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

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

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

Problem 55

Chapter 23, Problem 55

Rationalize the ranking of increasing reaction rate of the benzylic halides shown.

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Analyze the structure of the benzylic halides provided. Benzylic halides are compounds where a halogen atom is attached to a benzylic carbon, which is the carbon directly adjacent to a benzene ring. The reaction rate is influenced by the stability of the carbocation formed during the reaction.

Consider the mechanism of the reaction. Benzylic halides typically undergo reactions such as SN1 or SN2. In an SN1 reaction, the rate depends on the stability of the carbocation intermediate, while in an SN2 reaction, steric hindrance and the accessibility of the electrophilic carbon play a significant role.

Evaluate the substituents on the benzene ring and the benzylic carbon. Electron-donating groups (e.g., -CH3, -OCH3) on the benzene ring stabilize the carbocation intermediate through resonance, increasing the reaction rate. Electron-withdrawing groups (e.g., -NO2, -CF3) destabilize the carbocation, decreasing the reaction rate.

Assess the steric hindrance around the benzylic carbon. For SN2 reactions, bulky substituents near the benzylic carbon can slow down the reaction by making it harder for the nucleophile to attack.

Rank the benzylic halides based on the combined effects of carbocation stability (for SN1) and steric hindrance (for SN2). Consider the electronic effects of substituents and the accessibility of the benzylic carbon to rationalize the order of increasing reaction rate.

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Key Concepts

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

Benzylic Halides

Benzylic halides are organic compounds where a halogen atom is attached to a carbon atom that is directly bonded to a benzene ring. This structure allows for resonance stabilization, which can significantly influence the reactivity of the halide. The presence of the aromatic ring can stabilize the transition state during reactions, making benzylic halides generally more reactive than their aliphatic counterparts.

Reaction Mechanisms

Understanding the reaction mechanisms, particularly nucleophilic substitution (SN1 and SN2), is crucial for rationalizing reaction rates. In SN1 reactions, the rate-determining step involves the formation of a carbocation, which is more stable for benzylic halides due to resonance. In contrast, SN2 reactions involve a direct attack by the nucleophile, where steric hindrance plays a significant role in determining the reaction rate.

Carbocation Stability

Carbocation stability is a key factor in determining the reaction rates of benzylic halides. Benzylic carbocations are particularly stable due to resonance, as the positive charge can be delocalized into the aromatic ring. This stability leads to faster reaction rates in mechanisms that involve carbocation formation, such as SN1 reactions, compared to less stable carbocations formed from primary or secondary halides.

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Determining Carbocation Stability

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Textbook Question

A chemist attempted to oxidize a primary alcohol to a carboxylic acid using chromic acid. The product shown was obtained as a major component in the mixture. Suggest an arrow-pushing mechanism that accounts for its formation. [Think about what chromic acid would normally do to a phenol and make a list of bonds formed and bonds broken.]

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Textbook Question

Phenol oxidation can be coupled with other reactions to form new C―C bonds using reactions studied previously. Predict the product of the following series of reactions.

(a)

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Textbook Question

Identify the aromatic product that would result from the oxidation of the triol with an excess of PCC.

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Textbook Question

Phenol oxidation can be coupled with other reactions to form new C―C bonds using reactions studied previously. Predict the product of the following series of reactions.

(b)

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Textbook Question

Rationalize the fact that 1,4-dihydroxybenzene melts at a significantly higher temperature than 1,2-diydroxybenzene.

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Textbook Question

In light of your answers to Assessments 24.54 and 24.55, rank the following based on the rate of protonation of the alkene (1 = most basic, 6 = least basic). [Ignore the fact that the alkene may not be the most basic site in the molecule.]

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