Alkyne Reactions: Alkyne Oxymercuration using HgSO4, H2O, H2SO4

Borane (BH3), sodium hydroxide (NaOH) and hydrogen peroxide (H2O2) in the presence of alkynes react to form ketones and aldehydes:

This reaction proceeds with a Markovnikov addition, resulting in the addition of a double bonded O atom on the most substituted carbon. When the carbons are equally substituted, a mixture of ketones is formed:

Markovnikov Addition

Mixture of Ketones

The reaction mechanism is depicted below:

In the first step, electrons from the alkyne bond react with the mercury atom (Hg) from mercuric acetate (HgSO4).

In the second step, water attacks the newly formed mercuronium ion, adding a water functional group to the more substituted carbon.

In the third step, another water molecule deprotonates the newly added water moiety.

In the fourth step, the carbon-carbon double bond breaks, grabbing the proton from H2SO4. As this occurs, the free electrons from the OH group come down and form a double bond with the carbon.

In the fifth step, the negatively charged HSO4- approaches the mercury and causes the electrons from the carbon-mercury bond to be returned to carbon, renewing the carbon-carbon double bond. This forces the electrons from the carbon-oxygen double bond to go back to the oxygen as free electrons.

In the sixth step, the electrons from the alkene bond attack a proton residing on a nearby acid molecule (H3O+). This allows for the free electrons on the oxygen atom to come down and form a double bond, while being deprotonated by a nearby water molecule.

The reagents used for this reaction are HgSO4, H2SO4 or H3O+, and H2O.