Overall reaction: propene is converted to 2-propanol. Water adds with Markovnikov orientation and the mercury is removed by sodium borohydride.
Oxymercuration–demercuration is a two-step laboratory method for adding water across a carbon–carbon double bond. The result is a Markovnikov alcohol — the hydroxyl group lands on the more substituted carbon — just like acid-catalyzed hydration. What makes the method valuable is a single, decisive advantage: it does not proceed through a free carbocation, so it never gives rearranged products.
1. The reaction adds H–OH across the alkene with Markovnikov orientation.
The net transformation is hydration: one hydrogen and one hydroxyl group are placed on the two carbons of the former double bond. The regiochemistry follows Markovnikov's rule. Treating 1-butene with mercuric acetate in water and then sodium borohydride gives 2-butanol, not 1-butanol, because the OH goes to the more substituted (internal) carbon.
1-Butene to 2-butanol. The hydroxyl is delivered to the more substituted carbon of the alkene.
The two reagent lines matter. Step 1, mercuric acetate [Hg(OAc)2] in water, performs the oxymercuration and installs both the OH and a carbon–mercury bond. Step 2, sodium borohydride (NaBH4), is the demercuration that strips the mercury out and replaces it with hydrogen.
2. Step one builds a bridged mercurinium ion, not a carbocation.
When Hg(OAc)2 ionizes it releases +HgOAc, an electrophile. The alkene's π electrons attack this electrophile, but instead of forming an open cation the mercury bridges both alkene carbons in a three-membered ring. This bridged species is called a mercurinium ion, and it behaves much like the bromonium ion of halogen addition: the positive charge is shared and there is no exposed, planar carbocation.
The alkene is converted to the Markovnikov alcohol. In between sits a bridged mercurinium ion that never lets a free carbocation form.
This bridged intermediate is the mechanistic heart of the reaction. Every downstream advantage — the regioselectivity and the absence of rearrangement — traces back to the fact that charge is locked inside a Hg-bridged ring rather than sitting on a single sp2 carbon.
3. Water opens the ring at the more substituted carbon, setting Markovnikov selectivity.
Although the mercurinium ion is bridged, the positive charge is not shared equally. The more substituted carbon carries more of the partial positive charge because it better stabilizes it, so that carbon is more electrophilic. Water acts as the nucleophile and attacks there, by backside approach, opening the ring. The result is an organomercurial alcohol: OH on the more substituted carbon, Hg on the less substituted carbon.
1-Methylcyclohexene gives 1-methylcyclohexanol. Water opens the mercurinium at the more substituted carbon, so the OH lands on the ring carbon bearing the methyl group.
Because water attacks the bridged ion from the face opposite mercury, the OH and Hg add to opposite faces — a net anti addition. Note, however, that the carbon–mercury stereocenter is later scrambled during the radical demercuration step, so the overall stereochemistry of the product is usually not emphasized in this reaction.
4. Sodium borohydride removes the mercury and delivers hydrogen.
The organomercurial from step one is not the final product; it still carries a carbon–mercury bond. Adding NaBH4 reduces that C–Hg bond, replacing mercury with hydrogen. The overall bookkeeping is now complete: OH on the more substituted carbon (from step one) and H on the less substituted carbon (from this step) — a Markovnikov hydration.
Contrast the Markovnikov product of oxymercuration (2-butanol) with the anti-Markovnikov alcohol that hydroboration would give.
This is exactly the opposite regiochemistry from hydroboration–oxidation, which delivers OH to the less substituted carbon. The two methods are complementary tools: choose oxymercuration for a Markovnikov alcohol, hydroboration for an anti-Markovnikov one.
5. No carbocation means no rearrangement — the key advantage.
Acid-catalyzed hydration forms a true carbocation, which can shift a hydride or an alkyl group to become more stable before water traps it. That gives rearranged, often unexpected products. Oxymercuration avoids this entirely: the bridged mercurinium ion never releases a free carbocation, so there is nothing to rearrange.
3,3-Dimethyl-1-butene gives the unrearranged alcohol 3,3-dimethyl-2-butanol. Acid-catalyzed hydration of the same alkene would form a secondary cation that shifts a methyl group to a tertiary cation, yielding rearranged 2,3-dimethyl-2-butanol.
This example is the classic diagnostic. The neopentyl-type substrate 3,3-dimethyl-1-butene is a rearrangement trap: any free secondary carbocation next to that quaternary carbon will promptly undergo a methyl shift. Oxymercuration returns the clean, unrearranged Markovnikov alcohol, proving no open cation was ever present.
6. Swapping water for an alcohol gives an ether (alkoxymercuration).
The nucleophile that opens the mercurinium ion does not have to be water. If the reaction is run in an alcohol solvent (ROH) instead, the alcohol oxygen attacks the ring and the product, after demercuration, is an ether rather than an alcohol. This variant is called alkoxymercuration–demercuration and follows the same Markovnikov, no-rearrangement logic.
Alkoxymercuration uses an alcohol as the nucleophile; the mechanism is identical to oxymercuration, with ROH replacing H2O to give a Markovnikov ether.
7. Summary
Oxymercuration–demercuration is a reliable, rearrangement-free way to install a Markovnikov alcohol on an alkene. Step one, Hg(OAc)2 in water, forms a bridged mercurinium ion that water opens at the more substituted carbon, setting Markovnikov regiochemistry through a net anti addition. Step two, NaBH4, removes the mercury and delivers hydrogen. Because charge is trapped in the bridged ring and never becomes a free carbocation, the reaction never rearranges — the decisive advantage over acid-catalyzed hydration. Run the same chemistry in an alcohol solvent and you get an ether instead of an alcohol. Pair this method with hydroboration–oxidation, its anti-Markovnikov complement, and you can place a hydroxyl group on either carbon of an alkene at will.
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2-Butanol. The reaction is a Markovnikov hydration, so water opens the mercurinium ion at the more substituted (internal) carbon, placing the OH there and giving the secondary alcohol rather than 1-butanol.
A bridged mercurinium ion. Because the positive charge is shared inside a three-membered Hg ring rather than sitting on a free planar carbocation, there is no open cation to undergo hydride or alkyl shifts, so no rearrangement occurs.
They are complementary. Oxymercuration gives the Markovnikov alcohol (OH on the more substituted carbon), while hydroboration–oxidation gives the anti-Markovnikov alcohol (OH on the less substituted carbon).
The alcohol becomes the nucleophile (alkoxymercuration), so the product is an ether instead of an alcohol. The Markovnikov selectivity and the absence of rearrangement are unchanged.
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