Ch. 23 - Benzene I: Aromatic Stability and Substitution Reactions

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 23 - Benzene I: Aromatic Stability and Substitution Reactions

Problem 80

Chapter 22, Problem 80

Indigo is a dye that was originally isolated from coal tar. In 1905, Johann Friedrich Wilhelm Adolf von Baeyer won a Nobel Prize for a method that allowed indigo to be isolated from plants [a green chemist ahead of his time.] If you nitrated indigo using the reaction learned in this chapter, at which carbon would you expect the nitro group to attach?

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Identify the structure of indigo from the image. Indigo is composed of two indole units connected by a double bond. Each indole unit consists of a benzene ring fused to a five-membered nitrogen-containing ring.

Understand the concept of nitration. Nitration is an electrophilic aromatic substitution reaction where a nitro group (NO2) is introduced to an aromatic ring, typically using a mixture of concentrated nitric acid and sulfuric acid.

Determine the most reactive site for electrophilic substitution on the indigo molecule. In aromatic systems, the most electron-rich positions are typically the most reactive towards electrophiles. In indigo, the electron-donating effect of the nitrogen atom in the indole ring increases the electron density on the benzene ring.

Consider the directing effects of substituents. The nitrogen atom in the indole ring is an ortho/para director, meaning it will direct the incoming nitro group to the ortho or para positions relative to itself. However, due to steric hindrance and resonance stabilization, the para position (relative to the nitrogen) is often more favorable.

Predict the position of nitration. Based on the structure of indigo and the directing effects of the nitrogen atom, the nitro group is most likely to attach at the para position of the benzene ring in the indole unit, which is the position opposite to the nitrogen atom in the five-membered ring.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. In the case of nitration, the nitronium ion (NO2+) acts as the electrophile. Understanding EAS is crucial for predicting where the nitro group will attach on indigo, as the stability of the resulting intermediates and the directing effects of substituents influence the reaction outcome.

Aromaticity and Stability

Aromatic compounds, like indigo, exhibit unique stability due to their cyclic, planar structure and delocalized π-electrons, which follow Huckel's rule (4n + 2 π-electrons). This stability affects the reactivity of the compound during electrophilic substitution reactions. The position of substituents on the aromatic ring can either enhance or diminish this stability, guiding where new groups, such as nitro groups, will preferentially attach.

Ortho/Para vs. Meta Directing Effects

In electrophilic aromatic substitution, substituents on the aromatic ring can be classified as ortho/para-directing or meta-directing based on their electronic effects. Electron-donating groups typically direct incoming electrophiles to the ortho and para positions, while electron-withdrawing groups direct them to the meta position. Understanding these directing effects is essential for predicting the site of nitration on indigo, which has existing substituents that influence the reaction.

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Related Practice

Textbook Question

Fluoxetine, an antidepressant, is better known as Prozac®. Suggest reagents that could be used to make the indicated C–O bond of fluoxetine. [Note that CF₃, an electron-withdrawing group, is para to where the new bond will be formed.]

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Textbook Question

Protonation of aniline slows electrophilic aromatic substitution and directs electrophiles to the meta position. Why?

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Textbook Question

Would you expect chlorination to occur ortho, para, or meta to the pyridinium ion in the following molecule? Explain your answer.

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Textbook Question

Suggest a synthesis of each of the molecules shown beginning with benzene.

(c)

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Textbook Question

Bromination of phenol or aniline does not require the use of a Lewis acid catalyst and often results in trihalogenation. Why?

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Textbook Question

Acetohexamide is used to treat type 2 diabetes that cannot be well controlled by diet alone. Fill in the blanks of the synthetic scheme used to synthesize acetohexamide. [Hints have been provided below each step.]

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