In a substituted cyclohexane with a bulky tert-butyl group and a smaller ethyl group, which configuration minimizes steric hindrance and maximizes stability in the chair conformation?

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tert-butyl in the equatorial position and ethyl in the axial position

tert-butyl in the axial position and ethyl in the equatorial position

Both groups in equatorial positions

Both groups in axial positions

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Answer

In the chair form the bulky tert‑butyl group should occupy an equatorial position to avoid 1,3‑diaxial interactions, which are the main source of steric strain. The smaller ethyl group can also be placed equatorially, giving both substituents the least steric crowding. This arrangement keeps the two groups on the same side of the ring (cis) but with both equatorial, so the chair is the most stable. For example, a 4‑tert‑butyl‑5‑ethyl‑cyclohexane adopts a chair where both substituents are equatorial, minimizing clash and maximizing stability.

Detailed Explanation

The tert‑butyl group is large and prefers the equatorial position because it has more space and avoids 1,3‑diaxial interactions. Other options are incorrect because Putting the bulky tert‑butyl in the axial position creates large clashes with rings that are close to the chair; Having both groups equatorial seems nice, but it forces the chair to adopt a less favorable tilt.

Key Concepts

Conformational Analysis of Cyclohexane Derivatives

Ring Flipping Mechanism

Stereochemistry in Chair Conformations

Topic

Cyclohexane Chair Conformations

Difficulty

hard level question

Cognitive Level

understand

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In a molecule of 1-methylcyclohexane, which conformation minimizes steric hindrance and torsional strain while maximizing stability after a ring flip?

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In a cyclohexane derivative featuring both a chlorine atom and a methyl group, which substituent position (axial or equatorial) would likely lead to a more stable chair conformation?

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Arrange the following conformational changes of cyclohexane from least to most stable: A. Chair conformation, B. Twist-boat conformation, C. Boat conformation, D. Half-chair conformation.

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In the chair conformation of cyclohexane, substituents in the ____ position are generally more stable due to reduced steric interactions compared to those in the axial position.

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