Textbook Question
Make models of the following compounds, and predict the products formed when they react with the strong bases shown.
(b) meso-1,2-dibromo-1,2-diphenylethane + (CH3CH2)3N:
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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 27c
Make models of the following compounds, and predict the products formed when they react with the strong bases shown.
(c) (d,l)-1,2-dibromo-1,2-diphenylethane + (CH3CH2)3N:
Verified step by step guidance1
Step 1: Analyze the structure of (d,l)-1,2-dibromo-1,2-diphenylethane. This compound contains two bromine atoms attached to adjacent carbon atoms (C1 and C2) in a 1,2-relationship. Each carbon also has a phenyl group (C6H5) attached, and the compound exists as a pair of enantiomers (d and l forms).
Step 2: Recall the mechanism of an E2 elimination reaction. E2 eliminations require a strong base (in this case, triethylamine, (CH3CH2)3N) and a β-hydrogen that is anti-coplanar to the leaving group (bromine). Anti-coplanar geometry is crucial for the reaction to proceed efficiently.
Step 3: Identify the β-hydrogens on the molecule. For each bromine atom, look at the adjacent carbon (the β-carbon) and determine if it has hydrogens that are anti-coplanar to the bromine. In this case, the β-hydrogens are located on the same carbons as the phenyl groups.
Step 4: Predict the product of the E2 elimination. When the β-hydrogens are removed along with the bromine atoms, a double bond will form between C1 and C2. The resulting product will be a trans-stilbene (E-1,2-diphenylethene), as the anti-coplanar geometry favors the formation of the trans isomer.
Step 5: Consider stereochemical constraints. Since the starting material is (d,l)-1,2-dibromo-1,2-diphenylethane, the reaction will proceed through the anti-coplanar pathway for each enantiomer, leading to the same trans-stilbene product. Syn-coplanar elimination is unlikely due to the rarity of this pathway and the free rotation around the single bonds.
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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 bimolecular reaction where a strong base abstracts a proton from a β-carbon, leading to the simultaneous removal of a leaving group from the α-carbon. This results in the formation of a double bond. The reaction is stereospecific, often requiring an anti-coplanar arrangement of the leaving group and the hydrogen being removed for optimal overlap of orbitals.
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09:36
Drawing the E2 Mechanism.
Anti-Coplanar vs. Syn-Coplanar
In E2 reactions, the terms anti-coplanar and syn-coplanar refer to the spatial arrangement of atoms involved in the elimination process. Anti-coplanar configurations, where the leaving group and the hydrogen are on opposite sides, are favored and lead to more stable products. In contrast, syn-coplanar configurations, where these groups are on the same side, are less common and typically occur when rotation around a bond is restricted.
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03:19The Anti-Coplanar Requirement
Steric Hindrance and Conformational Analysis
Steric hindrance refers to the repulsion between bulky groups in a molecule that can affect reactivity and product formation. In conformational analysis, understanding the spatial arrangement of atoms in a molecule is crucial, especially in determining whether a compound can adopt the necessary geometry for E2 elimination. This analysis helps predict whether the reaction will proceed via an anti-coplanar or syn-coplanar pathway.
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03:29Understanding what a conformer is.
Related Practice
Textbook Question
Show that the (S,S) enantiomer of this (R,R) diastereomer of 1-bromo-1,2-diphenylpropane also undergoes E2 elimination to give the cis diastereomer of the product. (We do not expect these achiral reagents to distinguish between enantiomers.)
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Textbook Question
Make models of the following compounds, and predict the products formed when they react with the strong bases shown.
(d)
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Textbook Question
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
Make models of the following compounds, and predict the products formed when they react with the strong bases shown.
(a)
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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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