Question

The dehydration of aldol product of two acetaldehyde molecules through Elcb mechanism affords formation of

• A. 1-Butenal
• B. 2-Butenal
• C. 3-Butenal
• D. 4-Butenal

Answer 1

The Correct Option is B

The question involves understanding the aldol condensation and subsequent dehydration reactions of acetaldehyde, followed by the Elcb (Elimination conjugate base) mechanism. Let's break down the solution step by step:

Step 1: Aldol Addition Reaction

When two molecules of acetaldehyde (CH₃CHO) undergo aldol addition, they form β-hydroxybutanal. This occurs through the enolate ion of one molecule of acetaldehyde attacking the carbonyl carbon of another molecule.

The reaction can be represented as:

\(2\text{ CH}_3 \text{CHO} \rightarrow \text{CH}_3\text{CH(OH)CH}_2\text{CHO}\)

Step 2: Dehydration via Elcb Mechanism 

The β-hydroxybutanal undergoes dehydration via the Elcb mechanism. In this process, the hydroxyl group is converted to a water molecule and released, forming a double bond between the α and β carbon atoms.

The resulting dehydrated product is an α,β-unsaturated aldehyde, specifically 2-Butenal (also known as Crotonaldehyde). The elimination reaction occurs as follows:

\(\text{CH}_3\text{CH(OH)CH}_2\text{CHO} → \text{CH}_3\text{CH=CHCHO} + \text{H}_2\text{O}\)

Step 3: Identifying the Correct Option

Among the provided options, 2-Butenal corresponds to the product formed via this dehydration process. The numbering of carbon atoms in 2-Butenal is done from the aldehyde functional group as position 1, confirming that the double bond appears between the second and third carbon atoms, hence the name 2-Butenal.

Conclusion

Based on the mechanism and product formation explained, the correct answer to the question is 2-Butenal.