Question

When ethanal and propanal undergo aldol condensation with dilute NaOH, the pair of products obtained by cross condensation reaction is

• A. but-2-enal and 2-methylpent-2-enal
• B. but-2-enal and pent-2-enal
• C. 2-methylbut-2-enal and pent-2-enal
• D. 2-methylbut-2-enal and 2-methylpent-2-enal

Hint:

Cross aldol of ethanal and propanal gives 2-methylbut-2-enal and pent-2-enal.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
In a cross aldol condensation, each aldehyde with alpha hydrogens can act as the enolate (nucleophile) and attack the other aldehyde. Four products form: two self-condensation and two cross products. We need the two cross products.

Step 2: Cross product 1:
Enolate of propanal attacks ethanal: \(\text{CH}_3\text{CH}_2\text{CHO}\) gives \(\text{CH}_3\text{CH}^-\text{CHO}\), which adds to \(\text{CH}_3\text{CHO}\) to give \(\text{CH}_3\text{CH(OH)CH(CH}_3)\text{CHO}\). Dehydration gives \(\text{CH}_3\text{CH=C(CH}_3)\text{CHO}\), which is 2-methylbut-2-enal.

Step 3: Cross product 2:
Enolate of ethanal attacks propanal: \(\text{CH}_3\text{CH}_2\text{CH(OH)CH}_2\text{CHO}\). Dehydration gives \(\text{CH}_3\text{CH}_2\text{CH=CHCHO}\), which is pent-2-enal.

Step 4: Compare with options:
But-2-enal (A and B) is the self-condensation product of ethanal. 2-Methylpent-2-enal (A and D) is the self-condensation product of propanal. So only C lists the two cross products.

Final Answer:
The cross products are 2-methylbut-2-enal and pent-2-enal. \[ \boxed{\text{(C) }\text{2-methylbut-2-enal and pent-2-enal}} \]