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 32

Chapter 3, Problem 32

Use your models to do a chair–chair interconversion on each ring of the conformation of cis-decalin shown in Figure 3-27. Draw the conformation that results.

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Identify the structure of cis-decalin from the image. Cis-decalin consists of two cyclohexane rings fused in such a way that the hydrogen atoms at the bridgehead carbons are on the same side of the molecule.

Understand the concept of chair–chair interconversion. This involves flipping the cyclohexane chair conformation, which changes axial positions to equatorial and vice versa.

Begin with the first cyclohexane ring. Perform a chair flip by inverting the positions of the substituents. Axial positions become equatorial, and equatorial positions become axial.

Repeat the chair flip process for the second cyclohexane ring. Ensure that the bridgehead hydrogens remain on the same side to maintain the cis configuration.

Draw the resulting conformation after both chair flips. The new conformation should still represent cis-decalin, with the bridgehead hydrogens on the same side, but with altered axial and equatorial positions for other substituents.

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Key Concepts

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

Chair Conformation

Chair conformation is a three-dimensional arrangement of carbon atoms in cyclohexane and its derivatives, where the carbon atoms adopt a staggered configuration. This minimizes steric strain and torsional strain, making it the most stable conformation. In the context of decalin, understanding chair conformations is crucial for visualizing how the rings can interconvert and how substituents are positioned.

Cis-Trans Isomerism

Cis-trans isomerism refers to the spatial arrangement of substituents around a double bond or a ring structure. In cis-decalin, the two hydrogen atoms at the bridgehead carbons are on the same side of the ring system. This concept is essential for understanding the stereochemistry of decalin and how it affects the molecule's conformational flexibility during chair-chair interconversion.

Bridgehead Carbons

Bridgehead carbons are the carbon atoms located at the junction of two rings in a bicyclic compound. In cis-decalin, these carbons are critical for determining the molecule's conformation and stability. Their positioning influences the ability of the rings to undergo chair-chair interconversion, as the steric interactions and strain around these carbons dictate the preferred conformations.

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Related Practice

Textbook Question

a. There are 18 isomeric alkanes of molecular formula C₈H₁₈. Draw and name any eight of them.

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Textbook Question

Name the following compounds.

(c)

(d)

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Textbook Question

Draw and name the six isomeric cyclopentanes of molecular formula C₇H₁₄. These will include four constitutional isomers, of which two show geometric (cis-trans) stereoisomerism.

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Textbook Question

Draw the most stable conformation of

c. trans-1-tert-butyl-3-(1,1-dimethylpropyl)cyclohexane.

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Textbook Question

Which of the following structures represent the same compound? Which ones represent different compounds?

(a)

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