📚 Learning Guide
Cycloalkanes Naming
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In a cyclohexane derivative with two substituents, one methyl group at the 1-position and another at the 3-position, which configuration will be observed if the methyl at the 1-position is axial and the methyl at the 3-position is equatorial?

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Choose the Best Answer

A

Cis configuration

B

Trans configuration

C

Both configurations are equivalent

D

No configuration due to steric hindrance

Understanding the Answer

Let's break down why this is correct

Answer

The axial methyl at C‑1 points upward, while the equatorial methyl at C‑3 points downward in the chair form of cyclohexane. Because one substituent is above the ring plane and the other below it, the two methyl groups lie on opposite sides of the molecule. This arrangement is called the trans configuration, meaning the substituents are on opposite faces of the ring. For example, in a 1‑methyl‑3‑methylcyclohexane drawn in the chair conformation, the 1‑methyl is axial up and the 3‑methyl is equatorial down, giving a trans relationship.

Detailed Explanation

In cyclohexane, the axial position points straight up or down along the ring axis. Other options are incorrect because The cis option assumes both methyl groups share the same side of the ring; Saying the configurations are equivalent ignores the fixed orientation of each substituent.

Key Concepts

Cis-Trans Isomerism
Cycloalkane Derivatives
Topic

Cycloalkanes Naming

Difficulty

medium level question

Cognitive Level

understand

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