Major Product of E2 Elimination in a Cyclohexane Ring Using NaOEt/EtOH

In the given reaction, the substrate is a substituted cyclohexyl chloride treated with sodium ethoxide (NaOEt) in ethanol. Sodium ethoxide is a strong base, and under these conditions the reaction proceeds predominantly through an E2 elimination mechanism.

Correct Answer

Correct Option: (A)

Understanding the Reaction Conditions

Sodium ethoxide is a strong base and favors a concerted E2 elimination. In an E2 reaction, the base abstracts a β-hydrogen while the leaving group departs simultaneously. No carbocation intermediate is formed, making the reaction stereospecific.

For cyclohexane systems, an additional stereochemical requirement exists. The leaving group and the β-hydrogen must be arranged in an trans-diaxial (anti-periplanar) orientation. Only when this geometry is available can elimination occur efficiently.

Why Option (A) is the Major Product

After the appropriate chair conformation is considered, the chlorine atom and the adjacent β-hydrogen become trans-diaxial. Elimination therefore occurs across these adjacent carbon atoms, producing the alkene shown in Option (A).

This product satisfies both the stereochemical requirement of the E2 mechanism and the formation of the more stable alkene.

Explanation of Every Option

Option (A)

This is the correct answer. The product is formed by trans-diaxial elimination of H and Cl in the required anti-periplanar arrangement. The resulting alkene is the preferred E2 product under the given reaction conditions.

Option (B)

This option is incorrect because the required trans-diaxial geometry is not satisfied. Formation of this alkene would require elimination from an unfavorable stereochemical arrangement, making it much less likely.

Option (C)

This option is incorrect because it places the double bond at a position that is not formed from the available β-hydrogen in the reactive chair conformation. It does not correspond to the preferred E2 pathway.

Option (D)

This option is also incorrect. Although it represents an alkene, it cannot be obtained directly through the favored trans-diaxial elimination of the given substrate under the reaction conditions.

Concept Behind E2 Elimination in Cyclohexane

Unlike open-chain compounds, cyclohexane derivatives must satisfy a strict conformational requirement for E2 elimination. The leaving group must occupy an axial position, and the β-hydrogen removed by the base must also be axial on the adjacent carbon but oriented anti to the leaving group. This trans-diaxial arrangement ensures proper orbital overlap during the concerted elimination process.

Whenever a suitable chair conformation is available, elimination occurs rapidly to produce the corresponding alkene. Therefore, conformational analysis is often more important than simply applying Zaitsev’s rule.

Final Answer

Correct Option: (A)