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Ch.3 - Structure and Stereochemistry of Alkanes
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Non è quello che usi tu?Cambia libro di testo
Tutti i libri di testoWade 9th EditionCh.3 - Structure and Stereochemistry of AlkanesProblema 46e,f
Capitolo 3, Problema 46e,f

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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1
Begin by understanding the structure of cyclohexane and its chair conformations. Cyclohexane can adopt two chair conformations that are interconvertible. Each carbon atom in the chair conformation has two types of positions for substituents: axial (pointing up or down, perpendicular to the ring) and equatorial (pointing outwards, around the equator of the ring).
For cis-1-ethyl-4-methylcyclohexane, identify the positions of the substituents. 'Cis' indicates that both substituents are on the same side of the cyclohexane ring. Draw the first chair conformation with the ethyl group at the 1-position and the methyl group at the 4-position, both in axial positions. Then, draw the second chair conformation with both substituents in equatorial positions.
Label the substituents in each conformation as axial or equatorial. In the first conformation, both ethyl and methyl groups are axial. In the second conformation, both are equatorial.
Determine the more stable conformation for cis-1-ethyl-4-methylcyclohexane. Generally, equatorial positions are favored for larger groups due to less steric hindrance. Therefore, the conformation with both substituents in equatorial positions is more stable.
For trans-1-ethyl-4-methylcyclohexane, 'trans' indicates that the substituents are on opposite sides of the ring. Draw the first chair conformation with the ethyl group axial at the 1-position and the methyl group equatorial at the 4-position. Then, draw the second chair conformation with the ethyl group equatorial at the 1-position and the methyl group axial at the 4-position. Label the substituents and determine the more stable conformation, which typically has the larger group (ethyl) in the equatorial position.

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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 carbon atoms positioned above and below the plane. This conformation is crucial for understanding the spatial arrangement of substituents and their interactions in cyclohexane derivatives.
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Understanding what a conformer is.

Axial and Equatorial Positions

In the chair conformation of cyclohexane, substituents can occupy either axial or equatorial positions. Axial positions are perpendicular to the ring plane, while equatorial positions are roughly parallel. Equatorial positions are generally more stable due to reduced steric hindrance, making it important to identify these positions when assessing the stability of different conformations.
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Equatorial Preference

Cis and Trans Isomerism

Cis and trans isomerism refers to the relative orientation of substituents around a cyclohexane ring. In cis isomers, substituents are on the same side of the ring, while in trans isomers, they are on opposite sides. This distinction affects the stability and spatial arrangement of the chair conformations, influencing which conformation is more stable for a given compound.
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Is the following cyclohexane cis or trans?
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a. Draw the two chair conformations of cis-1,3-dimethylcyclohexane, and label all the positions as axial or equatorial.

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