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.
Ch. 7 - Structure and Synthesis of Alkenes; Elimination
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
Chapter 7, Problem 31a
Give the expected product(s) of E2 elimination for each reaction. (Hint: Use models!)
(a) 
Verified step by step guidance1
Step 1: Identify the substrate and the leaving group. In this case, the substrate is a cyclohexane ring with a bromine atom (Br) as the leaving group. The reaction is an E2 elimination, which requires a strong base (NaOCH3) and an anti-periplanar geometry between the β-hydrogen and the leaving group.
Step 2: Locate the β-hydrogens on the carbon atoms adjacent to the carbon bearing the bromine atom. In this structure, there are two β-hydrogens: one on the axial position and one on the equatorial position.
Step 3: Determine the anti-periplanar relationship. For E2 elimination to occur, the β-hydrogen and the leaving group (Br) must be anti-periplanar, meaning they are on opposite sides of the plane of the ring. In this case, the axial β-hydrogen is anti-periplanar to the bromine atom.
Step 4: Remove the bromine atom and the anti-periplanar β-hydrogen. This elimination results in the formation of a double bond between the two carbons involved in the elimination process.
Step 5: Draw the product. The double bond will form in the ring, resulting in a cyclohexene structure. Ensure the stereochemistry of the remaining substituents is preserved in the product.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
E2 Elimination Mechanism
The E2 elimination mechanism is a concerted reaction where a base removes a proton from a β-carbon while a leaving group departs from the α-carbon, resulting in the formation of a double bond. This mechanism requires the base to be strong and the substrate to have a suitable leaving group. The stereochemistry of the reaction is crucial, as it typically favors anti-periplanar geometry for optimal overlap of orbitals.
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Drawing the E2 Mechanism.
Stereochemistry in E2 Reactions
Stereochemistry plays a significant role in E2 reactions, as the elimination must occur from an anti-periplanar arrangement. This means that the hydrogen atom being removed and the leaving group must be positioned opposite each other in a staggered conformation. Understanding the spatial arrangement of substituents on the cyclohexane ring is essential for predicting the correct product of the elimination reaction.
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Cyclohexane Conformation
Cyclohexane can adopt various conformations, with the chair conformation being the most stable. In this conformation, substituents can occupy axial or equatorial positions, influencing the reactivity in elimination reactions. Recognizing the preferred positions of the bromine and hydrogen atoms in the chair form is vital for determining which hydrogen will be eliminated during the E2 reaction with NaOCH3.
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Related Practice
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.
(b)
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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.
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.
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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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