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Ch. 7 - Structure and Synthesis of Alkenes; Elimination
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 7 - Structure and Synthesis of Alkenes; EliminationProblem 22a
Chapter 7, Problem 22a

When (1-bromoethyl)cyclohexane is heated in methanol for an extended period of time, five products result: two ethers and three alkenes. Predict the products of this reaction, and propose mechanisms for their formation.

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Step 1: Recognize the reaction conditions. Heating (1-bromoethyl)cyclohexane in methanol suggests a solvolysis reaction, where methanol acts as both the solvent and nucleophile. This indicates that the reaction likely proceeds via an SN1/E1 mechanism due to the polar protic solvent and the secondary alkyl halide.
Step 2: Propose the first step of the mechanism. The bromine atom in (1-bromoethyl)cyclohexane is a good leaving group. Under heat, the C-Br bond breaks heterolytically, forming a secondary carbocation intermediate on the cyclohexane ring. This carbocation is stabilized by hyperconjugation and inductive effects.
Step 3: Predict the formation of ethers. Methanol, acting as a nucleophile, can attack the carbocation to form an ether product. Since methanol can attack from either side of the planar carbocation, two stereoisomeric ethers (R and S configurations) are formed.
Step 4: Predict the formation of alkenes. The carbocation can also undergo elimination (E1 mechanism). A proton from a β-carbon (adjacent to the carbocation) is removed by methanol acting as a base, leading to the formation of alkenes. Three possible alkenes can form due to different β-hydrogens being eliminated, including cis/trans isomers if applicable.
Step 5: Summarize the products. The reaction yields two ethers (stereoisomers) and three alkenes (including possible cis/trans isomers). The mechanisms involve both substitution (SN1) and elimination (E1) pathways, with methanol playing dual roles as a nucleophile and base.

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Key Concepts

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

Nucleophilic Substitution

Nucleophilic substitution is a fundamental reaction in organic chemistry where a nucleophile attacks an electrophile, replacing a leaving group. In the case of (1-bromoethyl)cyclohexane reacting with methanol, methanol acts as a nucleophile, leading to the formation of ethers through an SN1 or SN2 mechanism, depending on the reaction conditions.
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Nucleophiles and Electrophiles can react in Substitution Reactions.

Elimination Reactions

Elimination reactions involve the removal of a small molecule from a larger one, typically resulting in the formation of alkenes. In this scenario, heating (1-bromoethyl)cyclohexane can promote elimination, where the bromine atom is lost along with a hydrogen atom from an adjacent carbon, leading to the formation of alkenes through either E1 or E2 mechanisms.
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Reaction Mechanisms

Understanding reaction mechanisms is crucial for predicting the products of organic reactions. Mechanisms describe the step-by-step process of how reactants transform into products, including the intermediates formed. In this case, both nucleophilic substitution and elimination pathways must be considered to explain the formation of the observed ethers and alkenes.
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Related Practice
Textbook Question

Give the substitution and elimination products you would expect from the following reactions.

a. 3-bromo-3-ethylpentane heated in methanol

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Textbook Question

When (1-bromoethyl)cyclohexane is heated in methanol for an extended period of time, five products result: two ethers and three alkenes. Predict which of the three alkenes is the major elimination product.

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Textbook Question

Predict the elimination products of the following reactions. When two alkenes are possible, predict which one will be the major product. Explain your answers, showing the degree of substitution of each double bond in the products.

a. 2-bromopentane + NaOCH3

b. 3-bromo-3-methylpentane + NaOMe (Me = methyl,CH3)

c. 2-bromo-3-ethylpentane + NaOH

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Textbook Question

Give the substitution and elimination products you would expect from the following reactions.

c. 1-bromo-2-methylcyclohexane + silver nitrate in water (AgNO3 forces ionization)

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Textbook Question

Give the substitution and elimination products you would expect from the following reactions.

b. 1-iodo-1-phenylcyclopentane heated in ethanol

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Textbook Question

Under second-order conditions (strong base/nucleophile), SN2 and E2 reactions may occur simultaneously and compete with each other. Show what products might be expected from the reaction of 2-bromo-3-methylbutane (a moderately hindered 2° alkyl halide) with sodium ethoxide.

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