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 43

Chapter 23, Problem 43

Beginning with benzene, synthesize the benzyl bromide shown.

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Start with benzene and perform a Friedel-Crafts alkylation using isopropyl chloride (CH3CHClCH3) and AlCl3 as a catalyst to introduce the isopropyl group onto the benzene ring, forming isopropylbenzene.

Next, perform a chlorination reaction on isopropylbenzene using Cl2 and FeCl3 to introduce a chlorine atom at the para position relative to the isopropyl group, forming para-chloroisopropylbenzene.

To introduce the bromine atom, perform a free radical bromination on the benzylic position of para-chloroisopropylbenzene using N-bromosuccinimide (NBS) and light (hv) to form para-chloro-1-bromo-2-isopropylbenzene.

Ensure that the reaction conditions are controlled to favor substitution at the benzylic position rather than the aromatic ring.

Finally, verify the structure of the synthesized compound to ensure it matches the target molecule, which is para-chloro-1-bromo-2-isopropylbenzene.

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Key Concepts

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

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring, such as benzene. This reaction is crucial for synthesizing various derivatives of benzene, including benzyl bromide. Understanding the mechanisms of EAS, including the formation of the sigma complex and the role of catalysts, is essential for predicting the outcomes of reactions involving aromatic compounds.

Bromination of Aromatic Compounds

Bromination is a specific type of electrophilic aromatic substitution where bromine acts as the electrophile. In the synthesis of benzyl bromide, bromine can be introduced to the benzene ring through the use of bromine (Br2) in the presence of a Lewis acid catalyst, such as FeBr3. This process highlights the importance of understanding how to generate and stabilize the electrophile for successful substitution reactions.

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions involve the replacement of a leaving group by a nucleophile. In the context of synthesizing benzyl bromide, once the benzene ring is functionalized to form benzyl bromide, a nucleophilic substitution can occur where a bromide ion (Br-) replaces a hydrogen atom. Familiarity with the mechanisms of both SN1 and SN2 reactions is vital for understanding how to manipulate the reactivity of the benzyl group in further synthetic steps.

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Nucleophiles and Electrophiles can react in Substitution Reactions.

Related Practice

Textbook Question

The following steps were used in the synthesis of the antimalarial thiaplakortone A. Identify the missing reagents, (a) and (b),

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Textbook Question

The benzylic bromide shown undergoes neither S_N1 nor S_N2 substitution reactions. Explain.

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Textbook Question

In Chapter 13, we learned that epoxide opening can give different products, depending on whether the reaction occurs under acidic or basic conditions. Explain why the epoxide shown opens identically under either set of conditions.

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Textbook Question

When (R)-(1-bromoethyl)benzene is treated with sodium cyanide, a single enantiomer is produced. However, upon treatment of the same molecule with water, a mixture of two enantiomers is obtained.

(a) Explain these results.

(b) Why is only partial racemization sometimes observed?

Textbook Question

In Chapter 23, we learned about the electrophilic aromatic nitration reaction. When phenol is subjected to these conditions with a large excess of nitric acid, a molecule called picric acid is produced. Predict the product of this reaction and explain why the pK_a value of this compound is 0.38.

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Textbook Question

The following reaction proceeds in good yield, despite requiring high temperature and high concentration. Identify the product you’d expect to obtain in this reaction.

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