Organic Chemistry · Alkenes: Reactions and Synthesis
Hydration of Alkenes: Addition of H2O by Oxymercuration
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hydration Addition of water across a multiple bond to form an alcohol Full entry → means adding water across a double bond — the net result is an alcohol. oxymercuration Addition of mercury(II) and water across the alkene Full entry →–demercuration Replacement of the C–Hg bond by a C–H bond using NaBH4 Full entry → is a two-step way to hydrate an alkene:
- Oxymercuration: the alkene reacts with mercuric acetate, Hg(OAc)2, in water (usually with THF as a cosolvent). The mercury adds to one carbon and water adds to the other, forming an organomercury alcohol.
- Demercuration: sodium borohydride (NaBH4) replaces the carbon–mercury bond with a carbon–hydrogen bond.
The net transformation is the Markovnikov addition Addition where the electrophile's "positive end" goes to the more substituted carbon Full entry → of water:
alkene + H2O 1. Hg(OAc)2; 2. NaBH4⟶ alcohol
The hydroxyl group lands on the more substituted carbon of the double bond — the same regiochemistry as acid-catalyzed hydration — but the reaction goes through a cyclic mercurinium ion A positively charged three-membered ring with mercury bridging two carbons Full entry → rather than a free carbocation, so no rearrangements occur. This makes oxymercuration–demercuration the most reliable way to make Markovnikov alcohols from alkenes.
Why this matters
Acid-catalyzed hydration (water plus sulfuric acid) also gives Markovnikov alcohols, but it passes through a carbocation, and carbocations can rearrange by hydride or methyl shifts — especially when a tertiary cation is not directly available. Oxymercuration–demercuration gives the same alcohol without ever forming a free carbocation, so the product you predict from the alkene's structure is the product you actually get. This predictability is essential in synthesis, where a single misplaced methyl group changes the whole molecule. It also introduces you to organometallic intermediates (C–Hg bonds), a theme that recurs in later chapters on organometallic reagents.
The college version
Core Concepts
Step 1: Oxymercuration (the mechanism)
Mercuric acetate is an electrophile: the mercury is electron-poor. The alkene's π electrons attack the mercury, and the acetate leaves, forming a mercurinium ion — a positively charged three-membered ring with mercury bridging the two alkene carbons. This is exactly analogous to the bromonium ion from halogenation. Water then attacks the more substituted carbon of the mercurinium ion from the back side, and deprotonation gives an organomercury alcohol:
alkene + Hg(OAc)2 + H2O ⟶ HO-alkene-HgOAc + HOAc
Because the ring is bridged, the same anti-addition stereochemistry seen in halogenation applies: water and mercury add from opposite faces.
Step 2: Demercuration
Sodium borohydride, NaBH4, is a source of hydride. It reduces the carbon–mercury bond to a carbon–hydrogen bond, replacing the –HgOAc group with a hydrogen atom. The mechanism is a radical process on the mercury, but the practical takeaway is simple: the C–Hg bond becomes a C–H bond, and the carbon keeps its stereochemistry. The alcohol survives because borohydride does not reduce alcohols or alkenes under these mild conditions.
Regiochemistry and why no rearrangements occur
The hydroxyl group goes to the more substituted carbon because that carbon bears more positive charge in the mercurinium ion (it is more carbocation-like), so water attacks there. The hydrogen from demercuration lands on the less substituted carbon. The net product is the Markovnikov alcohol — but with a crucial difference from acid-catalyzed hydration: no carbocation ever forms, so hydride and methyl shifts cannot scramble the carbon skeleton. Whenever an alkene could rearrange under acidic conditions, oxymercuration–demercuration is the tool of choice.
Practical notes
Mercuric acetate is a mercury(II) salt; mercury compounds are toxic, so this reaction is performed with care in a fume hood, and the borohydride step converts the mercury to metallic mercury, which is removed by filtration. THF is added because it dissolves both the water-soluble mercury salt and the organic alkene, keeping the mixture homogeneous.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Oxymercuration–demercuration | Acid-catalyzed hydration | Same Markovnikov regiochemistry, but oxymercuration never forms a carbocation, so no rearrangements |
| Oxymercuration (Markovnikov) | Hydroboration–oxidation (anti-Markovnikov) | The –OH goes to the more substituted carbon in oxymercuration and to the less substituted carbon in hydroboration–oxidation |
| Mercurinium ion | Carbocation | The bridged mercurinium ion cannot rearrange; a carbocation can |
| Hg(OAc)2 step | NaBH4 step | The mercury step adds Hg and –OH; the borohydride step only replaces C–Hg with C–H |
| Hydration of alkene | Hydration of alkyne | Alkynes need different conditions (often Hg2+ catalysis) and give carbonyl compounds, not alcohols |
| "Mercury is toxic" as a lab instruction | A clinical statement | This is a general laboratory-safety principle; mercury poisoning treatment is outside the scope of this study guide |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of the double bond as two friends holding hands. A mercury "king" comes along and sits between them, holding both their hands (that is the mercurinium ring). A water balloon then attaches to the friend who has more friends already (the more substituted carbon). Finally, a helper molecule (borohydride) quietly replaces the mercury king with a plain hydrogen. The net effect: the double bond turned into an –OH on the busier carbon and an H on the other — and because the mercury king never let go, the friends never had a chance to shuffle their positions (no rearrangements).
