Ch.3 - Structure and Stereochemistry of Alkanes

Wade9th EditionOrganic ChemistryISBN: 9780135213728Non è quello che usi tu?Cambia libro di testo

Wade 9th Edition

Ch.3 - Structure and Stereochemistry of Alkanes

Problema 29a,b

Capitolo 3, Problema 29a,b

Draw the two chair conformations of each of the following substituted cyclohexanes. In each case, label the more stable conformation.
a. cis-1-ethyl-2-methylcyclohexane
b. trans-1,2-diethylcyclohexane

Guida verificata passo dopo passo

Identify the substituents and their positions on the cyclohexane ring for each compound. For cis-1-ethyl-2-methylcyclohexane, the ethyl and methyl groups are on adjacent carbons and on the same side of the ring. For trans-1,2-diethylcyclohexane, the ethyl groups are on adjacent carbons but on opposite sides of the ring.

Draw the basic cyclohexane chair conformation. Remember that cyclohexane can exist in two chair conformations that interconvert through a ring-flip.

For cis-1-ethyl-2-methylcyclohexane, place the ethyl group on carbon 1 and the methyl group on carbon 2. In the first chair conformation, place both substituents in axial positions. In the second chair conformation, perform a ring-flip to place both substituents in equatorial positions.

For trans-1,2-diethylcyclohexane, place one ethyl group on carbon 1 in an axial position and the other ethyl group on carbon 2 in an equatorial position in the first chair conformation. Perform a ring-flip to switch the positions of the ethyl groups, placing the first ethyl group in an equatorial position and the second in an axial position in the second chair conformation.

Evaluate the stability of each conformation by considering steric hindrance. Generally, the conformation with the larger substituents in equatorial positions is more stable due to reduced steric strain. Label the more stable conformation for each compound.

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Chair Conformation

Chair conformation is a three-dimensional shape that cyclohexane adopts to minimize steric strain and torsional strain. It resembles a chair, with alternating axial and equatorial positions for substituents. Understanding this conformation is crucial for analyzing the stability of substituted cyclohexanes.

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03:29

Understanding what a conformer is.

Axial and Equatorial Positions

In cyclohexane's chair conformation, substituents can occupy axial positions, which are parallel to the ring's axis, or equatorial positions, which are around the equator of the ring. Equatorial positions are generally more stable due to reduced steric hindrance, making them preferable for larger substituents.

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04:02

Equatorial Preference

Cis and Trans Isomerism

Cis and trans isomerism refers to the relative positioning of substituents on a cyclohexane ring. In cis isomers, substituents are on the same side, while in trans isomers, they are on opposite sides. This affects the stability and spatial arrangement of the chair conformations, influencing which conformation is more stable.

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00:44

Is the following cyclohexane cis or trans?

Pratica correlata

Domanda del libro di testo

Draw the most stable conformation of

b. trans-1-tert-butyl-2-methylcyclohexane.

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Domanda del libro di testo

Use your results from Problem 3-27 to complete the following table. Each entry shows the positions of two groups arranged as shown. For example, two groups that are trans on adjacent carbons (trans-1,2) must be both equatorial (e,e) or both axial (a,a).

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Domanda del libro di testo

a. Draw both chair conformations of cis-1,4-dimethylcyclohexane, and determine which conformer is more stable.

b. Repeat for the trans isomer.

c. Predict which isomer (cis or trans) is more stable.

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Domanda del libro di testo

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)