Oxymercuration of Alkenes: Step-by-Step Mechanism

In organic chemistry 1, there are typically two ways (exluding acid-catalyzed hydration reactions that can be problematic) to hydrate an alkene; oxymercuration and hydroboration. This post will focus on the mechanism of oxymercuration.

Oxymercuration of an Alkene

This process involves the addition of mercuric acetate (Hg(OAc)₂) and water to an alkene, resulting in the formation of an organomercury intermediate which is subsequently hydrolyzed to yield an alcohol. The mechanism can be broken down into several key steps:

Step 1: Electrophilic Attack

To start the reaction, we begin with mercuric acetate. The simplest theory on how this works is that the mercuric acetate slightly dissociates to give a positively charged species that can be attacked by the π electrons of the alkene. Once the alkene attacks the mercury center, a mercurinium ion is formed. Take note of this particular part of the reaction. We have seen something similar before when halogenating an alkene; think of the bromination of an alkene and the bromonium ion that is subsequently formed. The same consequences from the bromonium ion will come into play here. Expert tip: many reactions in organic have similarities and you can often expect these to give similar properties. The mercurinium ion intermediate is a three-membered ring, where the mercury atom is positively charged and bonded to two carbons of the alkene. Take a look below.

Illustration of the oxymercuration process, highlighting the formation of the mercurinium ion. Mercury(II) acetate ionizes to the electrophilic +HgOAc, which bridges the alkene π bond, resulting in a mercurinium ion intermediate with cyclohexene.

Step 2: Nucleophilic Attack

Following the formation of the mercurinium ion, water (acting as a nucleophile) attacks the more substituted carbon of the cyclic intermediate. Important note: an alcohol can also act as a nucleophile here and the product would then be an ether. This regioselectivity occurs due to the stability of the more substituted carbocation that forms after the opening of the ring, following Markovnikov's rule. Take a look below.

Oxymercuration Process: Step 2 - In this stage, water attacks the mercurinium ion from the face opposite to the Hg atom. For cyclohexene, water approaches anti to Hg, resulting in a trans arrangement of Hg and OH2. In 1-methylcyclohexene, regioselectivity occurs as water targets the more substituted carbon, following Markovnikov's rule, without rearrangement.

Step 3: Deprotonation

After the nucleophilic attack, a proton is removed from the newly formed alcohol, resulting in the formation of a stable alcohol product. This step typically involves the use of a base (often the acetate ion from the initial mercuric acetate) to facilitate the deprotonation process.

Step 4: Hydrolysis of the Organomercury Intermediate

The final product is an alcohol, and the organomercury compound can be further treated with sodium borohydride (NaBH4) or another reducing agent to remove the mercury atom, yielding the final alcohol without any mercury contamination.

Shortened Description of Oxymercuration Steps 3-4 In the oxidation-reduction sequence of oxymercuration, acetate ion deprotonates the oxonium ion to form a neutral organomercury alcohol intermediate. NaBH₄ then replaces the mercury group with hydrogen, resulting in 1-methylcyclohexanol with Markovnikov orientation and no carbocation rearrangement.

Conclusion

Oxymercuration is an effective method for the hydration of alkenes, providing a straightforward route to synthesize alcohols while adhering to Markovnikov's rule. The use of mercuric acetate allows for selective addition, and subsequent reduction effectively removes mercury, resulting in a clean alcohol product.