Ch. 24 - Benzene II: Reactions Influenced by the Aromatic Ring

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

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Mullins 1st Edition

Ch. 24 - Benzene II: Reactions Influenced by the Aromatic Ring

Problem 21

Chapter 23, Problem 21

In light of Figure 24.22, provide a mechanism by which para-dihydroxybenzene is oxidized to para-quinone.

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Identify the starting material as para-dihydroxybenzene, which has two hydroxyl groups attached to a benzene ring in the para position.

Recognize the oxidizing agent as chromic acid (H₂CrO₄), which is commonly used to oxidize alcohols to carbonyl compounds.

Understand that the mechanism involves the oxidation of the hydroxyl groups to carbonyl groups, forming para-quinone.

Propose that the first step involves the formation of a chromate ester intermediate, where the hydroxyl oxygen attacks the chromium atom, forming a bond and releasing water.

Suggest that the chromate ester undergoes a two-electron oxidation, where the C-O bond is cleaved, forming a carbonyl group and reducing the chromium species, resulting in the formation of para-quinone.

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Key Concepts

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

Oxidation-Reduction Reactions

Oxidation-reduction (redox) reactions involve the transfer of electrons between species, leading to changes in oxidation states. In the context of organic chemistry, oxidation often refers to the loss of hydrogen or the gain of oxygen, while reduction is the opposite. Understanding these processes is crucial for analyzing how para-dihydroxybenzene can be converted to para-quinone through the removal of electrons and protons.

Mechanism of Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a fundamental reaction mechanism in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. In the case of para-dihydroxybenzene, the hydroxyl groups can activate the ring towards electrophilic attack, facilitating the formation of para-quinone. Recognizing the role of substituents in directing the reaction is essential for understanding the mechanism involved.

Resonance and Stability of Intermediates

Resonance refers to the delocalization of electrons within a molecule, which can stabilize reaction intermediates. In the oxidation of para-dihydroxybenzene, the formation of a resonance-stabilized carbocation intermediate is key to the reaction pathway. Understanding how resonance affects the stability of intermediates helps predict the feasibility and outcome of the oxidation process leading to para-quinone.

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Stability of Conjugated Intermediates

Related Practice

Textbook Question

Predict the product when the dihydroxybenzene shown is treated with a single equivalent of both base and haloalkane.

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Textbook Question

Predict the product of the following substitution/addition reactions involving phenoxides.

(a)

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Textbook Question

Assessment 24.22 asked why meta-dihydroxybenzene could not be oxidized to meta-quinone. The attempted oxidation instead gives rise to three different quinone products. Suggest a mechanism for the formation of each.

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Textbook Question

Oxidation of a phenol derivative produces the lactone shown. Provide an arrow-pushing mechanism that rationalizes this outcome.

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Textbook Question

(a) Based on Figure 24.23, explain why meta-dihydroxybenzene is not oxidized to meta-quinone.

(b) If a meta-quinone is not produced, what would you expect the product of the oxidation of meta-dihydroxybenzene to be?

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Textbook Question

Predict the product of the following substitution/addition reactions involving phenoxides. [Because this problem represents a review of current and previous material, section numbers have been provided for your reference.]

(d)

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