Question

Which one of the following is reduced with \(\mathrm{Zn}-\mathrm{Hg} / \mathrm{HCl}\) to give the corresponding hydrocarbon? (A) Butan-2-one (B) Acetic acid (C) Acetamide (D) Ethyl acetate

Short answer

Butan-2-one (A) is reduced with Zn-Hg / HCl to give the corresponding hydrocarbon, as it contains a carbonyl group, which can undergo reduction. The other compounds (B, C, and D) are not effective candidates for reduction with this reagent.

Step-by-step solution

Analyze Butan-2-one

Butan-2-one is a ketone, containing a carbonyl group, which can be reduced by Zn-Hg / HCl to its corresponding hydrocarbon. Therefore, this compound is a potential candidate.

Analyze Acetic acid

Acetic acid is a carboxylic acid, and Zn-Hg / HCl is not very effective in reducing carboxylic acids to their corresponding hydrocarbons. Therefore, this compound is not likely to be reduced by Zn-Hg / HCl.

Analyze Acetamide

Acetamide is an amide, and Zn-Hg / HCl is not very effective in reducing amides to their corresponding hydrocarbons. Therefore, this compound is not likely to be reduced by Zn-Hg / HCl.

Analyze Ethyl acetate

Ethyl acetate is an ester, and Zn-Hg / HCl is not very effective in reducing esters to their corresponding hydrocarbons. Therefore, this compound is not likely to be reduced by Zn-Hg / HCl. Considering the analysis above, the correct answer is: (A) Butan-2-one

Key concepts

Zn-Hg/HCl Reduction

The reduction of organic compounds using zinc amalgam (Zn-Hg) and hydrochloric acid (HCl) is a classic method known in organic chemistry for its ability to selectively reduce certain functional groups. This reduction process is widely known as the Clemmensen reduction.

In its operational simplicity, a carbonyl compound is treated with a zinc mercury amalgam, which serves as the reducing agent, and the acidic environment provided by HCl. This causes the carbonyl group, typically found in aldehydes and ketones, to be reduced to a methylene group (-CH₂).

Why is this important for students to understand? Well, identifying the right functional group that will react is crucial in organic synthesis. For example, in a ketone like butan-2-one, the Clemmensen reduction will result in a hydrocarbon where the double-bonded oxygen of the carbonyl is completely removed, leading to butane.

Carbonyl Group Reductions

Carbonyl groups, characterized by a carbon atom double-bonded to an oxygen atom (C=O), are a central feature of various organic compounds like aldehydes, ketones, carboxylic acids, esters, and amides.

Each type of carbonyl-containing compound has different properties and reactivity, which governs how they can be reduced. For instance, while ketones and aldehydes can be readily reduced to alcohols by common reducing agents like sodium borohydride (NaBH₄), the Clemmensen reduction specifically converts them into hydrocarbons. Understanding how and why these reductions occur is a cornerstone of organic chemistry, enabling students to predict the outcomes of reactions and plan multi-step syntheses.

Moreover, this knowledge is directly applicable to problem-solving in exams like the JEE MAIN and ADVANCED, where students may be asked to identify which compounds are reducible under certain conditions, as demonstrated in the earlier textbook exercise.

Organic Reactions

Organic reactions are the backbone of organic chemistry, with a multitude of reaction types, including addition reactions, substitution reactions, elimination reactions, and more. In the context of JEE MAIN and ADVANCED, having a solid understanding of these reactions not only helps in performing laboratory syntheses but also in scoring well on the exams.

The recognition of functional groups and the predictions about their behavior is a critical skill. For example, knowing that Zn-Hg/HCl will not effectively reduce carboxylic acids, amides, or esters, as mentioned in the exercise, is a useful shortcut for students. It saves time and provides a conceptual framework for understanding which reagents to use in various scenarios.

The brilliance of organic chemistry lies in the predictability of these reactions once you grasp the underlying principles. Encouraging students to delve deep into why certain reactions occur can foster a more intuitive understanding of organic chemistry, which is far more valuable than rote memorization for exams and practical applications alike.