Which one of the following is not formed when acetone reacts with 2-pentanone in the presence of dilute NaOH followed by heating?

When acetone (I: CH₃COCH₃) reacts with 2-pentanone (II: CH₃CH₂COCH₂CH₃) in the presence of dilute NaOH followed by heating, various aldol condensation and cross-aldol products are possible, as shown in the image.

The four structures formed from combinations of I and II are shown:

1. I + II product
2. II + I product
3. I + I product
4. II + II product

Based on the mechanisms of aldol and cross-aldol condensations, each product is formed by the reaction between different enolate and carbonyl partners.

Careful inspection of each product shows that all four are possible except for the product that would require a structure impossible by this mechanism. The structure not formed when acetone reacts with 2-pentanone in the presence of dilute NaOH followed by heating is the product derived solely from II + II (the last structure in the image):

II + II product (formed from two molecules of 2-pentanone only) is NOT formed in the cross-aldol reaction between acetone and 2-pentanone. Only cross-products (I+II, II+I) and self-condensation products involving acetone (I+I) are possible with both reactants present. The self-condensation product of 2-pentanone alone (II+II) would require only 2-pentanone without acetone present.

Answer: The product from II + II (self-condensation of 2-pentanone) is not formed in this reaction.