Question

A hydrocarbon \(\mathrm{C}_{6} \mathrm{H}_{10}\) does not react with ammonical solution of \(\mathrm{Cu}^{+}\) ions; it adsorbs \(2 \mathrm{~mol}\) of hydrogen on catalytic hydrogenation to give 2 -methylpentane. The hydrocarbon adds one molecule of water on treatment with \(\mathrm{Hg}^{2+} / \mathrm{H}_{2} \mathrm{SO}_{4}\). The structure of given compound is: (a) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) (b) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\) (c) C=CCC(C)C=C (d) \(\mathrm{HC} \equiv \mathrm{C}-\mathrm{CH}_{2}-\mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\)

Short answer

The structure is (a) \( \mathrm{H}_{2}\mathrm{C} = \mathrm{CH} - \mathrm{CH} = \mathrm{C}\left(\mathrm{CH}_{3}\right)_{2} \).

Step-by-step solution

Understanding the Chemical Properties

We are given that the hydrocarbon does not react with ammoniacal solution of \( \mathrm{Cu}^{+} \), which indicates that there are no terminal alkynes present as they would form copper acetylides.

Hydrogenation Reaction Analysis

The hydrocarbon adsorbs 2 moles of hydrogen to form 2-methylpentane, which indicates the presence of two degrees of unsaturation, possibly from double bonds or alkynes.

Addition of Water Reaction Analysis

The hydrocarbon can add one molecule of water in the presence of \( \mathrm{Hg}^{2+} / \mathrm{H}_{2} \mathrm{SO}_{4} \), which suggests the presence of a double bond that can be hydrated.

Identifying the Structure

Considering the above points, the structure must have two double bonds or one triple bond that results in the complete saturation to form a single bond structure of 2-methylpentane.

Structure Comparisons with Options

Option (a) has two double bonds, option (b) has a triple bond that is not terminal, option (c) can be checked for the possibility of two double bonds, and option (d) has a terminal triple bond. According to conditions, options with terminal alkynes are incorrect due to lack of reaction with \( \mathrm{Cu}^{+} \) and cannot be the answer.

Key concepts

Hydrocarbon Structure Analysis

When analyzing hydrocarbons, the structure provides essential clues about the molecule's properties and reactivity. Hydrocarbons, such as the one with the formula \(\mathrm{C}_{6} \mathrm{H}_{10}\), can include double or triple bonds leading to unsaturation. Identifying these features helps in determining the overall geometry and potential reactions.
Understanding why a hydrocarbon does not react with certain reagents, like the ammoniacal solution of \( \mathrm{Cu}^{+} \), is crucial. This lack of reaction indicates the absence of terminal alkynes since they would usually form copper acetylides in such solutions. Hence, it helps in narrowing down potential structures of the hydrocarbon.
Moreover, structural analysis involves determining the degree of unsaturation. It is revealed by understanding that adsorbing 2 moles of hydrogen via catalytic hydrogenation equates to two degrees of unsaturation. This unsaturation could be due to either multiple double bonds or a triple bond, influencing the structural configuration of the hydrocarbon.

Catalytic Hydrogenation

Catalytic hydrogenation is a crucial reaction in organic chemistry that helps transform unsaturated compounds into saturated ones. The process involves the addition of hydrogen to the molecule in the presence of a catalyst, such as platinum, palladium, or nickel.
In the given hydrocarbon problem, the compound \(\mathrm{C}_{6} \mathrm{H}_{10}\) adsorbs 2 moles of hydrogen to form 2-methylpentane. This indicates that the original hydrocarbon had unsaturated bonds, which could be either double or triple bonds. Through hydrogenation, these double or triple bonds are converted into single bonds, resulting in a saturated hydrocarbon structure.

• The catalyst used facilitates the reaction, lowering the activation energy and allowing hydrogen to add across the multiple bonds effectively.
• Analyzing the product of hydrogenation helps deduce the presence and type of bonds in the original molecule even in complex hydrocarbons.

Students should note how hydrogenation reactions can simplify predicting the structure of an unknown hydrocarbon by revealing changes in structure through saturation.

Chemical Reaction Mechanisms

Chemical reaction mechanisms provide a pathway for understanding how reactions progress from reactants to products. In the context of the given exercise, analyzing mechanisms can simplify deducing unknown structures.
The addition of water to the hydrocarbon using \( \text{Hg}^{2+} / \text{H}_{2} \text{SO}_{4} \) suggests the presence of a double bond. Such a reaction typically involves the conversion of the double bond to an alcohol group via a hydration reaction.

• The mechanism likely involves an intermediate state where the double bond is attacked by the catalyst, leading to the addition of a water molecule.
• This addition results in the formation of an alcohol, modifying the structure and indicating the presence of a double bond initially.

Understanding reaction mechanisms requires knowing the types of intermediates formed and the stability they offer during the progression of the reaction. Hence, mechanisms can elucidate the original structures of hydrocarbons and provide insight into the specific reactions that can be performed on them.