Hydroboration-Oxidation: Mechanism & Stereochemistry | Chemca

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Hydroboration-Oxidation: Anti-Markovnikov Hydration

By Chemca Editorial Team • Last Updated: January 2026 • 10 min read

Hydroboration-Oxidation is a two-step organic reaction used to convert alkenes into primary or secondary alcohols. It is one of the most useful reactions because it achieves the net addition of water across a double bond with Anti-Markovnikov regioselectivity and Syn-stereoselectivity.

1. General Reaction

An alkene reacts with Borane ($BH_3$) or Diborane ($B_2H_6$) followed by oxidation with alkaline Hydrogen Peroxide.

$$3 R-CH=CH_2 + BH_3 \xrightarrow{\text{THF}} (R-CH_2-CH_2)_3B$$ $$(R-CH_2-CH_2)_3B \xrightarrow{H_2O_2, \ OH^-} 3 R-CH_2-CH_2-OH + B(OH)_3$$

Key Features:

Regiochemistry: Anti-Markovnikov ($H$ adds to more substituted carbon, $OH$ adds to less substituted carbon).
Stereochemistry: Syn-Addition ($H$ and $OH$ add to the same face of the alkene).
Rearrangement: None (No carbocation intermediate is formed).

2. Detailed Mechanism

Step 1: Hydroboration (Formation of Organoborane)

Borane ($BH_3$) is an electrophile (Lewis Acid). It adds to the C=C bond in a concerted manner via a four-centered transition state.

Regioselectivity: The Boron atom adds to the less sterically hindered carbon atom (Anti-Markovnikov).
Stereoselectivity: The H and B atoms add simultaneously to the same face (Syn).

$$R-CH=CH_2 + H-BH_2 \rightarrow \text{[Transition State]} \rightarrow R-CH_2-CH_2-BH_2$$

This process repeats until all three hydrogens on Boron are replaced by alkyl groups, forming a Trialkylborane ($R_3B$).

Step 2: Oxidation

The organoborane is treated with basic hydrogen peroxide. The hydroperoxide ion ($HOO^-$) attacks the boron atom.

$$R_3B + OOH^- \rightarrow [R_3B-OOH]^-$$

Migration: An alkyl group ($R$) migrates from Boron to Oxygen with Retention of Configuration. This step repeats for all three alkyl groups, forming a borate ester, which is then hydrolyzed to the alcohol.

3. Comparison of Hydration Methods

Reaction	Regiochemistry	Rearrangement?	Stereochemistry
Acid Catalyzed Hydration ($H_3O^+$)	Markovnikov	Yes (Carbocation)	Random
Oxymercuration-Demercuration	Markovnikov	No	Anti-Addition
Hydroboration-Oxidation	Anti-Markovnikov	No	Syn-Addition

4. Examples

A. Propene to 1-Propanol

$$CH_3-CH=CH_2 \xrightarrow{1. BH_3} \xrightarrow{2. H_2O_2/OH^-} CH_3-CH_2-CH_2-OH$$

(Acid hydration would give 2-Propanol)

B. 1-Methylcyclohexene

The reaction yields trans-2-methylcyclohexanol.

Reasoning: The H and OH add Syn (same face). If H and OH are Cis, and the Methyl group was originally in the plane, the Methyl group and the newly added OH group end up Trans to each other.

5. Role of Solvents

THF (Tetrahydrofuran): Borane ($BH_3$) is electron-deficient and dimerizes to toxic Diborane ($B_2H_6$). THF acts as a Lewis base, stabilizing $BH_3$ as a complex ($BH_3 \cdot THF$), making it safe and reactive.

Hydroboration Quiz

Test your concepts on Alkene Hydration. 10 MCQs with explanations.

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