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 81

Chapter 22, Problem 81

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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Identify the functional group in the molecule: The molecule contains a pyridinium ion, which is a pyridine ring with a positively charged nitrogen atom.

Understand the electronic effects: The pyridinium ion is an electron-withdrawing group due to the positive charge on the nitrogen, which decreases electron density on the aromatic ring.

Determine the directing effects: Electron-withdrawing groups typically direct electrophilic aromatic substitution reactions to the meta position relative to the group.

Consider the reaction conditions: The presence of FeCl3 and Cl2 indicates that this is a chlorination reaction, which is an electrophilic aromatic substitution.

Predict the site of chlorination: Based on the electron-withdrawing nature of the pyridinium ion, chlorination is expected to occur at the meta position relative to the pyridinium ion.

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Key Concepts

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

Electrophilic Aromatic Substitution (EAS)

Electrophilic Aromatic Substitution is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. The reaction typically involves the formation of a sigma complex, where the aromaticity is temporarily lost, followed by deprotonation to restore aromaticity. Understanding EAS is crucial for predicting the positions where substitution will occur on the aromatic ring.

Ortho, Para, and Meta Positions

In the context of aromatic compounds, ortho, para, and meta refer to the relative positions of substituents on the benzene ring. Ortho positions are adjacent to each other, para positions are opposite each other, and meta positions are one carbon apart. The directing effects of substituents, such as electron-donating or electron-withdrawing groups, influence the likelihood of substitution occurring at these positions.

Pyridinium Ion and Its Effects

The pyridinium ion is a positively charged nitrogen-containing heterocycle that acts as an electron-withdrawing group due to the electronegativity of nitrogen. This property affects the reactivity of the aromatic ring it is attached to, typically directing electrophilic substitution to the meta position. Understanding the influence of the pyridinium ion is essential for predicting the outcome of chlorination in the given molecule.

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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

How might you use ¹³C NMR spectroscopy to differentiate between the possible ortho, meta, and para products of the electrophilic aromatic substitution reaction shown?

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Textbook Question

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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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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