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Ch. 23 - Benzene I: Aromatic Stability and Substitution Reactions
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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All textbooksMullins 1st EditionCh. 23 - Benzene I: Aromatic Stability and Substitution ReactionsProblem 40b
Chapter 22, Problem 40b

Predict the major product of the following electrophilic aromatic substitution reactions.
(b) Chemical structure showing a benzene ring with a sulfur substituent and a reaction arrow indicating electrophilic aromatic substitution.

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1
Identify the type of reaction: The reaction shown is an electrophilic aromatic substitution (EAS) where a sulfonation reaction is taking place. The electrophile in this reaction is the sulfur trioxide (SO₃) in the presence of sulfuric acid (H₂SO₄).
Determine the directing effects of the substituent: The substituent on the benzene ring is a thioether group (S-CH₃). This group is an ortho/para director due to the lone pair of electrons on the sulfur atom, which can donate electron density into the benzene ring through resonance.
Predict the position of substitution: Since the thioether group is an ortho/para director, the incoming electrophile (SO₃) will preferentially attack the ortho and para positions relative to the sulfur atom on the benzene ring.
Consider steric and electronic factors: The para position is often favored over the ortho position due to less steric hindrance, making it the more likely site for substitution.
Draw the major product: The major product will be the para-substituted sulfonic acid derivative, where the SO₃H group is added to the para position relative to the thioether group on the benzene 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 (EAS)

Electrophilic Aromatic Substitution is a reaction where an electrophile replaces a hydrogen atom on an aromatic ring. The reaction involves the formation of a highly reactive carbocation intermediate, known as the sigma complex, which is stabilized by resonance. The final step is the deprotonation to restore aromaticity, resulting in the substitution of the electrophile onto the ring.
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Sulfonation Reaction

Sulfonation is a type of electrophilic aromatic substitution where sulfur trioxide (SO3) acts as the electrophile, typically in the presence of sulfuric acid (H2SO4). This reaction introduces a sulfonic acid group (SO3H) onto the aromatic ring. The process is reversible, and the position of substitution is influenced by existing substituents on the ring, which can direct the incoming group to ortho, meta, or para positions.
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Directing Effects of Substituents

Substituents on an aromatic ring can influence the position of incoming electrophiles during EAS reactions. Electron-donating groups, like alkyl or methoxy groups, typically direct electrophiles to ortho and para positions, while electron-withdrawing groups, such as nitro or carbonyl groups, direct them to the meta position. The sulfur substituent in the given structure is an electron-donating group, which will direct the sulfonation to the ortho and para positions.
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Related Practice
Textbook Question

Beginning with benzene, synthesize the following substituted benzenes. The ideal number of steps is indicated.

(d) 

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Textbook Question

Predict the major product of the following electrophilic aromatic substitution reactions.

(c)

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Textbook Question

With a small electron-donating group on the ring, it is possible to get as much as a 2:1 ratio of ortho to para. Why might this be?

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Textbook Question

Predict the products of the following electrophilic aromatic substitution reactions.

(b)

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Textbook Question

Predict the product of the following Friedel–Crafts acylation reactions.

(b)

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Textbook Question

In Section 23.7.4, we learned that Friedel–Crafts alkylation suffers from overalkylation.

(a) Draw the product of a Friedel–Crafts reaction that resulted in three methyl groups adding to the ring.

(b) Why is hard to stop at the addition of one alkyl group?