Oxymercuration-Demercuration: Hydration without Rearrangement
Oxymercuration-Demercuration is a two-step reaction sequence used to convert alkenes into alcohols. It essentially adds water across a double bond following the Markovnikov rule. Its primary advantage over acid-catalyzed hydration is that it proceeds without carbocation rearrangement.
1. General Reaction
The alkene is treated with mercuric acetate in aqueous THF, followed by reduction with sodium borohydride.
Conditions:
- Step 1 (Oxymercuration): Mercuric Acetate ($Hg(OAc)_2$) + Water ($H_2O$).
- Step 2 (Demercuration): Sodium Borohydride ($NaBH_4$) in basic medium.
- Net Result: Addition of $H-OH$ across the pi bond.
2. Detailed Mechanism
The absence of rearrangement is due to the formation of a cyclic intermediate.
Step 1: Formation of Mercurinium Ion
The electrophilic Mercury atom attacks the pi-bond. Instead of forming an open carbocation, it forms a three-membered cyclic cation called the Mercurinium Ion.
Step 2: Nucleophilic Attack (Anti-Addition)
Water (the nucleophile) attacks the more substituted carbon of the ring. This is because the more substituted carbon bears a greater share of the positive charge (Markovnikov-like transition state). The attack occurs from the side opposite to the mercury bridge (Anti-attack).
Step 3: Demercuration
The organomercury intermediate is treated with $NaBH_4$. The mechanism involves the replacement of the $HgOAc$ group with a Hydrogen atom ($H$).
3. Stereochemistry
Anti-Addition Step
The initial addition of the $-OH$ group and the $-HgOAc$ group is strictly Anti. However, the subsequent demercuration step is not always stereospecific (it involves radicals), leading to a mixture of stereoisomers where the H replaces Hg.
4. Crucial Comparison: Why use this method?
Consider the hydration of 3,3-Dimethyl-1-butene ($t-Bu-CH=CH_2$).
| Method | Mechanism Characteristic | Product |
|---|---|---|
| Acid Catalyzed Hydration ($H_3O^+$) | Carbocation Rearrangement ($2^\circ \to 3^\circ$) | 2,3-Dimethyl-2-butanol (Rearranged) |
| Oxymercuration-Demercuration | No Rearrangement | 3,3-Dimethyl-2-butanol (Unrearranged) |
| Hydroboration-Oxidation | Anti-Markovnikov Addition | 3,3-Dimethyl-1-butanol |
5. Solvomercuration
If an alcohol ($R'OH$) is used as the solvent instead of water in the first step, the nucleophile attacking the mercurinium ion is the alcohol. This results in the formation of an Ether ($R-CH(OR')-CH_3$). This variation is called Alkoxymercuration-Demercuration.
Oxymercuration Quiz
Test your concepts on Markovnikov hydration. 10 MCQs with explanations.
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