Alkenes do not undergo hydration upon treatment with water even at very high temperatures.  There are two approaches to hydrating alkenes:
1. Reaction of the alkene with water in the presence of a strong-acid catalyst.
2. Using an indirect method, such as oxymercuration-reduction, also known as oxymercuration-demercuration or, simply, oxymercuration.

net reaction:

The protocol is carried out in two stages:
Stage 1: Oxymercuration
The alkene is treated with aqueous mercury (II) acetate, Hg(OAc)2, which converts the alkene into an organomercury compound (see organometallic compound).

Stage 2: Reduction (Demercuration)
The organomercury compound is treated with a reducing agent, usually NaBH4, which converts the organomercury compound into an alcohol.

Stage 1:

The reaction is an anti addition.

Stage 2:

Stage 1 + Stage 2 =

The net reaction is a Markovnikov addition:

see also hydroboration-oxidation

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