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.

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Definition Definition Organic compounds that have a carbonyl group, C = O, as their functional group. The carbonyl group in ketones is placed somewhere in the middle of the molecular structure, which means, the C=O is attached to two alkyl groups or benzene rings. Just like all other homologous series, the naming of ketones is done using a common suffix “-one”. The general molecular formula is C n H 2n O. Functional group of a ketone

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.

Expert Solution & Answer

Explanation of Solution

The formation of indigo is represented as follows:

EBK A SMALL SCALE APPROACH TO ORGANIC L, Chapter 45, Problem 1Q , additional homework tip 1

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:

EBK A SMALL SCALE APPROACH TO ORGANIC L, Chapter 45, Problem 1Q , additional homework tip 2

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:

EBK A SMALL SCALE APPROACH TO ORGANIC L, Chapter 45, Problem 1Q , additional homework tip 3

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

EBK A SMALL SCALE APPROACH TO ORGANIC L, Chapter 45, Problem 1Q , additional homework tip 4

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

EBK A SMALL SCALE APPROACH TO ORGANIC L, Chapter 45, Problem 1Q , additional homework tip 5

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

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