Problem 113 - A Complete Resource Book in Chemistry for JEE Main

Question

A hydrocarbon of molecular formula \(\mathrm{C}_{7} \mathrm{H}_{12}\) on catalytic hydrogenation over platinum gives \(\mathrm{C}_{7} \mathrm{H}_{16}\). The parent hydrocarbon adds bromine and also reacts with \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right] \mathrm{OH}\) to give a precipitate. The parent hydrocarbon is (a) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{C} \equiv \mathrm{CH}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{2} \mathrm{CH}=\mathrm{CH}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CHCH}\left(\mathrm{CH}_{3}\right)_{2}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}^{2}=\mathrm{CHCH}=\mathrm{CHCH}_{3}\)

Step-by-Step Solution

Answer

The parent hydrocarbon is (a) \( (\mathrm{CH}_{3})_{3} \mathrm{CCH}_{2} \mathrm{C} \equiv \mathrm{CH} \).

Step 1: Hydrogenation Reaction Analysis

The given hydrocarbon has the formula \( \mathrm{C}_{7} \mathrm{H}_{12} \). During catalytic hydrogenation, it converts into \( \mathrm{C}_{7} \mathrm{H}_{16} \), reflecting the addition of 4 hydrogen atoms. Since alkenes add hydrogen to become saturated (alkenes to single bonds), the hydrocarbon is an alkene or alkyne before hydrogenation.

Step 2: Bromine Addition Reaction

The hydrocarbon reacts with bromine, which indicates the presence of a double or triple bond, suggesting unsaturation. Alkenes and alkynes decolorize bromine due to their ability to add bromine across double or triple bonds.

Step 3: Reaction with Tollens' Reagent

The hydrocarbon also reacts with \( [\mathrm{Ag}(\mathrm{NH}_{3})_{2}] \mathrm{OH} \) resulting in a precipitate, which suggests the presence of a terminal alkyne group. Tollens' reagent reacts with terminal alkynes to form a silver acetylide that precipitates.

Step 4: Compare with Options

Examine the molecular structures in the options. Option (a) \( (\mathrm{CH}_{3})_{3} \mathrm{CCH}_{2} \mathrm{C} \equiv \mathrm{CH} \) fits all criteria. It is a terminal alkyne, allowing it to hydrogenate to \( \mathrm{C}_{7} \mathrm{H}_{16} \), to react with bromine, and to give a precipitate with Tollens' reagent.

Key Concepts

Catalytic Hydrogenation

Catalytic hydrogenation is a process where unsaturated hydrocarbons, such as alkenes and alkynes, add hydrogen (H₂) across their double or triple bonds. This reaction takes place in the presence of a catalyst, typically platinum, palladium, or nickel, transforming these compounds into saturated hydrocarbons. The introduction of hydrogen converts multiple bonds to single bonds, making the compound more stable.
In the exercise, a hydrocarbon \( \mathrm{C}_{7} \mathrm{H}_{12} \) undergoes catalytic hydrogenation to become \( \mathrm{C}_{7} \mathrm{H}_{16} \). This conversion signifies the addition of four hydrogen atoms, shedding light on the hydrocarbon's initial unsaturation. This process is crucial for understanding the structure of compounds, as it reveals the presence of double or triple bonds in the original molecule.

Bromine Addition

When a hydrocarbon reacts with bromine, it usually involves a phenomenon known as bromine addition, indicating the presence of unsaturated bonds. Bromine, a reddish-brown liquid, loses its color as it reacts with alkenes and alkynes. This color change is a useful test for unsaturation, as the carbon-carbon double or triple bonds readily add bromine molecules, forming a dibromo compound.
In the exercise, the hydrocarbon's reaction with bromine confirms the presence of either a double or a triple bond, showcasing the compound's capability to engage in addition reactions, which is a telltale sign of unsaturation.

Tollens' Reagent

Tollens' reagent is a solution of ammoniacal silver nitrate, denoted as \( [\mathrm{Ag}(\mathrm{NH}_{3})_{2}] \mathrm{OH} \), which is famously used to distinguish between different types of hydrocarbons based on their structural properties. It reacts distinctively with terminal alkynes, leading to the formation of a precipitate called silver acetylide.
This reaction is a key characteristic of terminal alkynes, where the \(-\mathrm{C} \equiv \mathrm{CH} \) group reacts with Tollens’ reagent, precipitating out the silver as a solid. In the given problem, the formation of a precipitate upon reaction with Tollens' reagent strongly indicates the presence of a terminal alkyne, helping in identifying the correct structure of the hydrocarbon involved.

Terminal Alkynes

Terminal alkynes are a class of hydrocarbons where the triple bond is located at the end of the carbon chain. They possess a structural unit \(-\mathrm{C} \equiv \mathrm{C}-\mathrm{H} \), making them unique in their chemical behavior when compared with other alkynes.
The presence of the hydrogen atom bonded to the carbon in the triple bond makes terminal alkynes distinct. They can undergo various reactions, such as hydrogenation and bromine addition, like other unsaturated hydrocarbons.

• Their most notable reaction is with Tollens' reagent, forming a silver precipitate of silver acetylide that confirms their uniqueness.
• This precipitate forms because the terminal hydrogen atom in the alkyne can be abstracted by the silver ion, showcasing a clear identification method for terminal alkynes.

In the context of the exercise, this property helps in identifying option (a) \((\mathrm{CH}_{3})_{3} \mathrm{CCH}_{2} \mathrm{C} \equiv \mathrm{CH}\) as the correct hydrocarbon, confirmed by its reactions with hydrogen, bromine, and Tollens' reagent.