See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Syn addition refers to the addition of two groups to the same face of a double bond simultaneously or in a concerted/suprafacial manner. Step 1 - Analyze each reaction: (a) Dihydroxylation with dilute KMnO4: This reaction proceeds via a cyclic manganate ester intermediate, which forces both OH groups to add to the same face of the double bond. This is a classic syn addition. (b) Catalytic hydrogenation with D2 over Ni: Hydrogen (deuterium) adds to the surface of the metal catalyst, and both D atoms are delivered to the same face of the double bond. This is a syn addition. (c) Hydroboration-oxidation with B2H6 then H2O2/NaOH: Hydroboration proceeds via a four-membered cyclic transition state where B and H add simultaneously to the same face of the alkene (syn addition). The subsequent oxidation retains stereochemistry. Overall result is syn addition. (d) Bromination with Br2 in CCl4: This reaction proceeds through a cyclic bromonium ion intermediate. The second bromide ion (Br-) then attacks the bromonium ion from the backside (anti to the bromonium bridge) in an SN2-like fashion. This results in ANTI addition of the two bromine atoms across the double bond, NOT syn addition. Step 2 - Conclusion: Reactions (a), (b), and (c) all involve syn addition. Reaction (d) — bromination via bromonium ion intermediate — involves anti addition, so syn addition does NOT take place in reaction (d). Therefore, the correct answer is D.