Chapter 45, Problem 1Q

Interpretation Introduction

Interpretation: The product formed after the crossed-aldol condensation between 2-nitrobenzaldehyde and acetone in the first step for synthesis of indigo needs to be determined.

Concept Introduction:In crossed-aldol condensation, two different carbonyl compounds (both with alpha hydrogen atom) undergo condensation reaction together. There is a possibility of 4 products in such type of condensation reaction.

Explanation of Solution

The formation of indigo is represented as follows:

The first step of the reaction is crossed aldol condensation. The given reactants are 2-nitrobenzaldehyde and acetone or propan-2-one. Both of the reactantshasa carbonyl group thus, both are carbonyl compounds. Since, both the carbonyl compounds are different thus, crossed aldol condensation is possible.

The structure of reactants is represented as follows:

Here, the removal of alpha hydrogen from 2-nitrobenzaldehyde is not possible because the negative charge on the double bond is not stable. Thus, the hydroxide ion of water removes the alpha hydrogen of acetone or propan-2-one.

The mechanism for the crossed aldol condensation will be as follows:

Now, the negative charge will attack the carbonyl group of 2-nitrobenzaldehyde as follows and results in the formation of a hydroxy ketone.

Thus, the product formed in the first step will be as follows:

After this series of condensations, tautomerizations, a retro Claisen condensation and elimination reactions result in the formation of indigo.