Textbook Question
Draw 1,2,3,4,5,6-hexamethylcyclohexane with all the methyl groups
a. in axial positions.
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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 24
Table 3-6 shows that the axial–equatorial energy difference for methyl, ethyl, and isopropyl groups increases gradually: 7.6, 7.9, and 8.8 kJ/mol (1.8, 1.9, and 2.1 kcal/mol). The tert-butyl group jumps to an energy difference of 23 kJ/mol (5.4 kcal/mol), over twice the value for the isopropyl group. Draw pictures of the axial conformations of isopropylcyclohexane and tert-butylcyclohexane, and explain why the tert-butyl substituent experiences such a large increase in axial energy over the isopropyl group.
Verified step by step guidance1
To draw the axial conformation of isopropylcyclohexane, start by sketching a cyclohexane chair conformation. Place the isopropyl group on one of the axial positions, which are perpendicular to the plane of the ring.
For tert-butylcyclohexane, similarly draw a cyclohexane chair conformation and place the tert-butyl group in an axial position. The tert-butyl group is larger and bulkier than the isopropyl group.
The axial position in a cyclohexane chair conformation is less stable for larger groups due to steric hindrance and 1,3-diaxial interactions. These interactions occur between the axial substituent and the hydrogen atoms on the same side of the ring.
The tert-butyl group experiences a larger increase in axial energy compared to the isopropyl group because it is bulkier, leading to greater steric hindrance and more significant 1,3-diaxial interactions.
The energy difference between axial and equatorial positions is much larger for tert-butyl groups because the equatorial position allows the bulky group to avoid steric clashes, making it significantly more stable than the axial position.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Axial and Equatorial Positions
In cyclohexane, substituents can occupy two distinct positions: axial and equatorial. Axial substituents are oriented perpendicular to the plane of the ring, while equatorial substituents extend outward, parallel to the plane. The stability of these positions varies, with equatorial generally being more stable due to reduced steric hindrance and torsional strain.
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04:02
Equatorial Preference
Steric Hindrance
Steric hindrance refers to the repulsion between atoms that occurs when they are brought close together, particularly in bulky groups. In the context of cyclohexane, larger substituents like tert-butyl experience significant steric hindrance when in the axial position, leading to increased energy and instability compared to smaller groups like isopropyl, which can fit more comfortably in the axial position.
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02:53Understanding steric effects.
Energy Differences in Conformations
The energy difference between axial and equatorial conformations is quantified in kJ/mol or kcal/mol. This difference arises from the steric interactions and torsional strain experienced by substituents in the axial position. For example, the tert-butyl group has a much larger energy difference due to its size, resulting in a more significant preference for the equatorial position compared to smaller groups like isopropyl.
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09:55Explaining how A-Values are related to cyclohexane flip energy
Related Practice
Textbook Question
Draw 1,2,3,4,5,6-hexamethylcyclohexane with all the methyl groups
b. in equatorial positions.
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Textbook Question
Draw the most stable conformation of
c. cis-1-tert-butyl-4-isopropylcyclohexane.
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Textbook Question
Draw the most stable conformation of
a. ethylcyclohexane.
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Textbook Question
Draw the most stable conformation of
b. 3-isopropyl-1,1-dimethylcyclohexane.
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Textbook Question
Draw a Newman projection, similar to Figure 3-25 down the C1—C6 bond in the equatorial conformation of methylcyclohexane. Show that the equatorial methyl group is also anti to C5. (Using your models will help.)
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