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.
70
views
Ch. 7 - Structure and Synthesis of Alkenes; Elimination
Wade9th EditionOrganic ChemistryISBN: 9780135213728
Not the one you use?Change textbookSelect a different textbook
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 29
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.
Verified step by step guidance1
Step 1: Recall the requirements for an E2 elimination reaction. For an E2 reaction to occur, the leaving group (in this case, Br) and a β-hydrogen must be anti-periplanar (i.e., they must be in opposite planes and aligned in a staggered conformation). In cyclohexane rings, this typically means the leaving group and β-hydrogen must both occupy axial positions in the chair conformation.
Step 2: Analyze the stereochemistry of the given compound. The bromine atom is in an equatorial position in the chair conformation of 2-bromo-1,3-dimethylcyclohexane. This is evident from the wedge bond indicating equatorial placement.
Step 3: Check the positions of the β-hydrogens. For elimination to occur, there must be a β-hydrogen trans to the bromine atom that can adopt an axial position. However, in this case, the β-hydrogens are in equatorial positions, and no β-hydrogen is anti-periplanar to the bromine.
Step 4: Consider the possibility of conformational flipping. If the chair conformation flips, the bromine would move to an axial position. However, the β-hydrogens would then move to equatorial positions, and the anti-periplanar geometry required for E2 elimination would still not be achieved.
Step 5: Conclude why no E2 reaction occurs. Since the bromine and β-hydrogens cannot simultaneously adopt diaxial (anti-periplanar) positions in any chair conformation, the stereochemical requirements for E2 elimination are not met. Therefore, no alkene is produced under these conditions.
Verified video answer for a similar problem:
This video solution was recommended by our tutors as helpful for the problem above.
Video duration:
5mWas this helpful?
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
E2 Reaction Mechanism
The E2 (bimolecular elimination) reaction is a concerted process where a base abstracts a proton while a leaving group departs, resulting in the formation of a double bond. For an E2 reaction to occur, the substrate must have the appropriate geometry, typically requiring an anti-periplanar arrangement of the leaving group and the hydrogen being removed. This spatial requirement is crucial for the reaction to proceed efficiently.
Recommended video:
Guided course
09:36
Drawing the E2 Mechanism.
Chair Conformation of Cyclohexane
Cyclohexane can adopt a chair conformation, which minimizes steric strain and allows for staggered interactions between substituents. In this conformation, substituents can occupy axial or equatorial positions. The positioning of groups in the chair conformation significantly affects reactivity; axial substituents are more likely to participate in reactions like E2 due to their favorable orientation relative to hydrogen atoms.
Recommended video:
Guided course
01:05What is a chair conformation?
Steric Hindrance
Steric hindrance refers to the repulsion between bulky groups that can impede reactions. In the case of 2-bromo-1,3-dimethylcyclohexane, the presence of two methyl groups in axial positions creates significant steric strain, preventing the necessary anti-periplanar arrangement for the E2 reaction. This steric crowding inhibits the approach of the base to abstract the proton, thus preventing alkene formation.
Recommended video:
Guided course
02:53Understanding steric effects.
Related Practice
Textbook Question
Give the expected product(s) of E2 elimination for each reaction. (Hint: Use models!)
(a)
990
views
Textbook Question
Make models of the following compounds, and predict the products formed when they react with the strong bases shown.
(d)
813
views
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.
1334
views
Textbook Question
Predict the elimination products of the following reactions, and label the major products.
b. trans-1-bromo-2-methylcyclohexane + NaOCH3 in CH3OH
2927
views
Textbook Question
Predict the elimination products of the following reactions, and label the major products.
a. cis-1-bromo-2-methylcyclohexane + NaOCH3 in CH3OH
3191
views