Problem 50 - A Complete Resource Book in Chemistry for JEE Main

Question

Schiff's reagent gives pink colour with (a) acetone (b) acetaldehyde (c) acetic acid (d) methyl acetate

Step-by-Step Solution

Answer

Acetaldehyde (option b) gives a pink color with Schiff’s reagent.

Step 1: Understanding Schiff's Reagent

Schiff's reagent is a chemical test that is used to detect aldehydes. It produces a characteristic pink color when it reacts with aldehyde groups. This implies that in order for a compound to react with Schiff's reagent and give a pink color, it must contain an aldehyde group.

Step 2: Identifying Functional Groups

Look at the functional groups in the given options: (a) acetone is a ketone, (b) acetaldehyde is an aldehyde, (c) acetic acid is a carboxylic acid, and (d) methyl acetate is an ester. Since Schiff's reagent reacts specifically with aldehyde groups, compounds with these other functional groups won’t give a pink color with Schiff's reagent.

Step 3: Determining the Correct Option

Based on the functional groups, acetaldehyde (option (b)) is the only compound containing an aldehyde group. This means it will react with Schiff's reagent to produce a pink color. The other compounds do not have the aldehyde group needed to produce this reaction.

Key Concepts

Aldehydes Identification

Schiff's reagent is a useful tool when it comes to the identification of aldehydes. Aldehydes are an organic compound that contain a carbonyl group (0=H) bonded to one hydrogen atom. This specific structure is what allows aldehydes to react with Schiff's reagent, which results in a distinctive pink color. When conducting this test, if a pink hue emerges, it confirms the presence of an aldehyde group.
The identification relies on the selective nature of Schiff's reagent, which does not react with other carbonyl-containing compounds like ketones, acids, or esters. This selectivity is very useful in organic chemistry, especially when distinguishing between similar compounds. Therefore, understanding aldehyde structures and recognizing them in molecular formulas becomes crucial in accurately predicting the outcome when using Schiff's reagent.

Functional Groups in Organic Chemistry

Functional groups are specific groupings of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Each functional group has a specific structure and set of properties. In organic chemistry, understanding functional groups is crucial because they dictate how a molecule will behave chemically.
Some common functional groups include:

• Aldehydes: Characterized by the carbonyl group (0=H).
• Ketones: Feature a carbonyl group (0) bonded to two carbon atoms.
• Carboxylic Acids: Containing a carboxyl group (COOH).
• Esters: Formed from an alcohol and a carboxylic acid, typically written as RCOOR'.

In the context of Schiff's reagent, recognizing aldehyde as a functional group rather than confusing it with similar groups like ketones or carboxylic acids is essential. Correct identification ensures a more accurate application of chemical tests.

Chemical Tests for Carbonyl Compounds

Chemical tests are powerful in identifying and differentiating carbonyl-containing compounds like aldehydes and ketones. One of the most famous tests for aldehydes is using Schiff's reagent. Upon adding Schiff's reagent to a sample, an aldehyde will induce a color change to pink. This visual confirmation is straightforward and rapid, making it a preferred choice.
However, it is important to note how Schiff's reagent's lack of reaction with ketones is equally informative. Ketones contain a carbonyl group but are not as easily oxidized as aldehydes, which is why they do not react with Schiff's reagent to generate color. This non-reaction helps differentiate them from aldehydes, serving as a clear indicator of the type of carbonyl present.
Such chemical tests are vital in organic chemistry, providing quick qualitative analyses that guide further identification and study of compounds. Being aware of these tests enables chemists to utilize them effectively for precise analyses of organic substances.