Anti-Markovnikov hydration (concerted)
Hydroboration-oxidation of propene
How the Hydroboration-oxidation of propene works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: 1) BH₃·THF 2) H₂O₂, NaOH. Conditions: THF, 0–25 °C; then oxidative workup. Bonds formed: C–B then C–O; C–H; bonds broken: C=C π; B–H; C–B. Stereochemistry: Syn addition; retention at C–B oxidation. Rate and selectivity: Fast; B adds to the less hindered carbon.
At a glance
Step 1: BH₃ meets the alkene
BH₃ approaches the alkene; the π electrons and the B–H bond interact in a four-centered, fully concerted transition state — no carbocation ever forms.
Step 2: Four-centred concerted transition state
Boron bonds to the less hindered carbon while hydrogen delivers to the more substituted one, all in one step — this explains hydroboration's anti-Markovnikov regiochemistry and syn stereochemistry.
Step 3: Oxidative workup
Oxidative workup (H₂O₂/NaOH) replaces the C–B bond with C–OH with retention of configuration, giving the anti-Markovnikov alcohol.
Abbreviations and terms
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
- BH3
- Borane. Electron-deficient Lewis acid that adds H–B across alkenes.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- H2O2
- Hydrogen peroxide. Oxidant; its anion HOO⁻ is an excellent nucleophile.
- Markovnikov
- Markovnikov’s rule. In HX addition, H goes to the carbon with more H’s so the more stable cation forms.
- anti-Markovnikov
- Anti-Markovnikov. The opposite regiochemistry (radical HBr addition, hydroboration).
How it compares with related reactions
| Hydroboration-oxidation of propene | Acetic acid transfers a proton to water | Acid-catalyzed amide hydrolysis | |
|---|---|---|---|
| Reagents | 1) BH₃·THF 2) H₂O₂, NaOH | Acetic acid (pKa 4.76) + water | Amide, H₃O⁺ (HCl or H₂SO₄), H₂O |
| Conditions | THF, 0–25 °C; then oxidative workup | Aqueous solution, 25 °C | 6 M acid, reflux (hours) |
| Stereochemistry | Syn addition; retention at C–B oxidation | None (achiral species) | No stereocentre created; α-stereocentres retained |
| Rate / selectivity | Fast; B adds to the less hindered carbon | Equilibrium: Ka = 1.8 × 10⁻⁵, lies mostly left | Water attack / C–N cleavage rate-limiting; far slower than ester hydrolysis |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Hydroboration-oxidation of propene?
1) BH₃·THF 2) H₂O₂, NaOH. Conditions: THF, 0–25 °C; then oxidative workup.
Which bonds are formed and which are broken?
Formed: C–B then C–O; C–H. Broken: C=C π; B–H; C–B.
What is the stereochemical outcome?
Syn addition; retention at C–B oxidation
What controls the rate or the selectivity?
Fast; B adds to the less hindered carbon
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go