Q.27 The major product in the following reaction is
The major product is option (A), the alkene with an equatorial double bond and the isopropyl group trans (dashed) to it.
Reaction overview
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The substrate is a substituted cyclohexyl chloride treated with NaOEt/EtOH, a strong base and poor nucleophile that favors E2 elimination.
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In E2 on a cyclohexane ring, the base must abstract a β‑hydrogen that is anti‑periplanar to the leaving group, which in chair form means both must be axial.
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The conformation that places Cl axial also forces one specific β‑H axial; elimination of this pair gives a single, stereodefined alkene, matching structure (A).
Why option (A) is correct
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In the reactive chair, the chlorine is axial, and there is exactly one anti‑periplanar β‑H on the adjacent carbon, also axial.
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E2 abstraction of this axial β‑H by ethoxide, with simultaneous loss of Cl⁻, forms a double bond between the two carbons and preserves the configuration (dashed) of the isopropyl substituent, giving the trans‑alkene shown in (A).
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This product is both conformationally allowed (via axial–axial alignment) and relatively more substituted, so it is the Zaitsev alkene and hence the major product.
Why option (B) is incorrect
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Structure (B) corresponds to substitution (retention of sp³ carbon with a new group) rather than elimination, but NaOEt/EtOH under these sterically hindered secondary conditions strongly favors E2 over SN2.
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Moreover, forming (B) would require backside attack at a sterically congested axial center on a cyclohexane, which is disfavored compared to the concerted E2 pathway.
Why option (C) is incorrect
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Option (C) shows an alkene but with the stereochemistry of the isopropyl group inverted relative to that obtained from axial–axial elimination.
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To form (C), the system would need a different β‑H–Cl alignment that is not anti‑periplanar in any low‑energy chair, so this structure cannot be the direct E2 product and would not be formed in significant amount.
Why option (D) is incorrect
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Option (D) places the double bond at a different position on the ring (a less substituted alkene), which would arise from abstraction of a non‑axial β‑H or from a conformation not providing anti‑periplanar alignment.
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Because only the axial β‑H anti to axial Cl reacts efficiently in E2 on cyclohexane, formation of this less substituted alkene is both geometrically disfavored and thermodynamically less stable, so it is not the major product.
SEO‑friendly introduction
Finding the major product in NaOEt/EtOH elimination of substituted cyclohexyl chloride is a high‑yield concept for organic chemistry MCQs in competitive exams.
This detailed explanation shows how E2 anti‑periplanar geometry on a cyclohexane ring controls β‑hydrogen abstraction, stereochemistry and alkene position, and why option (A) is the correct major product while the other options are eliminated.
