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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:

Alkyne Reactions: Alkyne Oxymercuration using HgSO4, H2O, H2SO4 image2.png

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

Alkyne Reactions: Alkyne Oxymercuration using HgSO4, H2O, H2SO4 image2.png

Mixture of Ketones

Alkyne Reactions: Alkyne Oxymercuration using HgSO4, H2O, H2SO4 image3.png

The reaction mechanism is depicted below:

Alkyne Reactions: Alkyne Oxymercuration using HgSO4, H2O, H2SO4 image1.png

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.