogens compared to terminal alkynes.

1-butyne is an example of a terminal alkyne, while 2-butyne represents an internal alkyne. These subtle structural differences allow chemists to employ specific reactions for identification and characterization.

Reagent Analysis

Reagent analysis is critical in organic chemistry, especially when distinguishing between similar compounds like 1-butyne and 2-butyne. Understanding how these reagents interact with different alkynes can facilitate targeted experiments and accurate identifications.

• Bromine in CCl₄: Reacts with both terminal and internal alkynes, causing a color change, but it cannot distinguish between the two.
• Hydrogen and Lindlar Catalyst: This setup converts alkynes to alkenes but does not offer a way to differentiate them effectively.
• Dilute H₂SO₄ and HgSO₄: Both terminal and internal alkynes undergo Markovnikov hydration to form ketones, hence no differentiation is provided.
• Ammonical Cu₂Cl₂: This reagent is pivotal since it forms a copper-acetylide complex specifically with terminal alkynes. This creates a distinctive precipitate, marking it as the standout choice for distinguishing 1-butyne from 2-butyne.

In summary, reagent analysis offers a powerful approach to differentiating similar organic compounds by exploiting specific chemical interactions.

Organic Chemistry Reactions

Organic chemistry is a dynamic field that involves understanding and executing various reactions to manipulate organic molecules. The unique characteristics of different structural motifs, like triple bonds in alkynes, make these reactions both challenging and exciting.
With alkynes:

• The triple bond acts as a site for reactions, allowing additions and modifications under specific conditions.
• Hydrogenation, hydration, and halogenation are common reactions that modify the alkynes, yet they work similarly for both terminal and internal types, often requiring particular conditions or catalysts.
• The formation of complexes, such as the copper-acetylide with ammonical Cu₂Cl₂, exploits the unique acidity of terminal alkynes and is an example of targeted complexation used for differentiation.

Reactions in organic chemistry are not only about transformation but also provide pathways for distinguishing between different compounds. Such reactions are foundational techniques that chemists use to explore and innovate within this vast field.