Textbook Question
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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4. Alkanes and Cycloalkanes
Equatorial Preference
8:31 minutesProblem 74f
Textbook Question
Draw the two chair conformers for each of the following and indicate which conformer is more stable:
f. cis-1-ethyl-4-isopropylcyclohexane
Verified step by step guidance1
Step 1: Understand the structure of cis-1-ethyl-4-isopropylcyclohexane. The 'cis' designation means that both substituents (ethyl and isopropyl groups) are on the same side of the cyclohexane ring. This will be important when placing the groups in the chair conformations.
Step 2: Draw the basic chair conformation of cyclohexane. Label the carbon atoms in the ring to help with substituent placement. For example, assign carbon-1 to the position where the ethyl group is attached and carbon-4 to the position where the isopropyl group is attached.
Step 3: Place the substituents on the chair conformation. Since the substituents are 'cis,' both groups must either be in axial positions or both in equatorial positions. Draw one conformer with both substituents in axial positions and another conformer with both substituents in equatorial positions.
Step 4: Evaluate the stability of each conformer. Recall that larger groups prefer the equatorial position due to reduced steric hindrance. Compare the steric interactions in the axial positions versus the equatorial positions for both the ethyl and isopropyl groups.
Step 5: Conclude which conformer is more stable. The conformer with both substituents in equatorial positions will generally be more stable because it minimizes steric hindrance between the bulky groups and the axial hydrogens on the ring.
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Video duration:
8mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Chair Conformation
Chair conformation is a three-dimensional representation of cyclohexane that minimizes steric strain and torsional strain. In this conformation, the carbon atoms are arranged in a staggered manner, allowing for more stable interactions between substituents. Understanding chair conformations is crucial for analyzing the stability of cyclohexane derivatives, as different substituents can occupy axial or equatorial positions, affecting overall stability.
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Understanding what a conformer is.
Cis and Trans Isomerism
Cis and trans isomerism refers to the spatial arrangement of substituents around a ring or double bond. In the context of cyclohexane, 'cis' indicates that substituents are on the same side of the ring, while 'trans' means they are on opposite sides. This arrangement significantly influences the steric interactions between substituents, which in turn affects the stability of the conformers, making it essential to identify the correct isomer when analyzing stability.
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Steric Hindrance
Steric hindrance is the repulsion between atoms that occurs when they are brought close together, often due to the size of substituents. In cyclohexane derivatives, larger groups prefer equatorial positions to minimize steric interactions with other substituents. Recognizing which conformer has less steric hindrance is key to determining which chair conformation is more stable, as lower steric strain typically correlates with greater stability.
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02:53Understanding steric effects.
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