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Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and StructureProblem 50c
Chapter 4, Problem 50c

Which compound is more stable: cis-1-ethyl-2-methylcyclohexane or trans-1-ethyl-2-methylcyclohexane?

Verified step by step guidance
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Analyze the structure of cyclohexane: Cyclohexane adopts a chair conformation to minimize steric strain and torsional strain. Substituents on the cyclohexane ring can occupy either axial (parallel to the ring's axis) or equatorial (perpendicular to the ring's axis) positions.
Understand the steric interactions: Axial substituents experience 1,3-diaxial interactions, which are steric clashes with hydrogen atoms on the same side of the ring. Equatorial substituents are generally more stable because they avoid these interactions.
Examine the cis-1-ethyl-2-methylcyclohexane: In the cis isomer, both substituents (ethyl and methyl) are on the same side of the ring. One substituent will occupy an axial position, and the other will occupy an equatorial position in the chair conformation. This arrangement leads to steric strain due to the axial substituent's 1,3-diaxial interactions.
Examine the trans-1-ethyl-2-methylcyclohexane: In the trans isomer, the substituents are on opposite sides of the ring. Both substituents can occupy equatorial positions in the chair conformation, minimizing steric strain and avoiding 1,3-diaxial interactions.
Compare the stability: The trans isomer is more stable than the cis isomer because it allows both substituents to occupy equatorial positions, reducing steric strain. In contrast, the cis isomer forces one substituent into the less favorable axial position, increasing steric strain.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Cis-Trans Isomerism

Cis-trans isomerism refers to the different spatial arrangements of atoms in a molecule. In cycloalkanes, 'cis' indicates that substituents are on the same side of the ring, while 'trans' means they are on opposite sides. This arrangement significantly affects the steric interactions and overall stability of the compound.
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Steric Strain

Steric strain arises when atoms are forced closer together than their preferred distance, leading to increased energy and decreased stability. In cyclohexane derivatives, steric strain can occur when bulky groups are positioned near each other, making the molecule less stable. Understanding steric interactions is crucial for predicting the stability of isomers.
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What is torsional strain?

Chair Conformation

The chair conformation is the most stable arrangement of cyclohexane, minimizing steric strain and torsional strain. In this conformation, substituents can occupy equatorial or axial positions, with equatorial positions generally being more stable for larger groups. Analyzing the chair conformations of cis and trans isomers helps determine their relative stability.
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Related Practice
Textbook Question

Draw the more stable chair conformer of cis-1-ethyl-2-methylcyclohexane.

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Textbook Question

For each of the following disubstituted cyclohexanes, indicate whether the substituents in the two chair conformers are both equatorial in one chair conformer and both axial in the other or one equatorial and one axial in each of the chair conformers:

a. cis-1,2-

b. trans-1,2-

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Textbook Question

For each of the following disubstituted cyclohexanes, indicate whether the substituents in the two chair conformers are both equatorial in one chair conformer and both axial in the other or one equatorial and one axial in each of the chair conformers:

e. cis-1,4-

f. trans-1,4-

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Textbook Question

Which has a higher percentage of the diequatorial-substituted conformer compared with the diaxialsubstituted conformer: trans-1,4-dimethylcyclohexane or cis-1-tert-butyl-3-methylcyclohexane?

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Textbook Question

For each of the following disubstituted cyclohexanes, indicate whether the substituents in the two chair conformers are both equatorial in one chair conformer and both axial in the other or one equatorial and one axial in each of the chair conformers:

c. cis-1,3-

d. trans-1,3-

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Textbook Question

Is each of the following a cis isomer or a trans isomer?

d.

e.

f.

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