Question

Identify the product 'B' in following reaction:
$2\text{CH}_3\text{CHO} \xrightarrow{\text{dil. NaOH}} \text{A} \xrightarrow[-\text{H}_2\text{O}]{\Delta} \text{B}$

• A. But-3-enal
• B. 3-Hydroxybutanal
• C. But-2-enal
• D. 4-Hydroxybutanal

Hint:

To quickly predict the final structure of any self-aldol condensation product, align the two reacting molecules together. Remove the carbonyl oxygen (O) from the first molecule and two $\alpha$-hydrogens ($\text{H}_2$) from the second molecule, then connect those positions directly with a double bond ($=\text{CH-}$).

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We need to determine the final organic compound 'B' synthesized via a sequence involving two molecules of acetaldehyde ($\text{CH}_3\text{CHO}$) reacting under basic, heated conditions.

Step 2: Key Formula or Approach:
An aldehyde containing at least one $\alpha$-hydrogen atom undergoes an

Aldol Condensation reaction when treated with a dilute base, followed by dehydration upon heating to yield an $\alpha,\beta$-unsaturated carbonyl compound.

Step 3: Detailed Explanation:

Step 1: Formation of Aldol Product 'A' In the presence of dilute NaOH, one molecule of acetaldehyde loses an $\alpha$-proton to form a carbanion, which then acts as a nucleophile and attacks the carbonyl carbon of a second acetaldehyde molecule: $$\text{CH}_3\text{CHO} + \text{CH}_3\text{CHO} \xrightarrow{\text{dil. NaOH}} \text{CH}_3\text{-CH(OH)-CH}_2\text{-CHO}$$ This $\beta$-hydroxyaldehyde intermediate is known as 3-hydroxybutanal (Product 'A').

Step 2: Elimination of Water to form Product 'B' When intermediate 'A' is heated ($\Delta$), it readily loses a molecule of water via elimination across the $\alpha$ and $\beta$ carbons to establish a stable conjugated double bond system: $$\text{CH}_3\text{-CH(OH)-CH}_2\text{-CHO} \xrightarrow{-\text{H}_2\text{O},\ \Delta} \text{CH}_3\text{-CH=CH-CHO}$$ The resulting systematic IUPAC name for this four-carbon chain containing an alkene and an aldehyde is

but-2-enal (commonly known as crotonaldehyde).

Step 4: Final Answer:
The final product 'B' is but-2-enal, which corresponds to option (C).