Problem 189 - A Complete Resource Book in Chemistry for JEE Main

Question

Acetylene does not react with \(\quad[\mathbf{2 0 0 2}]\) (a) \(\mathrm{Na}\) (b) ammonical \(\mathrm{AgNO}_{3}\) (c) \(\mathrm{HCl}\) (d) \(\mathrm{NaOH}\)

Step-by-Step Solution

Answer

Acetylene does not react with NaOH.

Step 1: Understand Acetylene's Reactivity

Acetylene ( C_2H_2 ) is a hydrocarbon with a triple bond between two carbon atoms. It is known to react with electrophiles due to its ability to donate electrons.

Step 2: Assess Each Reagent

We need to check which reagents typically react with compounds like acetylene. Sodium ( Na ) might react with acetylene to form sodium acetylide. Ammoniacal AgNO_3 can react with terminal alkynes like acetylene to form a precipitate. HCl can react with acetylene in the presence of a catalyst to form vinyl chloride. Sodium hydroxide ( NaOH ), a base, does not typically react with acetylene.

Step 3: Identify the Non-reactive Reagent

From the assessments in Step 2, we noted that sodium hydroxide ( NaOH ) does not typically react with acetylene. This is because NaOH doesn't provide the conditions needed for a reaction with a hydrocarbon like acetylene.

Key Concepts

Electrophilic reaction

Electrophilic reactions occur when an electron-rich atom or molecule reacts with an electron-deficient atom or group. In the context of acetylene, which is a simple alkyne ( C_2H_2 ), the triple bond between the two carbon atoms is rich in electrons. This electron density attracts electrophiles, which are species that seek additional electrons to achieve a more stable state.
Acetylene’s ability to react with electrophiles stems from this strong electron density around its carbon-carbon triple bond.
For example:

• HCl , when catalyzed, can break the triple bond and add to acetylene, forming a new molecule like vinyl chloride.

Acetylene's reactivity with electrophiles is a key feature, allowing it to partake in a wide variety of chemical reactions, which can be utilized in various industrial applications.

Sodium acetylide formation

Sodium acetylide is formed when acetylene reacts with sodium ( Na ). This reaction is significant because sodium can donate its electron to form a stable ion, Na^+ , leaving the acetylene molecule with an extra electron, giving rise to acetylide ions ( C_2H^- ).
This transformation involves the removal of a hydrogen atom from the acetylene, creating an acetylide ion, which is strong and nucleophilic.
In a nutshell, the process looks like: [ C_2H_2 + 2Na ightarrow 2Na^+ + C_2H^- ] The sodium acetylide formed can further participate in various chemical reactions, particularly as a building block in organic synthesis due to its nucleophilic nature.

Terminal alkyne precipitation

Terminal alkynes, such as acetylene, have a hydrogen atom bonded to a carbon atom that participates in the carbon-carbon triple bond. This configuration allows them to react with ammoniacal silver nitrate ( AgNO_3 ).
The reaction is distinguished by the formation of a precipitate due to the interaction of the terminal hydrogen, carbon carbon triple bond, and the silver ions.
Notably, the reaction can be represented as:

• Alkyne + AgNO_3 + NH_3 → Alkyne-Ag + NH_4^+/NO_3^−

The formation of a silver-based precipitate from terminal alkynes is a defining test for their presence and is often used in identification and analytical chemistry.

Hydrocarbon base interaction

Hydrocarbons can interact with various bases, but this interaction heavily depends on the specific type of base and hydrocarbon involved. In the case of sodium hydroxide ( NaOH ), a strong base, acetylene, a hydrocarbon, typically does not react.
The reason for this is that NaOH does not provide the necessary conditions for stimulating a reaction with acetylene. Without a specific pathway or mechanism activated by the hydroxide ion, there is negligible interaction.
Thus, sodium hydroxide remains unreactive towards acetylene, illustrating its selectivity and the importance of choosing the right conditions and reagents when intending to engage hydrocarbons in reactions.