Problem 91 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the reagent from the following list which can easily distinguish between 1 -butyne and 2 -butyne? (a) bromine, \(\mathrm{CCl}_{4}\) (b) \(\mathrm{H}_{2}\), Lindlar catalyst (c) dilute \(\mathrm{H}_{2} \mathrm{SO}_{4}, \mathrm{HgSO}_{4}\) (d) ammonical coprocess chloride

Step-by-Step Solution

Answer

The reagent is (d) ammonical copper chloride.

Step 1: Understanding the Compounds

1-butyne has a terminal alkyne triple bond ( \( \text{C}_4\text{H}_7 \equiv \text{CH} \)), while 2-butyne has an internal alkyne triple bond ( \( \text{CH}_3\text{-C} \equiv \text{C}-\text{CH}_3 \)). The main difference lies in the location of the triple bond.

Step 2: Method Selection: Terminal vs Internal Alkyne

Terminal alkynes like 1-butyne react with ammonical copper chloride to form a precipitate because they have an acidic hydrogen atom on the terminal carbon, whereas internal alkynes like 2-butyne do not react.

Step 3: Analyzing Each Reagent

(a) Bromine in \( \text{CCl}_4 \) is used for testing unsaturation but does not distinguish between terminal and internal alkynes. (b) \( \text{H}_2 \) with Lindlar catalyst is used for converting alkynes to cis-alkenes but does not distinguish effectively between 1-butyne and 2-butyne. (c) Dilute \( \text{H}_2 \text{SO}_4, \text{HgSO}_4 \) converts alkynes to ketones or aldehydes and does not differentiate between the two. (d) Ammonical copper(I) chloride forms a complex with terminal alkynes but not with internal ones, effectively distinguishing them.

Step 4: Concluding the Reagent Choice

The only reagent that forms a specific result based on a terminal alkyne property is ammonical copper chloride (option d), which forms a precipitate with 1-butyne, not with 2-butyne.

Key Concepts

Terminal vs Internal Alkynes

In alkyne chemistry, understanding terminal and internal alkynes is crucial for differentiation. Terminal alkynes, such as 1-butyne, have a triple bond at the end of the carbon chain, marked by the \( \equiv \text{CH} \) group. On the other hand, internal alkynes, like 2-butyne, have the triple bond located between two carbon groups, represented by \( \text{C} \equiv \text{C} \).
The main distinction lies in the presence of a hydrogen atom bonded to the carbon with the triple bond in terminal alkynes. This hydrogen is slightly acidic, unlike in internal alkynes, where each carbon is bonded to other carbon groups, eliminating the acidic hydrogen.
Recognizing these differences helps in choosing suitable reagents for chemical reactions and differentiations, as certain reactions are exclusive to terminal alkynes due to their structure.

Ammonical Copper Chloride Reaction

The ammonical copper chloride reaction is a key method in alkyne differentiation, specifically for distinguishing terminal alkynes. This reaction takes advantage of the acidic hydrogen present in terminal alkynes.
When terminal alkynes like 1-butyne react with ammonical copper chloride, they form a copper-alkyne complex. This results in a reddish-brown precipitate, easily observed visually. The acid hydrogen participates in forming this complex, which does not occur with internal alkynes like 2-butyne.

• Terminal alkynes form a precipitate.
• Internal alkynes show no reaction.

This reactivity difference allows chemists to use ammonical copper chloride as a diagnostic tool in laboratories to identify terminal alkynes in a mixture of compounds.

Alkyne Triple Bond Identification

Identifying where an alkyne triple bond resides within a carbon chain affects both physical and chemical properties. Alkyne triple bonds consist of one sigma bond and two pi bonds, creating a linear structure that significantly influences reactivity.
Triple bonds can be located at terminal or internal positions on the carbon chain. The location affects properties such as electron density and acidity.

• Terminal alkynes: slightly acidic, enables interactions like forming metal acetylides.
• Internal alkynes: higher electron density around the triple bond, less acidic.

Understanding the bond's position informs the choice of reagents and anticipated reaction pathways, enabling more precise chemical synthesis and analysis.