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Ch.8 - Reactions of Alkenes
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.8 - Reactions of AlkenesProblem 58
Chapter 8, Problem 58

Cyclohexene is dissolved in a solution of lithium chloride in chloroform. To this solution is added one equivalent of bromine. The material isolated from this reaction contains primarily a mixture of trans-1,2-dibromocyclohexane and trans-1-bromo-2-chlorocyclohexane. Propose a mechanism to show how these compounds are formed.

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Step 1: Begin by identifying the key reactants and conditions. Cyclohexene is the starting material, bromine (Br₂) is the electrophile, and lithium chloride (LiCl) in chloroform is the solvent. The presence of LiCl suggests that chloride ions (Cl⁻) are available to participate in the reaction.
Step 2: Recognize that bromine reacts with cyclohexene via an electrophilic addition mechanism. Bromine forms a bromonium ion intermediate when it reacts with the double bond of cyclohexene. This occurs as the π-electrons of the double bond attack one bromine atom, while the other bromine atom becomes a leaving group.
Step 3: Consider the stereochemistry of the bromonium ion intermediate. The bromonium ion is cyclic and blocks one face of the molecule, forcing nucleophiles to attack from the opposite side. This anti-addition mechanism leads to trans products.
Step 4: Analyze the role of nucleophiles in the reaction. Bromide ions (Br⁻) and chloride ions (Cl⁻) are both present in the solution. These nucleophiles can attack the bromonium ion intermediate from the opposite side, leading to the formation of trans-1,2-dibromocyclohexane (when Br⁻ attacks) and trans-1-bromo-2-chlorocyclohexane (when Cl⁻ attacks).
Step 5: Summarize the mechanism. The reaction proceeds via electrophilic addition of bromine to cyclohexene, forming a bromonium ion intermediate. Nucleophilic attack by Br⁻ and Cl⁻ on the opposite side of the bromonium ion results in the formation of the two trans products observed in the reaction mixture.

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Key Concepts

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

Electrophilic Addition Reaction

Electrophilic addition is a fundamental reaction in organic chemistry where an electrophile reacts with a nucleophile, resulting in the addition of atoms or groups across a double bond. In the case of cyclohexene, the double bond acts as a nucleophile, attacking the electrophilic bromine molecule, leading to the formation of a bromonium ion intermediate.
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Bromonium Ion Mechanism

The bromonium ion mechanism involves the formation of a cyclic bromonium ion when bromine adds to the double bond of cyclohexene. This intermediate is crucial as it stabilizes the positive charge and allows for the subsequent attack by nucleophiles, such as chloride ions, leading to the formation of products like trans-1-bromo-2-chlorocyclohexane.
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Stereochemistry of Addition Reactions

Stereochemistry plays a vital role in determining the configuration of the products formed during addition reactions. In this case, the formation of trans-1,2-dibromocyclohexane and trans-1-bromo-2-chlorocyclohexane indicates that the addition of bromine and chloride occurs in a way that preserves the trans configuration, which is influenced by the mechanism of the bromonium ion and the subsequent nucleophilic attack.
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Related Practice
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The sex attractant of the housefly has the formula C23H46. When treated with warm potassium permanganate, this pheromone gives two products: CH3(CH2)12COOH and CH3(CH2)7COOH. Suggest a structure for this sex attractant. Explain which part of the structure is uncertain.

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