Problem 49 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The hydrocarbon \(A\), adds one mole of hydrogen in the presence of a platinum catalyst to form \(n\)-hexane. When \(A\) is oxidized vigorously with \(\mathrm{KMnO}_{4}\), a single carboxylic acid, containing three carbon atoms, is isolated. Give the structure of \(A\) and explain.

Step-by-Step Solution

Answer

The structure of \(A\) is 3-hexene.

Step 1: Identify the possible starting material before hydrogenation

The problem says that hydrocarbon \(A\) is converted into \(n\)-hexane, a saturated compound, upon hydrogenation. Therefore, \(A\) must be an unsaturated hydrocarbon like an alkene or an alkyne with six carbon atoms.

Step 2: Analyze the oxidation product details

The oxidation of hydrocarbon \(A\) gives a single carboxylic acid with three carbon atoms. When an unsaturated hydrocarbon is oxidized to form one carboxylic acid, it indicates a symmetrical structure or specific arrangement that leads to one oxidation product.

Step 3: Deduce the structure of \(A\)

Since reducing \(A\) forms \(n\)-hexane, \(A\) could be a hexene or alkyne. After considering the given oxidation product, conclude that \(A\) should be 3-hexene. When oxidized, its molecule splits at the double bond forming only one type of three-carbon carboxylic acid, which matches propanoic acid.

Step 4: Verify the logic and conclusion

In terms of chemical structure and reactions: \(A\) being 3-hexene results in the formation of propanoic acid upon oxidation due to its double bond at the central position in the chain. Hydrogenation converts it seamlessly to \(n\)-hexane, confirming the consistency of reactions with the problem's description.

Key Concepts

Hydrogenation

Hydrogenation is a key chemical reaction where unsaturated hydrocarbons, such as alkenes or alkynes, are converted to saturated hydrocarbons by the addition of hydrogen. This reaction usually requires a catalyst, like platinum, to proceed efficiently. During hydrogenation:

• Hydrogen molecules are added to the carbon-carbon multiple bonds (like double bonds in alkenes or triple bonds in alkynes).
• The result is the saturation of the hydrocarbon, transforming it into an alkane.

In the context of this exercise, hydrocarbon \(A\) likely contains a carbon-carbon double bond. Upon hydrogenation, it becomes \(n\)-hexane, a fully saturated hydrocarbon. Hydrogenation is widely used in industry, for example, in the production of margarine from vegetable oils.

Oxidation

Oxidation involves the addition of oxygen or the removal of hydrogen from a molecule. In organic chemistry, it often results in the conversion of simple hydrocarbons into more complex oxygen-containing compounds like alcohols, ketones, and carboxylic acids. A powerful oxidizing agent like potassium permanganate \(\mathrm{KMnO}_{4}\) is typically employed in these reactions.

• Oxidation can cleave carbon-carbon bonds, particularly at the site of carbon-carbon double bonds.
• In this problem, the oxidation of hydrocarbon \(A\) leads to the formation of a single carboxylic acid with three carbon atoms, indicating a specific reaction path.

This suggests that \(A\) is arranged symmetrically or has a specific configuration, resulting in a single product (propanoic acid). This conclusion is crucial for inferring the structure of hydrocarbon \(A\).

Carboxylic Acids

Carboxylic acids are organic compounds characterized by the presence of a carboxyl group \((-COOH)\). They are a significant product in the oxidation of hydrocarbons, especially when strong oxidizers like \(\mathrm{KMnO}_{4}\) are used. The appearance of a single three-carbon carboxylic acid as a product suggests that oxidation cleaved \(A\) at a specific point:

• This single product clue implies a symmetrical or strategic placement of the double bond in the structure of \(A\).
• Here, propanoic acid is the oxidation product, hinting at the specific reaction path taken by the molecule \(A\).

Understanding the formation and properties of carboxylic acids helps in deducing structures and predicting reactions of organic compounds.

Alkenes

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond \((C=C)\). They serve as key starting materials in various chemical reactions, including hydrogenation and oxidation. In this exercise, the identification of \(A\) involves examining alkenes:

• Six-carbon alkenes like hexenes can take various structural forms depending on the position of the double bond.
• The unique reactivity of alkenes comes from their double bonds, which make them susceptible to reactions like hydrogenation and oxidation.

In this problem, hydrocarbon \(A\) was determined to be 3-hexene due to its ability to become \(n\)-hexane after hydrogenation and form a specific carboxylic acid post-oxidation.

Catalyst Chemistry

In chemistry, catalysts are substances that speed up a reaction without being consumed in the process. They play a crucial role in reactions like hydrogenation, where platinum is commonly used. Catalysts work by lowering the activation energy required for a reaction, allowing:

• Reactions to occur under milder conditions, reducing energy costs.
• A more efficient and faster conversion of reactants to products.

For the hydrogenation of hydrocarbon \(A\), a platinum catalyst is employed to facilitate the addition of hydrogen and convert \(A\) to \(n\)-hexane. This illustrates the effectiveness and importance of catalysts in industrial and laboratory processes.