Ch. 7 - Structure and Synthesis of Alkenes; Elimination

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 7 - Structure and Synthesis of Alkenes; Elimination

Problem 30b

Chapter 7, Problem 30b

Predict which of the following compounds will undergo elimination with KOH faster, and explain why. Predict the major product that will be formed.

Verified step by step guidance

Step 1: Analyze the structures of the two compounds. Both are cyclohexane derivatives with a bromine atom and an isopropyl group attached. The difference lies in the stereochemistry of the bromine atom: one is in the axial position, and the other is in the equatorial position.

Step 2: Recall the mechanism of elimination (E2) with KOH. For E2 elimination, the anti-periplanar geometry is crucial. The β-hydrogen and the leaving group (Br) must be in opposite planes (anti-periplanar) for the elimination to occur efficiently.

Step 3: Evaluate the stereochemistry of the β-hydrogens relative to the bromine atom in each compound. In the compound with axial bromine, the β-hydrogens on adjacent carbons are equatorial, making them anti-periplanar to the bromine. In the compound with equatorial bromine, the β-hydrogens are axial, which are not anti-periplanar to the bromine.

Step 4: Predict which compound will undergo elimination faster. The compound with axial bromine will undergo elimination faster because the anti-periplanar geometry is readily achieved, whereas the equatorial bromine does not have β-hydrogens in the correct orientation for efficient elimination.

Step 5: Predict the major product formed. The elimination reaction will result in the formation of a double bond between the carbon bearing the bromine and one of the adjacent carbons. The major product will be determined by Zaitsev's rule, favoring the more substituted alkene.

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Key Concepts

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

Elimination Reactions

Elimination reactions involve the removal of a leaving group and a hydrogen atom from adjacent carbon atoms, resulting in the formation of a double bond. The most common types are E1 and E2 mechanisms, which differ in their reaction conditions and mechanisms. Understanding the conditions that favor each mechanism is crucial for predicting the rate and products of elimination reactions.

Sterics and Substitution Patterns

Steric hindrance refers to the spatial arrangement of atoms that can affect the reactivity of a molecule. In elimination reactions, more substituted carbon atoms tend to favor E2 mechanisms due to their ability to stabilize the transition state. Recognizing the steric effects of substituents on the carbon chain is essential for predicting which compound will undergo elimination faster.

Regioselectivity in Elimination

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others. In elimination reactions, the major product often forms from the more stable alkene, which is influenced by factors such as substitution and steric effects. Understanding the concept of regioselectivity helps in predicting the major product formed during the elimination process.

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Related Practice

Textbook Question

Give the expected product(s) of E2 elimination for each reaction. (Hint: Use models!)

(a)

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Textbook Question

Two stereoisomers of a bromodecalin are shown. Although the difference between these stereoisomers may seem trivial, one isomer undergoes elimination with KOH much faster than the other. Predict the products of these eliminations, and explain the large difference in the ease of elimination.

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Textbook Question

When the following stereoisomer of 2-bromo-1,3-dimethylcyclohexane is treated with sodium methoxide, no E2 reaction is observed. Explain why this compound cannot undergo the E2 reaction in the chair conformation.

Predict the elimination products of the following reactions, and label the major products.

b. trans-1-bromo-2-methylcyclohexane + NaOCH₃ in CH₃OH

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Textbook Question

Predict the major and minor elimination products of the following proposed reactions (ignoring any possible substitutions for now). In each case, explain whether you expect the mechanism of the elimination to be E1 or E2.

(a)

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Textbook Question

Give the expected product(s) of E2 elimination for each reaction. (Hint: Use models!)

(b)

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