Problem 188 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following: List I 1\. RCHO \(+\) 2R'OH dry HCl gas 2\. \(\mathrm{R}_{2} \mathrm{C}=\mathrm{O}+\mathrm{R}^{\prime} \mathrm{NH}_{2}\) 3\. \(\mathrm{RCH}=\mathrm{O}\) (or \(\mathrm{R}_{2} \mathrm{C}=\mathrm{O}\) ) \(+\) Phenyl hydrazine \(\mathrm{H}^{+}\) 4\. \(\mathrm{PhCHO}+\mathrm{NH}_{2} \mathrm{OH}\) \(\mathrm{H}^{+}\) List II (i) Benzaldehyde oxime (ii) A phenyl hydrazone (iii) A Schiff's base (iv) An acetal The correct matching is:

Step-by-Step Solution

Answer

1-(iv), 2-(iii), 3-(ii), 4-(i).

Step 1: Identify Reaction Types

Understand the type of reaction each compound in List I undergoes. For example, RCHO \(+\) 2R'OH in the presence of dry HCl is typical for acetal formation.

Step 2: Match Reaction 1

RCHO \(+\) 2R'OH dry HCl gas results in the formation of an acetal. Therefore, match Reaction 1 from List I to option (iv) in List II.

Step 3: Match Reaction 2

\(\mathrm{R}_{2}\mathrm{C}=\mathrm{O}+ \mathrm{R}^{\prime}\mathrm{NH}_{2}\) forms a Schiff's base. Hence, match Reaction 2 to option (iii) in List II.

Step 4: Match Reaction 3

\(\mathrm{RCH}=\mathrm{O} (\text{or} \mathrm{R}_{2}\mathrm{C}=\mathrm{O}) + \text{Phenyl hydrazine} \mathrm{H}^{+}\) leads to the formation of a phenyl hydrazone. So, match Reaction 3 to option (ii) in List II.

Step 5: Match Reaction 4

\(\mathrm{PhCHO}+\mathrm{NH}_{2}\mathrm{OH} \mathrm{H}^{+}\) forms benzaldehyde oxime. Therefore, match Reaction 4 to option (i) in List II.

Key Concepts

Functional Group Chemistry

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. They play a crucial role in organic chemistry by dictating the types of chemical reactions an organic compound can undergo. For example, the presence of an aldehyde group \((\text{RCHO})\) allows a compound to form acetals in the presence of alcohols and an acid catalyst like dry HCl gas.
This type of knowledge helps chemists predict how different molecules might react with each other under certain conditions.
Understanding functional groups is essential when exploring complex organic chemistry reactions and synthesis.

Chemical Reactions

In organic chemistry, chemical reactions involve the transformation of reactants into products through breaking and forming chemical bonds.
For instance, when an aldehyde reacts with an amine \((\text{R}'\text{NH}_2)\), it forms a Schiff's base through a condensation reaction, where a molecule of water is released. Each reaction has its specific requirements, catalysts, and conditions needed for the transformation to occur efficiently.
This fundamental understanding allows chemists to develop new compounds and understand complex pathways in organic synthesis.

Reaction Mechanisms

Reaction mechanisms are step-by-step descriptions of the process by which atoms and molecules undergo chemical transformations.
Understanding these mechanisms allows chemists to identify intermediate stages and predict the products of a reaction. For example, in forming an acetal from an aldehyde and alcohols, the aldehyde's oxygen first becomes protonated, increasing its electrophilic nature, before the alcohol nucleophilically attacks it. Step-by-step exploration of mechanisms provides insights that help fine-tune reactions and improve yields.

Acetals

Acetals are formed when aldehydes react with alcohols in the presence of an acid catalyst.
This reaction is commonly used in organic synthesis due to the stability it imparts to molecules. Acetals serve as protecting groups for carbonyl compounds because they remain stable under basic conditions but can be easily removed under acidic conditions.
The general reaction involves an aldehyde \((\text{RCHO})\) reacting with two molar equivalents of an alcohol \((2\text{R}'\text{OH})\) in the presence of a dry HCl gas, producing an acetal.

Schiff's Base

A Schiff's base is a type of imine formed when an aldehyde or ketone reacts with a primary amine \((\text{R}'\text{NH}_2)\).
These bases are characterized by the presence of a double bond between a carbon and a nitrogen atom \((\text{C}=\text{N})\). Schiff's bases are valuable intermediates in organic synthesis and have applications in the development of dyes and antibiotics.
In the reaction, a molecule of water is also produced, as it's a condensation reaction. They play important roles not only in chemistry but also in the biological realm.

Hydrazones

Hydrazones are formed when aldehydes or ketones react with hydrazines, such as phenylhydrazine, under acidic conditions.
This type of reaction is useful for characterizing carbonyl compounds, as hydrazones often have sharp melting points and distinct spectroscopic properties.
For example, when phenyl hydrazine reacts with an aldehyde \((\text{RCH}=\text{O})\) in the presence of acid, it forms a phenyl hydrazone. These compounds are also critical in medicinal chemistry and are used in various chemical screens for novel drug development.

Oximes

An oxime is a compound formed when hydroxylamine \((\text{NH}_2\text{OH})\) reacts with an aldehyde or ketone.
Oximes are noted for their role in the purification and characterization of carbonyl compounds, given their unique chemical properties and the ease with which they can be crystallized.
The reaction forms a compound featuring a \(\text{C}=\text{N}-\text{OH}\) group. For instance, benzaldehyde can form benzaldehyde oxime when it reacts with hydroxylamine under acidic conditions, giving chemists a useful derivative for analysis and study.

Aldehydes

Aldehydes are a class of organic compounds characterized by the presence of a carbonyl group \((\text{RCHO})\).
They are highly reactive and serve as pivotal intermediates in numerous chemical reactions. Aldehydes readily undergo nucleophilic addition reactions, which is why they are so versatile in organic synthesis.
In reactions like the formation of acetals, Schiff bases, hydrazones, and oximes, aldehydes react differently depending on the reagent and reaction conditions, demonstrating their importance and versatility in organic chemistry.

Amines

Amines are organic compounds derived from ammonia and contain a nitrogen atom bonded to one or more alkyl or aromatic groups.
Their basicity and ability to act as nucleophiles make them crucial in various chemical reactions. Primary amines \((\text{R}'\text{NH}_2)\) are especially important in forming Schiff's bases when they react with aldehydes or ketones.
Amines also react with different functional groups, offering pathways to synthesize a wide range of organic compounds. As essential building blocks in both synthetic and natural chemistry, they play a significant role in pharmaceuticals, dyes, and polymers.