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Organic Chemistry 1 · Alkene and Alkyne Chemistry

Hydration Without Rearrangements

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

Adding water (H and OH) across an alkene to make an alcohol can be done three ways. Acid-catalyzed hydration goes through a carbocation and can rearrange. adds water Markovnikov-style (OH to the more substituted carbon) through a bridged , so no rearrangements occur. adds water anti-Markovnikov (OH to the less substituted carbon) with , proceeding through intermediates and no carbocations. Together these two methods let you install an OH on either carbon of an alkene, both without rearrangements.

Why this matters

These two methods are workhorse tools in medicinal chemistry for installing an alcohol on a chosen carbon of a double bond without scrambling the carbon skeleton — critical when a drug's activity depends on a precise structure. (Mercury reagents are highly toxic, and hydrogen peroxide/borane are hazardous; any laboratory use must follow approved institutional safety documentation. This topic is conceptual only.)

The college version

1. Oxymercuration–Demercuration (Markovnikov, No Rearrangements)

In oxymercuration, the alkene reacts with mercuric acetate, Hg(OAc)₂, in aqueous THF. The π bond attacks the mercury to form a cyclic mercurinium ion — a three-membered ring with Hg bridging the two carbons, positive charge on Hg. Because this is a bridged cation rather than an open carbocation, no rearrangement is possible. Water attacks the more substituted carbon (which carries more positive character), opening the ring to give an organomercury alcohol. In demercuration, sodium borohydride (NaBH₄) replaces the HgOAc group with hydrogen. Net result: Markovnikov addition of water (H to the less substituted carbon, OH to the more substituted carbon), no rearrangements.

2. Hydroboration–Oxidation (Anti-Markovnikov, Syn Addition)

Hydroboration adds borane (BH₃, often written as diborane B₂H₆) across the alkene in a concerted, four-center step. Boron (Lewis-acidic, electron-poor) adds to the less substituted carbon while hydrogen adds to the more substituted carbon — anti-Markovnikov. Both B and H add to the same face (syn addition). The arises because the transition state puts partial positive charge on the more substituted carbon (better stabilized) while the bulky boron avoids the hindered, more substituted position. Repeating this with up to three alkenes builds a trialkylborane (R₃B). In the oxidation step, basic hydrogen peroxide (H₂O₂/NaOH) replaces the B with OH with retention of configuration, so the OH lands exactly where boron was (the less substituted carbon). Net: anti-Markovnikov addition of water with syn stereochemistry.

3. Why Neither Method Rearranges

Both methods avoid the open carbocation that acid-catalyzed hydration forms. Oxymercuration uses a bridged mercurinium ion; hydroboration is concerted (no cationic intermediate at all). Consequently, hydride and alkyl shifts are impossible, and the OH appears only on one of the original alkene carbons — never on a shifted carbon.

How it works

  1. Choose the method by the regiochemistry you want: oxymercuration for Markovnikov, hydroboration for anti-Markovnikov.
  2. Form the bridged (mercurinium) or concerted (borane) addition — neither produces a free carbocation.
  3. Replace the metal with OH (or H) in the second step.
  4. Confirm no hydride/alkyl shifts have occurred, so the OH stays on an original alkene carbon.

Common confusions

Do not confuseWithDifference
Oxymercuration (Markovnikov)Hydroboration (anti-Markovnikov)OH ends up on the more vs less substituted carbon
Mercurinium ionCarbocationBridged (no rearrangement) vs open (can rearrange)
Anti-MarkovnikovAnti additionAnti-Markovnikov is regiochemistry; anti addition is stereochemistry
HydroborationOxidation stepHydroboration adds B and H; oxidation swaps B for OH
Acid-catalyzed hydrationOxymercurationSame regiochemistry, but acid route can rearrange

Memory aids

"Oxymercuration = On the Old (more substituted) carbon; Hydroboration = Helps the Humble (less substituted) carbon." For syn: "Both atoms in hydroBoration land on the same face."

Quick review

Topic Recap

Oxymercuration–demercuration gives Markovnikov hydration through a mercurinium ion, and hydroboration–oxidation gives anti-Markovnikov hydration with syn addition through organoboranes. Both avoid carbocations and therefore avoid rearrangements, giving chemists regiochemical control that acid-catalyzed hydration cannot match.

Knowledge Check

  1. Why can acid-catalyzed hydration rearrange while oxymercuration cannot?
  2. In hydroboration–oxidation, which carbon ends up with the OH, and why?
  3. What is the role of NaBH₄ in oxymercuration–demercuration?
  4. State the stereochemistry of hydroboration–oxidation and the intermediate that carries the carbon.
  5. Give the major alcohol product of each method starting from 2-methyl-2-butene.

