oxymercuration

Oxymercuration is a two-stage way to add water across an alkene to make a Markovnikov alcohol — the same net result as acid-catalyzed hydration, but cleaner. It uses mercury(II) acetate, Hg(OAc)2, in water, followed by a reduction with sodium borohydride (NaBH4) that strips the mercury away. The reason chemists reach for it is that it gives the Markovnikov product without the rearrangements that plague the simple acid route.

The trick is that no free carbocation ever forms. The electrophilic mercury adds to the double bond and, rather than leaving an open cation, it bridges across both carbons as a three-membered mercurinium ion — a strained ring that traps the positive charge so it cannot wander or rearrange. Water then attacks this ring from the back side, opening it at the more-substituted carbon (where positive character is greatest), which gives Markovnikov orientation. The mercury, now hanging on the other carbon, is replaced by hydrogen in the NaBH4 step.

Oxymercuration matters as the reliable, rearrangement-free Markovnikov hydration. Pair it in your mind with hydroboration-oxidation, its anti-Markovnikov partner: between the two, you can install a hydroxyl on whichever carbon of an alkene you choose. The chief drawback is practical and modern — mercury salts are toxic and environmentally troublesome, so greener Markovnikov hydrations are often preferred today, but the reaction remains a clean teaching example of bridged-ion control of regiochemistry.