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 30a

Chapter 7, Problem 30a

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.

Verified step by step guidance

Step 1: Analyze the stereochemistry of the two bromodecalin isomers. In the first isomer (fast elimination), the bromine atom is in the axial position, while in the second isomer (slow elimination), the bromine atom is in the equatorial position.

Step 2: Recall the requirements for elimination reactions, specifically the E2 mechanism. The E2 mechanism requires a coplanar arrangement of the β-hydrogen and the leaving group (anti-periplanar geometry). This geometry is more easily achieved when the leaving group is in the axial position.

Step 3: In the fast elimination isomer, the axial bromine and the axial β-hydrogen on the adjacent carbon are anti-periplanar, making elimination via the E2 mechanism highly favorable. This leads to the formation of a double bond in the product.

Step 4: In the slow elimination isomer, the equatorial bromine and the β-hydrogen are not anti-periplanar. This misalignment makes elimination via the E2 mechanism less favorable, resulting in a slower reaction rate.

Step 5: Predict the products of elimination for both isomers. For the fast elimination isomer, the product will be a double bond formed between the two carbons involved in the elimination. For the slow elimination isomer, the same product will form, but the reaction will proceed at a slower rate due to the unfavorable geometry.

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Key Concepts

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

Stereoisomerism

Stereoisomerism refers to the phenomenon where compounds have the same molecular formula and connectivity of atoms but differ in the spatial arrangement of their atoms. In organic chemistry, stereoisomers can be classified into enantiomers and diastereomers, which can exhibit different chemical reactivities due to their distinct three-dimensional shapes. Understanding stereoisomerism is crucial for predicting the behavior of molecules in reactions, such as elimination.

Elimination Reactions

Elimination reactions involve the removal of a small molecule from a larger one, typically resulting in the formation of a double bond. In the context of the question, the elimination of bromine from bromodecalin with KOH leads to the formation of alkenes. The rate of elimination can vary significantly between stereoisomers due to factors such as steric hindrance and the stability of the resulting alkene, which are influenced by the spatial arrangement of substituents.

Zaitsev's Rule

Zaitsev's Rule states that in elimination reactions, the more substituted alkene product is favored due to its greater stability. This principle is particularly relevant when predicting the products of elimination reactions, as the stability of the alkene formed can influence the reaction rate. In the case of the bromodecalin stereoisomers, the faster elimination may lead to a more stable alkene, while the slower one may produce a less stable product, highlighting the importance of molecular structure in reaction outcomes.

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Related Practice

Textbook Question

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

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Textbook Question

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

(a)

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

a. cis-1-bromo-2-methylcyclohexane + NaOCH₃ in CH₃OH

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