Answers and Rationales

  1. Acid-catalyzed hydration forms an open carbocation, which can undergo hydride or alkyl shifts; oxymercuration forms a bridged mercurinium ion, which cannot rearrange.
  2. The less substituted carbon. Boron added there (anti-Markovnikov), and oxidation replaces B with OH with retention of configuration.
  3. NaBH₄ is the reducing agent in demercuration that replaces the HgOAc group with hydrogen.
  4. Syn addition (B and H add to the same face); the organoborane (trialkylborane) intermediate carries the carbon destined to become C–OH.
  5. Oxymercuration–demercuration and acid-catalyzed hydration give 2-methyl-2-butanol (Markovnikov); hydroboration–oxidation gives 3-methyl-2-butanol (anti-Markovnikov).
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Think of putting a sticker (OH) on a two-person bench. Acid-catalyzed hydration is like a clumsy worker who knocks one person off, and then the remaining person shuffles over before the sticker is applied — sometimes the sticker ends up in an unexpected spot (rearrangement).

Oxymercuration is like a gentle clamp that holds the bench in place (the mercury bridge) while the sticker is applied to the more crowded end, then the clamp is removed — no shuffling. Hydroboration is the mirror image: a "backwards" clamp holds things while the sticker is applied to the less crowded end. Both clamps prevent the shuffling that acid gives you.

Where it stops being exact: the "clamp" is not a physical brace — it is a bridged, electron-sharing arrangement (a mercurinium ion or a four-center borane transition state) that avoids ever forming a free, flat carbocation. Also, the sticker is not placed in one step; each method has two stages (addition, then replacement of the metal by OH or H).

Simple Example

1-methylcyclohexene + water:

  • Oxymercuration–demercuration → 1-methylcyclohexanol (Markovnikov, OH on the substituted carbon).
  • Hydroboration–oxidation → 2-methylcyclohexanol (anti-Markovnikov, OH on the less substituted carbon). Neither reaction gives a rearranged product.

Worked example

Oxymercuration–demercuration:

  1. The π bond (nucleophile) attacks Hg(OAc)₂ (electrophile); a mercurinium ion forms as the Hg–OAc bond breaks (acetate leaves).
  2. Water (nucleophile) attacks the more substituted carbon of the mercurinium ion from the face opposite the Hg bridge, opening the three-membered ring.
  3. Deprotonation gives a neutral organomercury alcohol (C–HgOAc and C–OH on adjacent carbons).
  4. NaBH₄ cleaves the C–Hg bond, installing H with the HgOAc removed (demercuration). Charge, atom, and octet accounting hold throughout; no carbocation is ever formed.

Hydroboration–oxidation:

  1. The π bond attacks the electron-deficient boron while a B–H hydride adds to the other carbon in one concerted, four-center transition state (B to less substituted C, H to more substituted C; syn).
  2. Two more alkenes add to the remaining B–H bonds, forming a trialkylborane.
  3. Basic H₂O₂ oxidizes the trialkylborane, replacing each C–B bond with C–OH (retention), then hydrolysis releases the alcohol. The OH occupies the less substituted carbon.

Key takeaways

  • High yield: Oxymercuration–demercuration = Markovnikov hydration with no rearrangements.
  • High yield: Hydroboration–oxidation = anti-Markovnikov hydration with syn addition and no rearrangements.
  • High yield: Neither method forms an open carbocation, which is why both beat acid-catalyzed hydration when rearrangements are a risk.
  • In hydroboration, boron adds to the less substituted carbon (anti-Markovnikov).
  • Oxidation of the organoborane replaces B with OH with retention of configuration.
  • High yield: Acid-catalyzed hydration (Markovnikov, rearrangements) vs oxymercuration (Markovnikov, no rearrangements) vs hydroboration (anti-Markovnikov, syn, no rearrangements).

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Practice Organic Chemistry 1

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Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Explain why acid-catalyzed hydration can rearrange, and how oxymercuration–demercuration avoids this while keeping Markovnikov regiochemistry.
  • Draw the oxymercuration–demercuration sequence, naming the mercurinium-ion and organomercury intermediates.
  • Draw the hydroboration–oxidation sequence and explain its anti-Markovnikov regiochemistry and syn stereochemistry.
  • Compare the three hydration methods (acid-catalyzed, oxymercuration, hydroboration) on regiochemistry, stereochemistry, and rearrangements.

Key vocabulary

Oxymercuration–demercuration
Two-step Markovnikov hydration via a mercurinium ion
Mercurinium ion
Bridged three-membered cation containing Hg
Hydroboration–oxidation
Two-step anti-Markovnikov hydration via organoboranes
Organoborane
Alkylborane intermediate (up to trialkylborane, R₃B)
Syn addition
Both new groups add to the same face
Regiochemistry
Which carbon gets which group

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