Ch.3 - Structure and Stereochemistry of Alkanes

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch.3 - Structure and Stereochemistry of Alkanes

Problem 47

Chapter 3, Problem 47

Using what you know about the conformational energetics of substituted cyclohexanes, predict which of the two decalin isomers is more stable. Explain your reasoning.

Verified step by step guidance

Begin by understanding the structure of decalin, which consists of two fused cyclohexane rings. The two isomers shown are trans-decalin and cis-decalin.

Consider the conformational stability of cyclohexane rings. Cyclohexane prefers to adopt a chair conformation because it minimizes steric strain and torsional strain.

In trans-decalin, both cyclohexane rings can adopt a chair conformation without any steric hindrance between the axial positions of the rings. This allows for a more stable conformation.

In cis-decalin, one of the cyclohexane rings must adopt a less stable conformation due to steric interactions between the axial positions of the rings. This results in increased steric strain.

Therefore, trans-decalin is generally more stable than cis-decalin because it allows both rings to remain in their preferred chair conformation, minimizing steric and torsional strain.

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Key Concepts

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

Conformational Energetics

Conformational energetics refers to the study of energy differences between different spatial arrangements of atoms in a molecule. In cyclohexanes, the chair conformation is typically the most stable due to minimized steric strain and torsional strain. Substituents prefer equatorial positions over axial ones to reduce steric hindrance, affecting the overall stability of the molecule.

Trans vs. Cis Isomers

Trans and cis isomers differ in the spatial arrangement of substituents across a double bond or a ring system. In decalin derivatives, trans isomers have substituents on opposite sides, while cis isomers have them on the same side. Trans isomers generally exhibit lower steric strain and are more stable due to less crowding and better distribution of substituents.

Steric Strain

Steric strain arises from the repulsion between atoms or groups that are too close to each other, leading to increased energy and decreased stability. In cyclohexane derivatives, steric strain is minimized when bulky groups are positioned equatorially rather than axially, as axial positions can lead to unfavorable interactions with other axial hydrogens or groups.

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Related Practice

Textbook Question

Draw the two chair conformations of each compound, and label the substituents as axial and equatorial. In each case, determine which conformation is more stable.

c. cis-1-ethyl-3-methylcyclohexane

d. trans-1-ethyl-3-methylcyclohexane

Convert each Newman projection to the equivalent line–angle formula, and assign the IUPAC name.

(c)

(d)

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Textbook Question

Draw the two chair conformations of each compound, and label the substituents as axial and equatorial. In each case, determine which conformation is more stable.

e. cis-1-ethyl-4-methylcyclohexane

f. trans-1-ethyl-4-methylcyclohexane

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Textbook Question

Convert each Newman projection to the equivalent line–angle formula, and assign the IUPAC name.

(a)

(b)

Convert each Newman projection to the equivalent line–angle formula, and assign the IUPAC name. g.

(g)

(h)

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Textbook Question

Draw the two chair conformations of each compound, and label the substituents as axial and equatorial. In each case, determine which conformation is more stable.

a. cis-1-ethyl-2-isopropylcyclohexane

b. trans-1-ethyl-2-isopropylcyclohexane

1033

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