Worked example
Example 1: 1-Methylcyclohexene by oxymercuration–demercuration
Predict the product of 1-methylcyclohexene treated with Hg(OAc)2 in water/THF, then NaBH4.
Reasoning. The mercurinium ion forms across C1–C2. C1 is the more substituted carbon (it bears the methyl), so water attacks C1 and the –OH lands there. Demercuration replaces the C–Hg bond at C2 with a C–H bond.
Answer. The product is 1-methylcyclohexan-1-ol (SMILES: CC1(O)CCCCC1) — a tertiary alcohol. Note that no rearrangement is possible here, and none occurs.
Example 2: The rearrangement trap: 3,3-dimethyl-1-butene
3,3-Dimethyl-1-butene (SMILES: C=CC(C)(C)C) has a terminal double bond. Predict its product under (a) acid-catalyzed hydration and (b) oxymercuration–demercuration.
Reasoning. Under acidic conditions, protonation of the terminal alkene would give a secondary carbocation at C2 — but a methyl shift converts it to a more stable tertiary cation, so the product rearranges. In oxymercuration, the mercurinium ion is bridged and cannot rearrange; water attacks the more substituted carbon (C2), and demercuration adds H to C1.
Answer. (a) Acid-catalyzed hydration gives the rearranged product 2,3-dimethyl-2-butanol (SMILES: CC(C)C(C)(C)O) via a 1,2-methyl shift. (b) Oxymercuration–demercuration gives the unrearranged product 3,3-dimethyl-2-butanol (SMILES: CC(O)C(C)(C)C). Same formula, different carbon skeleton — exactly why the method matters.
Example 3: Stoichiometry with dimensional analysis
What is the theoretical yield (in grams) of 1-methylcyclohexan-1-ol when 2.00 g of 1-methylcyclohexene (molar mass 96.17 g/mol) is hydrated by oxymercuration–demercuration?
Formula first. The reaction is 1:1, so moles of product equal moles of alkene. Moles from mass:
n = mM
then theoretical mass of product:
mproduct = n × Mproduct
Substitution. Moles of alkene: n = 2.00 g / 96.17 g mol-1 = 0.0208 mol. The product 1-methylcyclohexan-1-ol, C7H14O, has molar mass M = 7(12.01) + 14(1.008) + 16.00 = 114.18 g/mol:
mproduct = 0.0208 mol × 114.18 gmol = 2.37 g
Answer. The theoretical yield is 2.37 g; the isolated (actual) yield will be lower after workup and purification.
Key takeaways
- Oxymercuration–demercuration is a two-step hydration: Hg(OAc)2/H2O then NaBH4.
- Net result: Markovnikov alcohol — the –OH goes to the more substituted carbon.
- The intermediate is a cyclic mercurinium ion, so no rearrangements occur (unlike acid-catalyzed hydration).
- Addition is anti, though the final stereochemistry is often of little practical concern because a C–H bond forms in step 2.
- NaBH4 replaces C–Hg with C–H; the alcohol group is untouched.
- This is the preferred method for Markovnikov alcohols from alkenes that might otherwise rearrange (e.g., 3,3-dimethyl-1-butene).
- Mercury compounds are toxic: work in a fume hood with gloves; filter off the metallic mercury formed during demercuration.
- Compare: oxymercuration gives Markovnikov alcohols; hydroboration–oxidation (next topic) gives anti-Markovnikov alcohols; both avoid rearrangements.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What are the two steps of oxymercuration–demercuration, and what does each do?
Show answer
(1) Oxymercuration: Hg(OAc)2 and water add across the alkene to form an organomercury alcohol. (2) Demercuration: NaBH4 replaces the C–Hg bond with a C–H bond, giving the alcohol.
Where does the –OH end up on an unsymmetrical alkene?
Show answer
The –OH goes to the more substituted carbon (Markovnikov regiochemistry).
Why does oxymercuration–demercuration avoid carbocation rearrangements?
Show answer
The intermediate is a cyclic mercurinium ion that holds the two carbons in a bridged ring; no free carbocation exists, so hydride/methyl shifts are impossible.
What product does 3,3-dimethyl-1-butene give under oxymercuration–demercuration, and what would acid-catalyzed hydration give instead?
Show answer
Oxymercuration gives 3,3-dimethyl-2-butanol (unrearranged). Acid-catalyzed hydration gives 2,3-dimethyl-2-butanol (rearranged by a 1,2-methyl shift).
What does NaBH4 do to the organomercury intermediate, and why is the alcohol unaffected?
Show answer
NaBH4 replaces the C–Hg bond with a C–H bond (demercuration). It does not reduce the alcohol group under these mild conditions, so the –OH survives.
How many grams of 1-methylcyclohexene are needed to make 5.00 g of 1-methylcyclohexan-1-ol (assume 100% yield)?
Show answer
Product moles: 5.00 g / 114.18 g/mol = 0.0438 mol. Alkene needed: same moles, mass = 0.0438 mol × 96.17 g/mol = 4.21 g.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- hydration
- Addition of water across a multiple bond to form an alcohol
- oxymercuration
- Addition of mercury(II) and water across the alkene
- demercuration
- Replacement of the C–Hg bond by a C–H bond using NaBH4
- mercurinium ion
- A positively charged three-membered ring with mercury bridging two carbons
- Markovnikov addition
- Addition where the electrophile's "positive end" goes to the more substituted carbon
- organomercury compound
- A molecule with a carbon–mercury bond
Sources & references
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