📚 Learning Guide
Cycloalkane Conformations
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In a substituted cyclohexane, how does the axial position of a bulky substituent influence angle strain and overall stability compared to when it is in the equatorial position?

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Choose the Best Answer

A

The axial position increases angle strain and decreases stability.

B

The axial position decreases angle strain and increases stability.

C

The axial position has no effect on angle strain or stability.

D

The axial position decreases angle strain but decreases stability.

Understanding the Answer

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Answer

Bulky groups in a cyclohexane ring prefer the equatorial orientation because the axial position forces the bond to tilt toward the ring, creating a 60‑degree torsion that pulls the substituent into a crowded space and increases angle strain. In the axial position, the substituent’s bonds are forced into a gauche-like arrangement with neighboring hydrogens, raising steric hindrance and raising the ring’s overall energy. When the same group is placed equatorially, it lies farther from the ring’s interior, minimizing steric clashes and keeping bond angles closer to the ideal 109. 5°, so the ring experiences less angle strain and is more stable. For example, a tert‑butyl group in 1‑tert‑butylcyclohexane prefers the equatorial position, making the compound about 2–3 kcal/mol more stable than its axial isomer.

Detailed Explanation

An axial substituent sits directly above or below the ring, forcing bonds to bend toward the center. Other options are incorrect because Many think axial makes the ring easier to hold, ignoring that the axial group forces adjacent hydrogens to crowd, raising strain; The misconception is that axial and equatorial have the same effect.

Key Concepts

Substituent Positioning
Angle Strain
Topic

Cycloalkane Conformations

Difficulty

medium level question

Cognitive Level

understand

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