Problem 30 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reacts with both acetaldehyde and acetone? (a) Fehling's solution (b) \(\mathrm{CH}_{3} \mathrm{MgBr}\) (c) Tollen's reagent (d) Benedict solution

Step-by-Step Solution

Answer

The answer is (b) \( \mathrm{CH}_{3} \mathrm{MgBr} \).

Step 1: Understanding the Compounds

First, let's identify the types of reactions involved. Acetaldehyde is an aldehyde, while acetone is a ketone. Different reagents will react differently based on these functional groups.

Step 2: Reagent Analysis: Fehling's Solution

Fehling's solution is a chemical reagent used to differentiate between water-soluble aldehyde and ketone functional groups. Fehling's solution reacts with aldehydes, such as acetaldehyde, giving a red precipitate but does not react with ketones like acetone.

Step 3: Reagent Analysis: Grignard Reagent ( \( \mathrm{CH}_{3} \mathrm{MgBr} \))

Grignard reagents, such as \( \mathrm{CH}_{3} \mathrm{MgBr} \), are very reactive and can react with both aldehydes and ketones to form alcohols. Therefore, \( \mathrm{CH}_{3} \mathrm{MgBr} \) can react with both acetaldehyde and acetone.

Step 4: Reagent Analysis: Tollen's Reagent

Tollen's reagent is used to test for aldehydes, producing a silver mirror when it reacts with aldehydes. It reacts only with aldehydes like acetaldehyde but not with ketones like acetone.

Step 5: Reagent Analysis: Benedict's Solution

Benedict's solution is similar to Fehling's solution and reacts with aldehydes, producing a red or green precipitate, but not with ketones. Therefore, it can only react with acetaldehyde among the given options.

Step 6: Conclusion

Based on the analysis, only the Grignard reagent, \( \mathrm{CH}_{3} \mathrm{MgBr} \), can react with both an aldehyde (acetaldehyde) and a ketone (acetone).

Key Concepts

Aldehyde Reactions

Aldehyde reactions are pivotal in organic chemistry because aldehydes have a distinctive structure that makes them highly reactive. Aldehydes feature a carbonyl group ( \( C=O \) ) bonded to at least one hydrogen atom.
This makes them susceptible to nucleophilic attack, whereby a nucleophile donates an electron pair to form a chemical bond.

• A common type of aldehyde reaction is with Grignard reagents. Here, the carbonyl carbon in the aldehyde is attacked by the nucleophilic carbon in the Grignard reagent, leading to the formation of an alcohol after protonation.
• Another is the reaction with Fehling's or Tollen's reagent, which are classical tests for aldehydes. These reagents oxidize the aldehyde to a carboxylic acid.

Interestingly, aldehydes are typically reactive enough to interact with various reagents, providing extensive versatility for synthesis in chemical reactions. This makes knowing their behavior crucial for predicting reactions.

Ketone Chemistry

Ketones are another class of carbonyl-containing compounds, where the carbonyl carbon is bonded to two alkyl groups. This configuration renders them less reactive compared to aldehydes.
Despite this, ketones participate in several important reactions.

• Like aldehydes, ketones can undergo reactions with Grignard reagents to form tertiary alcohols. The nucleophilic Grignard reagent attacks the carbonyl carbon, and after subsequent protonation, the alcohol is produced.
• However, unlike aldehydes, ketones do not react with Fehling's or Tollen's reagents. This makes them easier to differentiate from aldehydes in organic analyses.

Grasping the chemistry of ketones is essential for organic synthesis and understanding how different reagents influence the transformation of various molecules. Through their reactivity and distinct characteristics, ketones illustrate the diverse nature of carbonyl chemistry in science.

Chemical Reagents Analysis

Chemical reagents serve as tools for identifying and transforming chemical compounds. Understanding their properties and specificities is key for effective chemical manipulation.

• Fehling's solution, primarily used for distinguishing aldehydes, functions by oxidizing these compounds while reducing copper(II) ions, resulting in a red precipitate. However, it does not react with ketones.
• Grignard reagents are powerful organometallic compounds used in the formation of carbon-carbon bonds. They react with both aldehydes and ketones, significantly expanding the repertoire of synthetic transformations.
• Tollen's reagent is another test for aldehydes. It produces a silver mirror upon reaction, indicating the presence of an aldehyde but not a ketone.

Chemical reagents play a crucial role in lab settings for synthesis, identification, and analysis, sparking advancements in chemical production and research. By learning each reagent’s specifics, one can predict and guide chemical reactions more accurately.