Oxymercuration Demercuration Hg Oac 2 H2o Nabh4 — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Oxymercuration-Demercuration (Hg(OAc)2, H2O/NaBH4) Oxymercuration-Demercuration (OMDM) is a two-step organic reaction that converts alkenes into alcohols by a Markovnikov addition of water across the double bond, without carbocation rearrangements. The first step, oxymercuration, involves the electrophilic addition of mercury(II) acetate to the alkene, followed by nucleophilic attack of water, forming a hydroxylmercurial intermediate. The second step, demercuration, involves the reduction of the carbon-mercury bond with sodium borohydride, replacing mercury with hydrogen. Typically, there are no distinct visual observations (like color changes or precipitates) during the reaction itself, as the intermediates are usually colorless or white solids that are transient. The reaction mixture remains heterogeneous or becomes clear during the process, depending on the solubility of the mercury species. The final product is a colorless liquid alcohol. The overall reaction is generally exothermic and spontaneous, driving the formation of the more stable alcohol product from the alkene. 1. Electrophilic attack of Hg(OAc)2 on the alkene's pi bond to form a cyclic mercurinium ion intermediate (a three-membered ring containing carbon, carbon, and mercury). 2. Nucleophilic attack by water (H2O) on the more substituted carbon of the mercurinium ion (Markovnikov's rule applies), opening the cyclic intermediate. This attack occurs from the face opposite to the mercury atom (anti-addition). 3. Deprotonation of the oxygen atom by a base (another water molecule or acetate) to yield a hydroxylmercurial compound. 4. The demercuration step involves the reduction of the carbon-mercury bond by sodium borohydride (NaBH4) in basic conditions. This is believed to occur via a radical mechanism, replacing the -HgOAc group with a hydrogen atom. Predicting carbocation rearrangements: Unlike acid-catalyzed hydration, OMDM does not involve discrete carbocation intermediates, so rearrangements do not occur. Incorrectly applying Markovnikov's rule: Ensure the -OH group adds to the more substituted carbon. Confusing with hydroboration-oxidation: OMDM is Markovnikov hydration, while hydroboration-oxidation is anti-Markovnikov. Incorrect stereochemical predictions: While the initial addition of Hg and OH is anti, the final H replacement can lead to racemization if a new chiral center is formed from an achiral alkene. Forgetting the two-step reagent system: Hg(OAc)2, H2O followed by NaBH4, NaOH.