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H₂O Addition

The addition of water to a C=C double bond yields an alcohol, being the opposite reaction to the dehydration of the latter.

Alcohol ⇄ olefin equilibrium: concentrated H₂SO₄ and heat dehydrate the alcohol; dilute H₂SO₄ hydrates the olefin

The electrophilic addition of water to olefins is thus reversible. The sense of the reaction depends on the amount of water in the medium.

The addition of water to olefins is carried out in acidic medium and the reaction proceeds through carbocations. As it is usual when carbocations are involved, the major product might be unexpected.

Hydration of 3-methyl-1-butene in acid: the expected product, 3-methyl-2-butanol, is minor; the major one is 2-methyl-2-butanol, formed by carbocation rearrangement

There exists an extremely interesting reaction that renders the formal addition of water to olefins but avoiding carbocations: oxymercuration-demercuration.

3-Methyl-1-butene with H₂O/H⁺ gives the rearranged alcohol; oxymercuration-demercuration ((AcO)₂Hg, NaBH₄, H₂O/HO⁻) gives 3-methyl-2-butanol without rearrangement

The mechanism resembles the opening of a bromonium ion or that of a protonated epoxide. The true carbocation is never formed and rearrangements are hence avoided.

The first step is the attack of the π cloud to the metal, a very electron-deficient centre, leading to a three-membered cyclic organomercury compound.

Attack of the alkene π cloud on mercury acetate: a three-membered cyclic mercurinium ion is formed

The organomercurial formed resembles a bromonium ion or a protonated epoxide; it is highly strained and opens in the same way, i.e. leading to the least unstable quasi-carbocation by an SN1-like mechanism.

Opening of the mercurinium ion by acetate: attack at the carbon that best supports the charge (secondary quasi-carbocation) is much faster than at the primary one

Finally, sodium borohydride breaks up the C-Hg bond and the metal is taken over by a hydrogen. The base saponifies (hydrolyzes) the acetate ester to render the alcohol.