Question

Write the chemical equation for the preparation of acetaldehyde and write a note on Aldol condensation and cross-Aldol condensation.

Hint:

Aldol condensation is widely used in organic synthesis for the formation of carbon-carbon bonds, especially in the preparation of conjugated α,β-unsaturated carbonyl compounds.

Answer 1

Preparation of Acetaldehyde:
Acetaldehyde can be prepared by the reduction of acetic acid using a reducing agent like lithium aluminium hydride (LiAlH₄): \[ \text{CH}_3\text{COOH} + \text{LiAlH}_4 \rightarrow \text{CH}_3\text{CHO} + \text{LiAlO}_2 \] Acetaldehyde can also be prepared by the catalytic dehydrogenation of ethanol: \[ \text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{Cu, 300°C}} \text{CH}_3\text{CHO} + \text{H}_2 \] Aldol Condensation:
Aldol condensation involves the reaction between two molecules of an aldehyde or ketone under basic conditions to form a β-hydroxy aldehyde or ketone, which can then undergo dehydration to form an α,β-unsaturated carbonyl compound. For example, the condensation of acetaldehyde (ethanal): \[ 2 \text{CH}_3\text{CHO} \xrightarrow{\text{NaOH}} \text{CH}_3\text{CH(OH)CH=CH}_2 \] On heating, the β-hydroxy aldehyde undergoes dehydration to form crotonaldehyde: \[ \text{CH}_3\text{CH(OH)CH=CH}_2 \xrightarrow{\Delta} \text{CH}_3\text{CH=CHCHO} \] Cross-Aldol Condensation:
In cross-Aldol condensation, two different aldehydes or ketones react in the presence of a base to form a mixture of products, resulting in the formation of α,β-unsaturated carbonyl compounds. For example, the reaction between acetaldehyde and benzaldehyde: \[ \text{CH}_3\text{CHO} + \text{C}_6\text{H}_5\text{CHO} \xrightarrow{\text{NaOH}} \text{CH}_3\text{CH(OH)CH=CHC}_6\text{H}_5 \] Upon heating, this β-hydroxy product undergoes dehydration to form the α,β-unsaturated compound: \[ \text{CH}_3\text{CH(OH)CH=CHC}_6\text{H}_5 \xrightarrow{\Delta} \text{CH}_3\text{CH=CHC}_6\text{H}_5 \] Final Answer: Acetaldehyde can be prepared by the reduction of acetic acid or by dehydrogenation of ethanol. Aldol condensation leads to the formation of β-hydroxy aldehydes or ketones, which can undergo dehydration to form α,β-unsaturated carbonyl compounds. Cross-Aldol condensation involves two different aldehydes or ketones and results in a mixture of products. Correct Answer: The chemical equations for the preparation of acetaldehyde and the steps for Aldol and Cross-Aldol condensation are shown above.