Oxymercuration Reactions: Mercuric Acetate, Water

There are no rearrangements since carbocations are not generated during this phase. The reaction is an anti addition, and it follows Markovnikov’s rule (hydroxyl group is always added to the carbon that is more substituted). When oxymercuration is followed by reductive demercuration, an oxymercuration–demercuration or oxymercuration–reduction reaction reaction occurs. In practice, the reaction is nearly always employed instead of oxymercuration.

Mechanism:

Oxymercuration can be broken down into three phases (the whole process is sometimes termed as deoxymercuration). The nucleophilic double bond attacks the mercury ion in the first step, ejecting an acetoxy group. The mercury ion’s electron pair then attacks a carbon in the double bond, resulting in a mercuronium ion with a positive charge on the mercury atom. The electrons from the double bond’s highest occupied molecular orbital are transferred to mercury’s vacant 6s orbital, while those from the double bond’s dxz (or dyz) orbital are given to mercury’s lowest unoccupied molecular orbital.

In the second step, the nucleophilic water molecule attacks the more substituted carbon, releasing the electrons involved in the mercury bond. The electrons collide with the mercury ion, neutralising it. The oxygen in the water molecule is now positively charged.

The negatively charged acetoxy ion that was expelled in the first step attacks a hydrogen of the water group in the third step, forming the waste product HOAc. The two electrons in the oxygen-attacked hydrogen bond collapse into the oxygen, neutralising its charge and producing the final alcohol product.

Hydration of Alkenes: Oxymercuration:

The reaction of an alkene with mercury(II) acetate in aqueous THF, followed by reduction with sodium borohydride, is known as oxymercuration. The end result is alcohol.It is critical to recognise the similarities between bromination and oxymercuration mechanisms. Recognizing these parallels allows you to reduce the amount of information you need to remember.

Mercuric acetate, or mercury(II) acetate, to use its preferred IUPAC name, is written as Hg(OAc)2; by comparing this formula to Hg(O2CCH3)2, you can equate Ac with OCCH3. In fact, Ac is an abbreviation for the acetyl group, which has the structure shown below, as are other similar abbreviations.

The act of adding electrophilic hydrogen from a non-nucleophilic strong acid (a reusable catalyst, examples of which include sulfuric and phosphoric acid) and applying appropriate temperatures to break the alkene’s double bond is known as electrophilic hydration. After forming a carbocation, water bonds with it to form a  1º, 2º, or 3º  alcohol on the alkane.

Oxymercuration-reduction:

In practice, the mercury adduct product of the oxymercuration reaction is almost always demercurated by treating it with sodium borohydride (NaBH4) in aqueous base. Demercuration involves the replacement of the acetoxymercury group with a hydrogen in a stereochemically insensitive reaction known as reductive elimination. An oxymercuration–reduction reaction occurs when oxymercuration is followed immediately by demercuration.As a result, the net addition of water across the double bond is the oxymercuration-reduction reaction. The demercuration step scrambles any stereochemistry established by the oxymercuration step, so that the hydrogen and hydroxy groups can be cis or trans from each other. Oxymercuration reduction is a popular laboratory technique for achieving Markovnikov selectivity in alkene hydration while avoiding carbocation intermediates and thus the rearrangement that can result in complex product mixtures.

Stereochemistry:

Oxymercuration is a very regioselective Markovnikov reaction; with the exception of extreme cases, the water nucleophile will always attack the more substituted carbon, depositing the resulting hydroxyl group there. Examining the three resonance structures of the mercuronium ion formed at the end of step one explains this phenomenon.An examination of these structures reveals that the positive charge of the mercury atom will occasionally reside on the more substituted carbon (approximately 4 percent of the time). This results in the formation of a temporary tertiary carbocation, which is a highly reactive electrophile. At this point, the nucleophile will attack the mercuronium ion. As a result, the nucleophile attacks the more substituted carbon, which has a more positive character than the less substituted carbon.In terms of stereochemistry, oxymercuration is an anti addition. Because of steric hindrance, the nucleophile cannot attack the carbon from the same face as the mercury ion, as shown in the second step. Simply put, there isn’t enough space on that side of the molecule to accommodate both a mercury ion and the attacking nucleophile. As a result, when free rotation is not possible, the hydroxyl and acetoxymercuric groups are always trans to each other.

Conclusion:

The reaction of an alkene with water to add hydroxyl and mercury groups is not the only example of oxymercuration. The carbon–mercury structure can be replaced by hydrogen spontaneously rather than waiting for a separate reduction step. As a result of this, mercury acts as a Lewis acid catalyst. Using an alkyne instead of an alkene, for example, produces an enol, which tautomerizes into a ketone. When you use alcohol instead of water, you get ether. Markovnikov’s rule is followed in both cases.