Question

What is the number of different products formed when a mixture of ethanal and propanal reacts with aq. NaOH after warming?

• A. 3
• B. 4
• C. 2
• D. 1

Answer 1

Answer: (A)

3

Answer 2

We analyze the aldol reaction when a mixture of ethanal (CH₃CHO) and propanal (CH₃CH₂CHO) is treated with aqueous NaOH and warmed. Since both aldehydes have α–hydrogens, they can form enolates and react in three different ways:

1. Self–condensation of ethanal: Two molecules of ethanal condense to give (after dehydration) 2–butenal (crotonaldehyde).

2. Self–condensation of propanal: Two molecules of propanal condense to give (after dehydration) 2–pentenal.

3. Cross–aldol reaction: One molecule’s enolate reacts with the carbonyl of the other. (Although there are two ways to form a cross product, they lead to the same product.)

   This gives a unique cross–aldol product (after dehydration).

Thus, there are three different products.

Core Explanation:

• Both ethanal and propanal can undergo aldol condensation (self and cross).
• Self condensation of ethanal → 2–butenal.
• Self condensation of propanal → 2–pentenal.
• Cross aldol reaction gives one unique product.