Oxymercuration reaction

In the second step, the nucleophilic water molecule attacks the more substituted carbon, liberating the electrons participating in its bond with mercury.

Oxymercuration is very regioselective and is a textbook Markovnikov reaction; ruling out extreme cases, the water nucleophile will always preferentially attack the more substituted carbon, depositing the resultant hydroxy group there.

This phenomenon is explained by examining the three resonance structures of the mercuronium ion formed at the end of the step one.

By inspection of these structures, it is seen that the positive charge of the mercury atom will sometimes reside on the more substituted carbon (approximately 4% of the time).

As illustrated by the second step, the nucleophile cannot attack the carbon from the same face as the mercury ion because of steric hindrance.

With 1-methyl-4-t-butylcyclohexene, oxymercuration yields only one product – still anti addition across the double bond – where water only attacks the more substituted carbon.

In demercuration, the acetoxymercury group is replaced with a hydrogen in a stereochemically insensitive reaction[6] known as reductive elimination.

Oxymercuration reduction is a popular laboratory technique to achieve alkene hydration with Markovnikov selectivity while avoiding carbocation intermediates and thus the rearrangement which can lead to complex product mixtures.

The carbon–mercury structure can undergo spontaneous replacement of the mercury by hydrogen, rather than persisting until a separate reduction step.