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 23

Chapter 23, Problem 23

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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Identify the starting material as meta-dihydroxybenzene, also known as resorcinol, which has hydroxyl groups at the 1 and 3 positions on the benzene ring.

Recognize that the oxidizing agent used is chromic acid (H2CrO4), which is a strong oxidizing agent capable of converting phenols to quinones.

Understand that the oxidation of resorcinol does not lead to a single meta-quinone product due to the instability of such a structure. Instead, the reaction proceeds through different pathways leading to multiple products.

Propose a mechanism for the formation of the first product: The hydroxyl group at the 1-position is oxidized to a carbonyl group, forming a para-quinone structure with the remaining hydroxyl group at the 3-position.

Suggest mechanisms for the formation of the other two products: The hydroxyl group at the 3-position is oxidized to a carbonyl group, forming an ortho-quinone structure with the remaining hydroxyl group at the 1-position. Additionally, a rearrangement or further oxidation could lead to the formation of a third quinone structure with different positioning of the carbonyl groups.

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Key Concepts

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

Dihydroxybenzene Structure

Dihydroxybenzene, also known as catechol, has two hydroxyl (-OH) groups attached to a benzene ring. The position of these groups significantly affects the compound's reactivity. In meta-dihydroxybenzene, the hydroxyl groups are located at the 1,3-positions, which influences the electron distribution and steric hindrance during oxidation reactions.

Oxidation Mechanism

Oxidation in organic chemistry typically involves the loss of electrons or an increase in oxidation state. In the case of meta-dihydroxybenzene, the oxidation process can lead to the formation of quinones, which are cyclic compounds with a conjugated double bond system. Understanding the specific pathways and intermediates formed during oxidation is crucial for predicting the products.

Quinone Formation

Quinones are a class of cyclic organic compounds characterized by a six-membered ring with two carbonyl groups. The formation of different quinone products from meta-dihydroxybenzene can occur through various mechanisms, including rearrangements and the involvement of radical intermediates. Each pathway can yield distinct quinone structures based on the positions of the hydroxyl groups and the reaction conditions.

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Oxidation of Phenols to Quinones Concept 1

Related Practice

Textbook Question

Suggest reagents and reaction conditions that would result in synthesis of the following bromoalkanes.

(c)

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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

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

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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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