Question

Acetaldehyde reacts with malonic acid in presence of sodium ethoxide at high temperature to give (a) acetic acid (b) propanoic acid (c) tartaric acid (d) crotonic acid

Short answer

Answer: Propanoic acid.

Step-by-step solution

Write down the reagents and their structures

First, let's write down the structures of the reactants: acetaldehyde, malonic acid, and sodium ethoxide. - Acetaldehyde: CH3CHO - Malonic acid: HOOC-CH2-COOH - Sodium ethoxide: C2H5O-Na+

Identify the nucleophile and electrophile

In the Claisen condensation, a nucleophile (in this case, malonic acid) attacks an electrophile (in this case, acetaldehyde). The sodium ethoxide serves as a base to deprotonate the nucleophile, making it more nucleophilic.

Deprotonation of malonic acid

Malonic acid has acidic protons at its carbonyl alpha position, which can be deprotonated by sodium ethoxide: \[C_2H_5O^{-}Na^+ + HOOC-CH_2-COOH \rightarrow C_2H_5OH + NaOOC-CH^-_2-COOH\]

Nucleophilic attack on acetaldehyde

Now that the malonic acid is deprotonated, it can act as a nucleophile and attack the electrophilic carbonyl carbon of acetaldehyde: \[CH_3CHO + NaOOC-CH^-_2-COOH \rightarrow CH_3CH(O^-)CH_2-COONa\]

Protonation of the oxygen

The oxygen anion generated in the previous step then takes a proton from either water or another molecule of malonic acid. The resulting product is a beta-keto ester: \[CH_3CH(O^-)CH_2-COONa + HOOC-CH_2-COOH \rightarrow CH_3CH(OH)CH_2-COONa + HOOC-CH_2-COO^-\]

Identify the final product

Upon heating, the beta-keto ester undergoes a decarboxylation reaction, in which the carboxylic acid group linked with an alpha-keto ester is removed as carbon dioxide (CO2). The final product is propanoic acid: \[CH_3CH(OH)CH_2-COONa \rightarrow CH_3CH_2COOH + NaOH + CO_2\] Based on the reaction mechanism and the resulting product, the correct option is: (b) propanoic acid

Key concepts

Acetaldehyde

Acetaldehyde is a small organic compound with the chemical formula \( \text{CH}_3\text{CHO} \). It is a highly reactive aldehyde, which means it contains a carbonyl group (a carbon atom double-bonded to an oxygen atom).

In organic reactions, the carbon atom of the carbonyl group is an electrophile. This makes acetaldehyde particularly prone to nucleophilic attacks, especially when it encounters compounds capable of donating electrons.

For acetaldehyde in reactions like Claisen condensation, its electrophilic nature is critical. It reacts promptly with nucleophiles, contributing to the formation of new carbon-carbon bonds.

• Electrophile: The carbon atom in the carbonyl group readily attracts nucleophiles.
• Role in reactions: Central player in polymerizations and condensation reactions like the Claisen condensation.

Malonic Acid

Malonic acid, with the formula \( \text{HOOC-CH}_2\text{-COOH} \), is a dicarboxylic acid featuring two carboxyl groups attached to a central methylene group. Its structure makes it a key player in many organic reactions.

The methylene group (\( \text{-CH}_2 \)) between the two carboxyl groups holds acidic hydrogen atoms. These protons are easily removed by a base such as sodium ethoxide, generating a carbanion. This carbanion is a potent nucleophile, making it highly reactive towards electrophiles such as acetaldehyde.

When involved in reactions like Claisen condensation, the deprotonated malonic acid acts as a nucleophile to attack acetaldehyde.

• Reactant: Provides nucleophilic carbanion for the reaction.
• Reactivity: Increased by deprotonation, enabling it to form strong carbon-carbon bonds.

Beta-Keto Ester

A beta-keto ester is a fascinating product often formed in condensation reactions like Claisen condensation. It includes both a ketone and an ester group. In our context, the beta-keto ester formed contains a ketone unit that is separated from the ester group by a single carbon atom (beta position).

The formation of a beta-keto ester arrives from the nucleophilic attack of the deprotonated malonic acid on acetaldehyde, leading to a compound with a newly formed carbon-carbon bond. This bond is critical in synthesizing complex molecules. Upon its formation, the molecule undergoes protonation and is subsequently ready for decarboxylation.

• Structure: Contains an ester group and a ketone group.
• Significance: Acts as an intermediate in various organic synthesis pathways.

Decarboxylation Reaction

The decarboxylation reaction is a process where a carboxyl group (\(-\text{COOH}\)) is removed from a molecule, releasing carbon dioxide (\( \text{CO}_2 \)). In the context of the reaction we are discussing, once the beta-keto ester is formed, it undergoes decarboxylation when heated.

This reaction simplifies the compound, generally leading to the formation of a smaller molecule. Decarboxylation in our demonstrated reaction results in the conversion of the beta-keto ester to propanoic acid. This step is particularly useful in synthetic chemistry as it helps to easily remove excess carboxyl groups, aiding in the refinement of molecular structure.

• Purpose: Simplification of molecular structure by removing carboxyl groups as \( \text{CO}_2 \).
• Outcome: Generation of a smaller, more stable molecule, like propanoic acid.