Q.15 Which one of the following is the major product of the hydrogenation reaction given below?

Under catalytic hydrogenation with H₂/Pd‑C in ethanol, only the isolated C=C double bond of the bicyclic acetal ring is reduced; the benzene ring and other saturated centers remain unchanged, giving product (A) as the major product.

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Introduction

Predicting the major product of a hydrogenation reaction with H₂/Pd‑C in ethanol is a classic problem used in JEE and JAM organic chemistry papers to test understanding of chemoselectivity and stereochemistry in catalytic reductions. Such questions focus on which multiple bonds actually get reduced and what configuration the new hydrogen atoms adopt at the saturated centers. Here, the substrate is an aromatic–alicyclic system containing a benzene ring fused to a bicyclic acetal with one C=C double bond and a primary alcohol side chain.

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Step‑by‑step analysis of the reaction

• Nature of reagent (H₂/Pd‑C):

  • Palladium on carbon is a heterogeneous catalyst that adds molecular hydrogen only to “accessible” C=C or C≡C bonds under mild conditions, while generally leaving the aromatic benzene ring intact unless very forcing conditions are used.

  • It does not normally cleave acetals or reduce stable primary alcohols; these functional groups survive unaltered during catalytic hydrogenation.

• Reactive site in the given substrate:

  • The benzene ring (anisole part) is aromatic and therefore far less reactive toward hydrogenation than an isolated double bond; it remains unchanged here.

  • The bicyclic acetal ring carries a single non‑aromatic C=C double bond, which is the most susceptible site to reduction by H₂/Pd‑C.

  • The CH₂OH substituent is a primary alcohol and is not reduced under these conditions.

• Stereochemistry of hydrogen addition:

  • On a rigid bicyclic system, hydrogen adds from the less hindered face of the double bond, producing a specific stereochemistry at the newly formed chiral centers (syn‑addition on the metal surface).

  • In the problem, the correct option must represent saturation of that C=C with hydrogens oriented from the less hindered side, while retaining all other functional groups.

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Why option (A) is correct

Option (A) shows:

• The benzene ring with a methoxy substituent unchanged (no loss of aromaticity).

• The bicyclic acetal ring with the former C=C now converted into a single bond, creating a saturated ring junction. The added hydrogen is drawn on the less hindered face (as given in the standard solution of such JAM‑style problems).

• The CH₂OH substituent is intact and remains as a primary alcohol attached to the same carbon as in the starting material.

This matches the expected chemoselective hydrogenation: only the isolated C=C is reduced, all other parts of the molecule are preserved, and the stereochemistry is consistent with syn‑addition on the less hindered face. Hence, (A) represents the major product.

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Why options (B), (C) and (D) are incorrect

Option (B)

• In (B), the bicyclic acetal ring appears with an altered relative stereochemistry at the ring junction and/or CH₂OH‑bearing carbon compared with the product expected from syn‑addition.

• Such a configuration would require either anti‑addition of hydrogen or post‑hydrogenation epimerization at a saturated stereocenter, neither of which is favored under mild H₂/Pd‑C conditions in ethanol.

• Therefore, (B) does not correctly represent the stereochemical outcome of catalytic hydrogenation and is not the major product.

Option (C)

• Option (C) shows the ring‑junction center saturated, but the CH₂OH group is replaced by CH₃, indicating reduction of a primary alcohol to a methyl group.

• H₂/Pd‑C does not reduce primary alcohols directly to alkanes; such a deoxygenation would require completely different reagents (e.g., HI/red P, or multi‑step sequences).

• Since the problem specifies only H₂/Pd‑C in ethanol, transformation of CH₂OH to CH₃ is unrealistic, making (C) incorrect.

Option (D)

• Option (D) keeps the benzene ring and acetal ring but again replaces the CH₂OH substituent with a CH₃ group, which is not achievable with simple catalytic hydrogenation.

• Moreover, the double bond in the bicyclic ring may not even be fully saturated in this drawing (depending on the exact figure in the exam), contradicting the primary effect of H₂/Pd‑C on an isolated C=C bond.

• Thus, (D) fails both on functional‑group change and on the pattern of unsaturation, so it cannot be the major product.

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Key takeaways for exams

• Under H₂/Pd‑C in ethanol, expect:

  • Reduction of isolated C=C/C≡C bonds.

  • Preservation of aromatic rings, acetals and alcohols under mild conditions.

  • Syn‑addition from the less hindered face on rigid ring systems, giving a predictable stereochemistry.

• For this problem, the only option that:

  • Saturates the non‑aromatic double bond,

  • Retains the benzene ring and CH₂OH group, and

  • Shows the correct stereochemistry at the new stereocenter
    is option (A), so (A) is the major product.