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

Question

Match the following: List I 1\. \(\mathrm{RCHO}+2 \mathrm{R}^{\prime} \mathrm{OH} \quad\) dry \(\mathrm{HCl}\) 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: 1 \(\begin{array}{lll}2 & 3 & 4\end{array}\) (a) (iv) (iii) (ii) (i) (b) (iv) (iii) (i) (ii) (c) (iii) (iv) (ii) (i) (d) (i) (ii) (iii) (iv)

Step-by-Step Solution

Answer

The correct match is option (b): 1-(iv), 2-(iii), 3-(ii), 4-(i).

Step 1: Reaction Analysis for Match 1

The compound in List I labeled as 1: \( \mathrm{RCHO}+2 \mathrm{R}^{\prime} \mathrm{OH} \quad \) in the presence of dry \( \mathrm{HCl} \) gas refers to an acetal formation reaction. When an aldehyde reacts with excess alcohol, especially in the presence of an acid catalyst like HCl gas, an acetal is typically formed. Hence, 1 matches with (iv).

Step 2: Reaction Analysis for Match 2

The compound labeled as 2: \( \mathrm{R}_{2} \mathrm{C}=\mathrm{O}+\mathrm{R}^{\prime} \mathrm{NH}_{2} \) describes the formation of a Schiff's base. When a ketone or aldehyde reacts with a primary amine, a Schiff's base (an imine) is formed. Thus, 2 matches with (iii).

Step 3: Reaction Analysis for Match 3

The compound labeled as 3: \( \mathrm{RCH}=\mathrm{O} \) (or \( \mathrm{R}_{2} \mathrm{C}=\mathrm{O} \)) + Phenyl hydrazine \( \mathrm{H}^{+} \) typically results in forming a phenyl hydrazone. The reaction of an aldehyde or ketone with phenyl hydrazine leads to a phenyl hydrazone. So, 3 matches with (ii).

Step 4: Reaction Analysis for Match 4

In the compound labeled as 4: \( \mathrm{PhCHO}+\mathrm{NH}_{2} \mathrm{OH} \) \( \mathrm{H}^{+} \), benzaldehyde reacts with hydroxylamine to form an oxime, particularly a benzaldehyde oxime. Therefore, 4 matches with (i).

Step 5: Compile Results and Select Answer Choice

Combining the findings from each step, we have the following matches: 1-(iv), 2-(iii), 3-(ii), 4-(i). Therefore, the correct answer is option (b).

Key Concepts

Acetal Formation

Acetal formation is a key reaction in organic chemistry, often used to protect aldehyde and ketone groups during synthetic procedures. In this reaction, an aldehyde (\( \text{RCHO} \)) or a ketone is treated with an excess of alcohol (\( \text{2 R}^{\prime}\text{OH} \)) in the presence of an acid catalyst, typically dry HCl gas. The purpose of the acid is to protonate the carbonyl oxygen, making the carbonyl carbon more electrophilic. The reaction begins with the formation of a hemiacetal intermediary, through the nucleophilic addition of the alcohol to the carbonyl carbon. The hemiacetal then reacts with another molecule of alcohol to form the acetal. Acetals are stable to bases and nucleophiles, making them useful as protective groups in multi-step synthesis. Key points about this process include:

• This reaction is reversible; to reverse it, often called hydrolysis, water is used under acidic conditions to convert the acetal back to the original carbonyl compound.
• Acetals are not typically formed from ketones because they are less reactive than aldehydes.

Schiff's Base Formation

Schiff's bases are important intermediates in organic synthesis and biochemical applications. They form through the condensation reaction of primary amines (\( \text{R}^{\prime}\text{NH}_2 \)) and carbonyl compounds such as aldehydes or ketones (\(\text{R}_2\text{C}=\text{O}\)). This reaction results in the formation of an imine linkage, often called a Schiff base. Here's more about the process:

• The nitrogen atom of the primary amine attacks the electrophilic carbonyl carbon, forming a tetrahedral intermediate.
• This intermediate then loses a molecule of water, which results in the formation of a double bond between the nitrogen and the former carbonyl carbon.
• Schiff's bases are characterized by their bright colors and are often used as dyes or pigments.

One of the key features of this reaction is that it is fully reversible, meaning that under certain conditions, a Schiff's base can be converted back into the original carbonyl compound and amine. The equilibrium of this reaction can be shifted in favor of the Schiff's base by removing water from the reaction.

Phenyl Hydrazone Synthesis

Phenyl hydrazones are derivatives of hydrazine where one of the hydrogens is replaced by a phenyl group. This reaction involves the condensation of carbonyl compounds, such as aldehydes or ketones (\(\text{RCH}= ext{O}\) or \(\text{R}_2 ext{C}= ext{O}\)), with phenyl hydrazines in the presence of acid (\(\text{H}^+\)). Steps of the reaction include:

• The carbonyl carbon is attacked by the nitrogen of phenyl hydrazine, creating a tetrahedral intermediate.
• After proton transfer and dehydration, the stable phenyl hydrazone is formed with a double bond joining carbon and nitrogen.

Phenyl hydrazone formations are extremely useful in identifying carbonyl compounds due to their sharp melting and boiling points, which are often employed in qualitative analysis. Furthermore, their formation is an important tool for the characterization of sugars in carbohydrate chemistry.

Benzaldehyde Oxime Formation

Formation of benzaldehyde oximes is a quintessential reaction in the field of organic chemistry, emphasizing the synthesis of oximes. These reactions involve the reaction of aldehydes or ketones, like benzaldehyde (\( \text{PhCHO} \)), with hydroxylamine (\( \text{NH}_2\text{OH} \)) under acidic conditions (\( \text{H}^+ \)). The reaction steps are straightforward but crucial:

• The mechanism starts with the nucleophilic attack of the hydroxylamine nitrogen onto the carbonyl carbon, resulting in a tetrahedral intermediate.
• This intermediate proceeds through dehydration, which establishes a stable carbon-nitrogen double bond, forming the oxime.

Oximes like benzaldehyde oxime have several useful applications, especially in pharmaceuticals, as well as in the purification and analysis of carbonyl compounds. They can also serve as intermediates in the synthesis of other nitrogen-containing compounds. The formation of oximes, therefore, not only provides an important method for identifying carbonyl groups but also opens pathways for further chemical modifications.