Problem 220 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is reduced with zinc and hydrochloric acid to give the corresponding hydrocarbon? (a) ethyl acetate (b) acetic acid (c) acetamide (d) butan-2-one

Step-by-Step Solution

Answer

Option (d) butan-2-one.

Step 1: Understanding the Reaction

The question asks which compound can be reduced with zinc and hydrochloric acid to form a hydrocarbon. Zinc and hydrochloric acid is often used to provide nascent hydrogen, which is a reducing agent capable of converting certain functional groups into hydrocarbons.

Step 2: Identifying Suitable Functional Groups for Reduction

Zinc and hydrochloric acid, as a reducing agent, will be effective only on certain functional groups like carbonyl compounds. Therefore, compounds with carbon-oxygen double bonds are potential candidates. Among the options, we need to identify which have this feature.

Step 3: Analyzing Each Compound

- Ethyl acetate (option a) is an ester: RCOOR'. It does have a carbon-oxygen double bond, but esters are not typically reduced to hydrocarbons by simple reagents like zinc/HCl. - Acetic acid (option b) is a carboxylic acid: RCOOH. Similar to esters, carboxylic acids are also not typically reduced to hydrocarbons by zinc/HCl. - Acetamide (option c) is an amide: RCONH2. Amides are less reactive and not reduced to hydrocarbons by zinc/HCl. - Butan-2-one (option d) is a ketone: RCOR'. Ketones contain a carbonyl group that can indeed be reduced to hydrocarbons by zinc/HCl.

Step 4: Choosing the Correct Option

Since butan-2-one is a ketone and has a carbonyl group that can be reduced to a hydrocarbon, option (d) butan-2-one is the correct choice.

Key Concepts

Functional Groups

Functional groups are specific groups of atoms within molecules that determine the characteristic chemical reactions of those molecules. They are the "functional" parts that participate in chemical reactions while the rest of the molecule stays largely intact.

Here are some common functional groups you may encounter:

• Hydroxyl group (-OH): Found in alcohols, makes the molecule polar and soluble in water.
• Carbonyl group (C=O): Present in aldehydes and ketones, known for participating in a variety of reactions.
• Carboxyl group (-COOH): Found in carboxylic acids, can donate a hydrogen ion (H+), rendering the molecule acidic.
• Amino group (-NH2): Characteristic of amines and amides, makes the molecule basic and increases water solubility.
• Ester group (-COOR'): Typically found in esters, formed by the reaction between carboxylic acids and alcohols.

Functional groups play a key role in organic chemistry as they mostly define the reactivity and properties of organic compounds. Recognizing functional groups helps predict the behavior and reactions of the molecules in which they are present.

Carbonyl Compounds

Carbonyl compounds are organic molecules that feature a carbonyl group (C=O), which consists of a carbon atom double-bonded to an oxygen atom. This group is highly reactive due to the polar nature of the carbon-oxygen double bond, making these compounds versatile in chemical reactions.

Carbonyl compounds are broadly classified into several types based on what is attached to the carbon of the carbonyl group:

• Aldehydes: Have at least one hydrogen attached to the carbonyl carbon (RCHO).
• Ketones: The carbonyl carbon is bonded to two carbon atoms (RCOR').
• Carboxylic Acids: Contain an -OH group attached to the carbonyl, making them acidic (RCOOH).
• Esters: Result from the reaction of an alcohol with a carboxylic acid, with a -COOR' group.
• Amides: The carbonyl carbon is bonded to a nitrogen (RCONH2), typical in peptides.

Among these, ketones and aldehydes are particularly susceptible to reduction reactions because of their relatively simple structure and high reactivity. The exercise specifically looks at ketones, such as butan-2-one, which can be reduced to hydrocarbons using reducing agents like zinc and hydrochloric acid (Zn/HCl).

Reducing Agents

Reducing agents are substances that cause reduction by donating electrons to another substance in a chemical reaction. In doing so, the reducing agent itself gets oxidized.

Important points about reducing agents:

• They are typically rich in electrons and have a strong tendency to donate them.
• They play a crucial role in redox reactions, where oxidation and reduction occur simultaneously.
• Common examples include hydrogen gas ( H_2), metals like zinc (Zn) and iron (Fe), and compounds like sodium borohydride (NaBH4) and lithium aluminum hydride (LiAlH4).
• In the exercise, zinc and hydrochloric acid provide nascent hydrogen, which acts as the reducing agent that converts carbonyl compounds into hydrocarbons.

Reducing agents are essential in industrial and laboratory settings for processes like metal extraction and organic synthesis. In the specific exercise context, a ketone such as butan-2-one can be reduced to its corresponding hydrocarbon using zinc and hydrochloric acid as the reducing agents, illustrating a practical utilization of reduction reactions in chemistry.