Problem 106 - A Complete Resource Book in Chemistry for JEE Main

Question

In the reaction $$ \mathrm{CH}_{3} \mathrm{CHO}+\mathrm{CH}_{2}(\mathrm{COOH})_{2} \stackrel{\Delta, \text { Pyridine }}{\longrightarrow} \mathrm{A} $$ The compound \(\mathrm{A}\) is (a) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCOOH}\) (b) \(\mathrm{COOHCH}=\mathrm{CHCOOH}\) (c) \(\mathrm{CH}_{3} \mathrm{COOH}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{COOH}\)

Step-by-Step Solution

Answer

The compound \( \mathrm{A} \) is option (a) \( \mathrm{CH}_{3} \mathrm{CH} = \mathrm{CHCOOH} \).

Step 1: Identify Reactants

The given reactants are \( \mathrm{CH}_{3} \mathrm{CHO} \), which is acetaldehyde, and \( \mathrm{CH}_{2}(\mathrm{COOH})_{2} \), which is malonic acid.

Step 2: Determine Reaction Type

This reaction is typically a Knoevenagel condensation reaction involving aldehydes and malonic ester or malonic acid under basic conditions provided by pyridine and heat (\( \Delta \)).

Step 3: Condensation Mechanism

Knoevenagel condensation generally involves the removal of a molecule of water, forming a double bond between the alpha carbon of the aldehyde and the carbon of the carboxylic acid component of malonic acid.

Step 4: Identify Product

In the Knoevenagel condensation of \( \mathrm{CH}_{3} \mathrm{CHO} \) and \( \mathrm{CH}_{2}(\mathrm{COOH})_{2} \), we form a \( \beta \)-unsaturated acid, specifically forming \( \mathrm{CH}_{3} \mathrm{CH} = \mathrm{CHCOOH} \).

Step 5: Match Product to Options

Compare the product \( \mathrm{CH}_{3} \mathrm{CH} = \mathrm{CHCOOH} \) with the given options. The product matches option (a).

Key Concepts

beta-unsaturated acid

Beta-unsaturated acids are compounds that contain a carbon-carbon double bond (also known as an alkene) on the beta position relative to a carboxylic acid group. This particular configuration can significantly influence the chemical properties and reactivity of the compound. In the given exercise, the beta-unsaturated acid formed is known as crotonic acid, with the structure \(\text{C}_3\text{H}_5\text{COOH}\).

When forming through a Knoevenagel condensation, the reaction introduces a double bond between the alpha carbon of the carbonyl compound and the beta carbon of the malonic acid, resulting in this unsaturated structure. These acids are often precursors for further synthetic transformations due to the reactivity of their double bond.

acetaldehyde

Acetaldehyde, with the formula \(\text{CH}_3\text{CHO}\), is an organic compound featuring a carbonyl group. It is a volatile and reactive aldehyde that commonly serves as an intermediate in various synthetic processes, including the formation of beta-unsaturated acids.

In the context of Knoevenagel condensation, acetaldehyde acts as a substrate providing one of the two carbon atoms that will eventually contribute to the formation of the carbon-carbon double bond in the final product. It is chosen for its reactivity which allows it to readily participate in reactions that require the formation of new carbon-carbon bonds.

malonic acid

Malonic acid, chemically represented as \(\text{CH}_2(\text{COOH})_2\), is a dicarboxylic acid with notable nucleophilic characteristics. It is often utilized in condensation reactions due to its ability to lose a molecule of water, facilitating a new carbon-carbon bond.

In the Knoevenagel condensation process, malonic acid acts alongside acetaldehyde, contributing its methylene group to form the backbone of the resultant beta-unsaturated acid. The presence of two carboxylic acid groups makes it easier to form enolates, which play a crucial role in the reaction progression, ultimately allowing malonic acid to form unsaturated products with ease.

pyridine in organic reactions

Pyridine is commonly used in organic reactions as a base, particularly due to its ability to accept protons, thus enhancing reaction conditions without interfering with the reactive sites of the main substrate.

In reactions like Knoevenagel condensation, pyridine acts to deprotonate the acidic methylene group of malonic acid, facilitating the formation of an enolate ion. This enolate is nucleophilic in nature and readily reacts with the electrophilic acetaldehyde to form the double bond characteristic of beta-unsaturated acids. Utilizing pyridine is advantageous as it offers mild basic conditions, avoiding overly aggressive reactions that could lead to unwanted side reactions or products.