Problem 44 - A Complete Resource Book in Chemistry for JEE Main

Question

The reagent used for the separation of acetaldehyde from acetophenone is (a) \(\mathrm{NH}_{2} \mathrm{OH}\) (b) \(\mathrm{NaHSO}_{3}\) (c) \(\mathrm{NaO} \mathrm{H} / \mathrm{I}_{2}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NHNH}_{2}\)

Step-by-Step Solution

Answer

The reagent used is \\(NaHSO_{3}\\).

Step 1: Understanding the Functional Groups

Acetaldehyde is an aldehyde, while acetophenone is a ketone. Aldehydes and ketones have distinct chemical properties due to the nature of their carbonyl groups.

Step 2: Identifying Suitable Reagents

To separate acetaldehyde from acetophenone, we need a reagent that can selectively react with the carbonyl group in acetaldehyde. This involves converting acetaldehyde into a non-carbonyl form while leaving acetophenone as is.

Step 3: Analyzing Reagent Reactivity

Analyze each reagent's reactivity:- \(NH_{2}OH\) forms oximes with aldehydes and ketones, but is not selective.- \(NaHSO_{3}\) forms bisulfite addition compounds selectively with aldehydes.- \(NaOH / I_{2}\) is used for haloform reactions primarily with methyl ketones.- \(C_{6}H_{5}NHNH_{2}\) (phenylhydrazine) forms hydrazones with carbonyl compounds.

Step 4: Choosing the Correct Reagent

Among the options, \(NaHSO_{3}\) is known for forming addition compounds specifically with aldehydes, which would selectively target acetaldehyde over acetophenone.

Key Concepts

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. In organic chemistry, understanding functional groups like aldehydes and ketones is key to predicting how substances will behave in a reaction.

Acetaldehyde contains the functional group known as an aldehyde, which has the structure \(-CHO\). In contrast, acetophenone contains a ketone group, represented by \(-CO-\). Both aldehydes and ketones include a carbonyl group, a carbon atom double-bonded to an oxygen atom, but their reactivity differs significantly.

This difference in reactivity hinges on the positioning of the carbonyl group: on an end carbon in aldehydes, and between two carbons in ketones. This affects their reactivity with nucleophiles and other reagents.

Reagent Selection

Reagent selection in organic synthesis is crucial because the right choice can lead to a successful separation or transformation of compounds. In our exercise, we needed to separate acetaldehyde from acetophenone by using a selective reagent.

When choosing a reagent, consider what specific transformations are needed. Not all reagents are selective; some may react with multiple functional groups indiscriminately. This can be a problem when trying to separate substances.

The correct reagent here - sodium bisulfite \(\mathrm{NaHSO}_{3}\) - is known to form addition compounds with aldehydes specifically, converting acetaldehyde into a non-carbonyl compound but not reacting with the ketone group in acetophenone. This makes \(\mathrm{NaHSO}_{3}\) the best choice for this separation task.

Carbonyl Compounds

Carbonyl compounds are a class of organic compounds characterized by the presence of a carbonyl group. These compounds, including aldehydes and ketones, play a crucial role in organic reactions.

In aldehydes, the carbonyl carbon is bonded to at least one hydrogen, while in ketones, it's flanked by other carbon atoms. The presence of the carbonyl group makes these compounds susceptible to nucleophilic addition reactions, where a nucleophile attacks the electrophilic carbon in the carbonyl group.

The differences in substitution (hydrogen vs carbon groups) in aldehydes and ketones affect their reactivity and the types of reagents they react with. Aldehydes are generally more reactive because they have less steric hindrance and electron donation to the carbonyl carbon than ketones. This is why sodium bisulfite can selectively react with acetaldehyde in the exercise. Understanding these subtle differences in carbonyl chemistry is key to mastering organic reactions.