Textbook Question
Which of the following cycloalkanes are capable of geometric (cis-trans) isomerism? Draw the cis and trans isomers.
b. 1-ethyl-3-methylcycloheptane
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Ch.3 - Structure and Stereochemistry of Alkanes
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 3, Problem 19
The heat of combustion of cis-1,2-dimethylcyclopropane is larger than that of the trans isomer. Which isomer is more stable? Use drawings to explain this difference in stability.
Verified step by step guidance1
Understand the concept of heat of combustion: The heat of combustion is the energy released when a compound is completely burned in oxygen. A higher heat of combustion indicates that the molecule is less stable because it has more stored energy to release during combustion.
Analyze the molecular structure of cis-1,2-dimethylcyclopropane: In the cis isomer, the two methyl groups are on the same side of the cyclopropane ring. This causes steric strain due to the close proximity of the bulky methyl groups, increasing the overall energy of the molecule.
Analyze the molecular structure of trans-1,2-dimethylcyclopropane: In the trans isomer, the two methyl groups are on opposite sides of the cyclopropane ring. This arrangement reduces steric strain compared to the cis isomer, making the trans isomer more stable.
Relate stability to heat of combustion: Since the cis isomer has more steric strain and is less stable, it has a higher heat of combustion. The trans isomer, being more stable, has a lower heat of combustion.
Use drawings to visualize the difference: Draw the cis and trans isomers of 1,2-dimethylcyclopropane. Highlight the steric interactions in the cis isomer (methyl groups close together) and the lack of such interactions in the trans isomer (methyl groups far apart). This visual representation helps explain why the trans isomer is more stable.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Isomerism
Isomerism refers to the phenomenon where compounds have the same molecular formula but different structural arrangements. In the case of cis and trans isomers, the spatial arrangement of substituents around a double bond or ring structure leads to distinct physical and chemical properties. Understanding isomerism is crucial for analyzing stability differences, as the arrangement can influence steric interactions and strain within the molecule.
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Heat of Combustion
The heat of combustion is the amount of energy released when a substance is completely burned in oxygen. It serves as an indicator of the stability of a compound; a higher heat of combustion typically suggests a less stable molecule, as more energy is released upon its conversion to more stable products. In comparing isomers, the heat of combustion can help determine which isomer is more stable based on the energy released during combustion.
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Steric Strain
Steric strain arises when atoms within a molecule are forced closer together than their preferred distance, leading to increased energy and decreased stability. In cyclopropane derivatives, the arrangement of methyl groups in cis and trans configurations affects steric interactions. The cis isomer experiences greater steric strain due to the proximity of the bulky methyl groups, making it less stable than the trans isomer, which allows for more spatial separation.
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Related Practice
Textbook Question
Which of the following cycloalkanes are capable of geometric (cis-trans) isomerism? Draw the cis and trans isomers.
c. 1-ethyl-3-methylcyclopentane
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Textbook Question
Draw 1,2,3,4,5,6-hexamethylcyclohexane with all the methyl groups
a. in axial positions.
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Textbook Question
trans-1,2-Dimethylcyclobutane is more stable than cis-1,2-dimethylcyclobutane, but cis-1,3-dimethylcyclobutane is more stable than trans-1,3-dimethylcyclobutane. Use drawings to explain these observations.
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Textbook Question
Which of the following cycloalkanes are capable of geometric (cis-trans) isomerism? Draw the cis and trans isomers.
a. 3-ethyl-1,1-dimethylcyclohexane
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Textbook Question
The cyclohexane chair shown in Figure 3-22 has the headrest to the right and the footrest to the left. Draw a cyclohexane chair with its axial and equatorial bonds, showing the headrest to the left and the footrest to the right.
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