Question

Which is the most suitable reagent among the following to distinguish between but-1-yne and but-2-yne? (A) \(\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O}\) (B) \(\mathrm{AgNO}_{3} / \mathrm{NH}_{4} \mathrm{OH}\) (C) \(\mathrm{NaOH}\) (D) None of these

Short answer

The most suitable reagent to distinguish between but-1-yne and but-2-yne is reagent (B): \(\mathrm{AgNO}_{3} / \mathrm{NH}_{4} \mathrm{OH}\), as it specifically reacts with terminal alkynes such as but-1-yne and doesn't react with but-2-yne.

Step-by-step solution

Identify the structure of but-1-yne and but-2-yne

But-1-yne has the molecular formula \(\mathrm{C}_{4}\mathrm{H}_{6}\), and contains a triple bond between the first two carbon atoms: HC≡C-CH2-CH3. But-2-yne has the same molecular formula but contains a triple bond between the second and third carbon atoms: CH3-C≡C-CH3.

Analyze the reaction with reagent (A)

Reagent (A) is \(\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O}\). It involves a halogen addition reaction with the alkynes. Both but-1-yne and but-2-yne would undergo the same reaction and the product wouldn't distinguish between the two molecules.

Analyze the reaction with reagent (B)

Reagent (B) is \(\mathrm{AgNO}_{3} / \mathrm{NH}_{4} \mathrm{OH}\). This reagent is a Tollens' reagent which is used to distinguish between terminal and non-terminal alkynes. Terminal alkynes have an acidic hydrogen that forms a water-soluble silver acetylide complex when treated with Tollens' reagent. But-1-yne is a terminal alkyne and reacts with Tollens' reagent, while but-2-yne doesn't react. This makes reagent (B) suitable for distinguishing between but-1-yne and but-2-yne.

Analyze the reaction with reagent (C)

Reagent (C) is \(\mathrm{NaOH}\). \(\mathrm{NaOH}\) being a strong base, it doesn't clearly distinguish between the two alkynes since both would undergo deprotonation in the presence of the base.

Choose the most suitable reagent

Based on the analysis in Steps 2, 3, and 4, the most suitable reagent to distinguish between but-1-yne and but-2-yne is reagent (B): \(\mathrm{AgNO}_{3} / \mathrm{NH}_{4} \mathrm{OH}\) due to its specific reaction with terminal alkynes.

Key concepts

But-1-yne vs But-2-yne

When comparing but-1-yne and but-2-yne, the primary difference lies in the position of the triple bond within the carbon chain.

But-1-yne features a triple bond between the first and second carbon atoms, denoted as HC≡C-CH₂-CH₃. This structure makes it a terminal alkyne, meaning the triple bond is at the end of the carbon chain. Terminal alkynes are characterized by their acidic hydrogen atom attached to the carbon with the triple bond.

In contrast, but-2-yne has its triple bond between the second and third carbon atoms, represented as CH₃-C≡C-CH₃, which makes it a non-terminal alkyne. The absence of an acidic hydrogen in non-terminal alkynes (like but-2-yne) is a critical factor that affects their chemical reactivity, particularly when interacting with certain reagents.

Tollens' Reagent Reaction

Tollens' reagent, which is often represented as \( \mathrm{AgNO}_3 / \mathrm{NH}_4 \mathrm{OH} \), plays a significant role in organic chemistry, especially for identifying the presence of a terminal alkyne group.

It relies on the principle that a terminal alkyne's acidic hydrogen atom can be abstracted to form a silver acetylide complex. This reaction is facilitated when Tollens' reagent reacts with an alkyne possessing an acidic hydrogen, leading to the generation of a water-soluble precipitate.

The beauty of this reaction is its selectivity; non-terminal alkynes, which lack an acidic hydrogen, will not react with Tollens' reagent, thereby providing a clear distinction. For students, understanding this selective reactivity is crucial for accurately identifying different types of alkynes in chemical testing.

Halogen Addition Reaction

The addition of halogens to alkynes, also known as the halogen addition reaction, is another interesting chemical process. When an alkyne is treated with a halogen, such as bromine represented by \( \mathrm{Br}_2 \), the reaction leads to the addition of the halogen atoms across the triple bond.

This reaction typically results in a dihalo product for alkynes and is not selective between terminal and non-terminal alkynes, making it unsuitable for distinguishing between compounds like but-1-yne and but-2-yne. Both alkynes would react similarly to form a tetra-substituted alkene, which makes it impossible to tell them apart using this method.

Understanding these basic reactions in organic chemistry helps students predict and interpret the outcomes of alkyne reactivity, which are imperative skills for future organic chemists.