Problem 120 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

The structure of alkyne is linear. Terminal alkyne is acidic in character. It reacts with base to give acid-base reaction. Alkyne is nucleophile and gives electrophilic as well as nucleophilic addition reaction. Terminal alkyne will react with which of the following? (a) Tollen's reagent (b) \(\mathrm{Sia}_{2} \mathrm{BH} / \mathrm{THE}\) (c) \(\mathrm{H}_{2} \mathrm{O} / \mathrm{HgSO}_{4} / \mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{~d})\) All of these

Step-by-Step Solution

Answer

None of the given choices specifically test the acidity of a terminal alkyne.

Step 1: Understanding Terminal Alkynes

Terminal alkynes have an acidic hydrogen atom at the terminal carbon. This acidic hydrogen can be removed by a strong base or react with a reagent that can detect acidic protons.

Step 2: Reaction with Tollen's Reagent

Tollen's reagent is used primarily to test for aldehydes. Terminal alkynes do not react with Tollen's reagent because it is specific for carbonyl compounds, and it does not involve the acidic hydrogen reaction with alkynes.

Step 3: Reaction with Sia_{2}BH / THF

The reagent Sia_{2}BH / THF is used for hydroboration-oxidation, which forms aldehydes from terminal alkynes. This reaction doesn't test acidity but is a reaction specific to alkynes.

Step 4: Reaction with H_2O / HgSO_4 / H_2SO_4

Combine terminal alkynes with H_2O / HgSO_4 / H_2SO_4 for hydration. The compound forms a ketone in an acid-catalyzed reaction, indicating an interaction with alkynes but not related to acidity.

Step 5: Conclusion of Reaction Possibilities

None of the given reactions depend on the terminal alkyne’s acidity. Instead, they focus on converting or testing the alkyne structure in different reactions.

Key Concepts

Acidic Hydrogen

Terminal alkynes are particularly interesting in organic chemistry due to the presence of an acidic hydrogen atom. This hydrogen is located on the terminal carbon - the very end of the alkyne chain. What makes it acidic? It's all about the stability of the resulting anion when the hydrogen is removed.

• The acidity arises from the ability of alkynes to stabilize the negative charge that forms after losing a hydrogen.
• When the acidic hydrogen is released, the electron density remains with the triple bond, which is more electronegative.
• This stabilization often allows a strong base to deprotonate the terminal alkyne.

Given how it interacts with bases, this feature can make terminal alkynes versatile in reactions, particularly important in synthetic applications where functional group transformations are key.

Electrophilic Addition

Electrophilic addition reactions are typical of compounds with double or triple bonds, like alkynes. Here, the unsaturated carbon chain acts significantly as a nucleophile due to its electron-rich nature. In electrophilic addition:

• Electrophiles are attracted to the electrons in the alkyne's multiple bonds.
• The reaction typically begins with the electrophile attacking the electron-rich triple bond.
• As the initial pi bond breaks, it forms a new bond with the electrophile.

For alkynes, common electrophilic addition reactions involve halogens, hydrogen halides, or water in the presence of catalysts. This process effectively converts the alkyne into alkenes or even further into alkanes, depending on the reaction's conditions and reagents.

Nucleophilic Addition

Though less common than with electrophiles, alkynes can participate in nucleophilic addition reactions. Here, the alkyne serves more as the accepting entity.

• Nucleophiles, which are rich in electrons, typically target the electron-deficient sites created by catalyst-induced polarization of the alkyne triple bond.
• Due to the strong nature of the alkyne's bonds, nucleophilic addition often requires the presence of a catalyst or specific reaction conditions to proceed.
• This type of addition enables the alkyne to be transformed into other functional groups, adding further diversity to its chemical repertoire.

By understanding nucleophilic addition, students can predict and control complex synthetic paths involving alkynes, crucial for advanced chemical synthesis.

Hydration of Alkynes

The hydration of alkynes is a fundamental reaction wherein water is added to the alkyne structure, usually leading to the formation of carbonyl compounds. In practical laboratory scenarios, this often involves terminal alkynes.

• Typical reagents for this process include HgSO_4, H_2SO_4, and water.
• The presence of these reagents facilitates the conversion of alkynes into ketones through a process known as keto-enol tautomerism.
• This reaction is acid-catalyzed and broadens the applicability of terminal alkynes by increasing their functional flexibility.

Understanding this reaction is essential as it sets the stage for further synthesis opportunities, making terminal alkynes vital building blocks in organic chemistry. Such reactions highlight the unique capacity of terminal alkynes to act as pivotal intermediates in chemical production and synthesis routes.