Q.59 The number of axial C-H bond(s) in the major product (P) of the given reaction is
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The major product is a tertiary alcohol in which the tert‑butyl group is locked equatorial and only two axial C–H bonds remain on the cyclohexane ring.
Mechanism of the reaction
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The starting compound is 4‑tert‑butylcyclohexanone, which adopts a chair conformation with the bulky tert‑butyl group equatorial to minimize 1,3‑diaxial strain.
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MeMgBr, a Grignard reagent, attacks the carbonyl carbon to give, after aqueous work‑up, a tertiary alcohol where the new \ceCH3 and \ceOH substituents occupy positions on C‑1 and C‑4 of the ring.
Stereochemistry and conformation
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In such substituted cyclohexanones, nucleophiles generally attack from the less hindered face; with an equatorial tert‑butyl group, axial attack at C‑1 is favored, giving the OH group equatorial in the alcohol product.
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The product thus has tert‑butyl and OH groups equatorial in the preferred chair; the small methyl group ends up axial because that conformation minimizes overall steric strain.
Counting axial C–H bonds
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In the final chair conformation, there are three axial positions not occupied by substituents: two of these bear hydrogens and one bears the axial methyl group.
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Therefore, the number of axial C–H bonds on the ring carbons is 2 (each at an axial CH position).
