Oxymercuration followed by demercuration is a versatile and widely-used laboratory method for the synthesis of
alcohols from
alkenes, two of the commonest types of organic chemical. Such a synthesis involves adding
water across the double bond in the alkene, and although it is sometimes possible to simply react the alkene with water itself (an acid-catalysed process), it is more often impossible, so other methods are required. Oxymercuration-demercuration is a general method for the synthesis of what may be called the "Markovnikov alcohol", in which the negative species OH
- adds to the more substituted carbon in an unsymmetrical alkene, according to
Markovnikov's rule.
Oxymercuration
The reaction is directed by the formation of a carbon-mercury bond, which is weak and thus easily broken. The alkene (in a suitable solvent, e.g. an ether) is first reacted with a source of mercury(II) ions, often mercury acetate, Hg(OAc)2. The alkene attacks the mercury and replaces one of its acetate groups, forming what is called a mercurinium ion. When this is attacked by water, the bond between mercury and the less substituted carbon is broken. The product of this is a relatively stable compound in which the alkene's double bond has been added across by OH on the more substituted carbon and by HgOAc on the less substituted carbon.
Demercuration
However we simply want an alcohol, not an organomercury compound, so the mercury is removed by the addition of a suitable base, e.g. sodium borohydride, NaBH4. A hydrogen atom from the base replaces the HgOAc group, and now we have our alcohol - we have added across the double bond an OH group to the more substituted carbon, and an H atom to the less substituted, so this is hydration, an addition of H2O.
For example, consider the hydration of 1-butene, H2C=CHCH2CH3, using this technique. The terminal carbon is the least substituted, so the product of oxymercuration is a molecule in which the terminal carbon is bonded to mercury and its neighbour is bonded to the hydroxy group: this is AcOHg-CH2CHOHCH2CH3. Demercuration then replaces the mercury with hydrogen to give the secondary alcohol 2-butanol, CH3CHOHCH2CH3.
Oxymercuration-demercuration is among the most useful laboratory-scale syntheses of alcohols from alkenes. In giving the "Markovnikov alcohol", it is complementary to the method known as hydroboration-oxidation, which gives the "anti-Markovnikov" alcohol in which the hydroxy group instead bonds to the less substituted carbon atom.