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 25b

Chapter 3, Problem 25b

Draw the most stable conformation of
b. 3-isopropyl-1,1-dimethylcyclohexane.

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Identify the structure of 3-isopropyl-1,1-dimethylcyclohexane. The cyclohexane ring is the base structure, with an isopropyl group at carbon 3 and two methyl groups at carbon 1.

Draw the cyclohexane ring in its chair conformation, which is the most stable conformation due to minimized steric strain.

Place the two methyl groups at carbon 1. Since they are on the same carbon, one will be axial and the other equatorial. To minimize steric hindrance, place the larger group (isopropyl) in the equatorial position.

Position the isopropyl group at carbon 3. To achieve the most stable conformation, place the isopropyl group in the equatorial position, as it is bulkier than a methyl group.

Verify the stability of the conformation by ensuring that the larger substituents (isopropyl and one of the methyl groups) are in equatorial positions, reducing 1,3-diaxial interactions and overall steric strain.

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Key Concepts

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

Cyclohexane Conformations

Cyclohexane can adopt several conformations, with the chair conformation being the most stable due to minimized steric strain and torsional strain. In the chair form, carbon atoms are staggered, reducing repulsion between hydrogen atoms. Understanding these conformations is crucial for determining the most stable structure of substituted cyclohexanes.

Axial and Equatorial Positions

In the chair conformation of cyclohexane, substituents can occupy axial or equatorial positions. Axial positions are perpendicular to the ring plane, while equatorial positions are parallel. Larger groups prefer equatorial positions to minimize steric hindrance, which is key to determining the most stable conformation of substituted cyclohexanes.

Steric Hindrance

Steric hindrance occurs when atoms or groups are too close, causing repulsion and destabilizing the molecule. In cyclohexane derivatives, placing bulky groups in equatorial positions reduces steric hindrance, leading to a more stable conformation. Recognizing steric effects is essential for predicting the stability of different conformations.

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Related Practice

Textbook Question

Name the following compounds. Remember that two up bonds are cis; two down bonds are cis; one up bond and one down bond are trans.

(e)

(f)

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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

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.

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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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Name the following compounds. Remember that two up bonds are cis; two down bonds are cis; one up bond and one down bond are trans.

(c)

(d)

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