What is Hydroboration Oxidation Reaction?
Hydroboration oxidation reaction is a two-step reaction in which alkene reacts with BH3, THF (tetrahydrofuran) and hydrogen peroxide in a basic medium to give alcohol.
General form of the reaction is given below –
(Image 2 to be added soon)
Or it can be represented directly as follows –
( Image 3 to be added soon)
For unsymmetrical alkene –
(Image 4 to be added soon)
Alkene (unsymmetrical) Primary alcohol
Hydroboration oxidation of symmetrical alkene and unsymmetrical alkene works differently. As the reaction proceeds with unsymmetrical alkene follows Anti-Markovnikov rule. Although as a product we always get alcohol from both alkenes.
Mechanism of Hydroboration Oxidation Reaction
It is necessary to understand the chemistry of BH3 before understanding the mechanism of hydroboration oxidation reaction. As boron has one p-orbital empty so BH3 acts as lewis acid. It exists as a dimer of BH3, it means it exists in the form of
B2H6. 2BH3 🡪 B2H6
It can exist as a lewis acid – base complex with THF (tetrahydrofuran).
(Image 5 to be added soon)
Mechanism of hydroboration oxidation reaction can be written as follows –
Step 1. Hydroboration
In this step hydroboration of the alkene takes place. Addition of borane to the alkene is initiated. In this process bond breaking and bond formation takes place at the same time so this reaction takes place as a concerted reaction. During this process carbocation is not formed, so no rearrangement takes place. Addition of boron on carbon takes place according to Anti-Markovnikov rule. Which states that the proton (or hydrogen atom) is added to the carbon atom with least number of hydrogen substituents in addition reactions of alkenes and alkynes. So, the hydrogen atom gets attached to the least substituted (by hydrogen) carbon atom. Reaction is given below –
(Image 6 to be added soon)
In the end of this step a trialkyl borane is formed which acts as electrophile in the next step.
Step 2. Oxidation
In this step oxidation of trialkyl borane takes place by hydrogen peroxide. Hydrogen peroxide acts as a nucleophile in this step. It donates electron pair to trialkyl borane which was formed in the 1st step (Hydroboration). While trialkyl borane acts as electrophile by accepting the electrons.
Reaction –
(Image 7 to be added soon)
(Image 8 to be added soon)
Trialkyl Borane Hydroperoxide
Now rearrangement of the above formed compound takes place. Which results in the formation of BR3O and removal of hydroxide ion. Reaction is given below –
(Image 9 to be added soon)
Now two more reactions of BR3O with hydroperoxide takes place which give trialkyl borate. Reaction is given below –
(Image 10 to be added soon)
Now trialkyl borate reacts with aqueous NaOH to give the alcohol and sodium borate. Reaction is given below –
(Image 11 to be added soon)
Application of Hydroboration Oxidation Reaction
It is used for the synthesis of alcohols from alkenes. It provides more stereospecific and regioselective alcohols than other oxidation reactions which are used in the formation of alcohol. It can also take place on alkynes and produces aldehydes. It is very common method or reaction used for synthesis of alcohols in laboratories.
It was all about Hydroboration oxidation reaction and its mechanism, if you are looking for more methods of preparation of alcohols or other name reactions then register yourself on Vedantu.com or download Vedantu learning app for class 6-10, IITJEE and NEET. You can check out other articles such as name reactions, Important name reactions for class 12 chemistry etc. available on Vedantu website.
FAQs on Hydroboration Oxidation Reaction
1. What is the hydroboration-oxidation reaction and can you provide an example?
The hydroboration-oxidation reaction is a two-step organic process used to convert an alkene into an alcohol. It involves an initial hydroboration step, where borane adds across the double bond, followed by an oxidation step that replaces the boron atom with a hydroxyl (-OH) group. For example, the hydroboration-oxidation of propene yields propan-1-ol, an anti-Markovnikov product.
Reaction: CH₃-CH=CH₂ → (1. BH₃/THF, 2. H₂O₂/OH⁻) → CH₃-CH₂-CH₂-OH.
2. Does the hydroboration-oxidation of alkenes follow Markovnikov's or Anti-Markovnikov's rule?
The reaction strictly follows the Anti-Markovnikov's rule. During the addition, the hydrogen atom from the borane reagent attaches to the carbon atom of the double bond that has fewer hydrogen atoms, while the boron atom attaches to the carbon with more hydrogen atoms. Consequently, the final alcohol has the -OH group on the less substituted carbon atom of the original double bond.
3. What are the essential reagents for the hydroboration-oxidation reaction?
The key reagents used in this two-step reaction are specific to each stage:
- Step 1 (Hydroboration): Diborane (B₂H₆) or a more stable borane-tetrahydrofuran (BH₃-THF) complex is used as the hydroborating agent. THF also serves as the solvent.
- Step 2 (Oxidation): A solution of hydrogen peroxide (H₂O₂) in a basic aqueous medium, such as sodium hydroxide (NaOH), is used to oxidise the intermediate organoborane.
4. How does the hydroboration-oxidation of an alkyne lead to an aldehyde or a ketone?
When an alkyne undergoes hydroboration-oxidation, an intermediate enol is formed. This enol is unstable and immediately rearranges (tautomerizes) into a more stable carbonyl compound. The final product depends on the starting alkyne:
- A terminal alkyne (with the triple bond at the end of the chain) yields an aldehyde.
- An internal alkyne (with the triple bond within the carbon chain) yields a ketone.
5. What is the stereochemistry of the hydroboration-oxidation reaction?
The hydroboration-oxidation reaction is stereospecific and proceeds via syn-addition. This means that in the first step, the hydrogen (H) and borane (BH₂) groups add to the same face of the alkene's planar double bond. The subsequent oxidation step replaces the boron atom with a hydroxyl group, preserving this initial stereochemistry, resulting in a syn-addition product.
6. Why is borane (BH₃) typically used as a complex with THF for this reaction?
Borane (BH₃) is an electron-deficient and highly reactive molecule that exists as a toxic gas dimer, diborane (B₂H₆). Using it as a complex with tetrahydrofuran (THF) is preferred because:
- Stability: THF, a Lewis base, donates electrons to the Lewis acidic boron atom, forming a stable and less reactive adduct (BH₃-THF).
- Safety and Handling: The complex is a liquid solution that is much safer and easier to handle in a laboratory setting than gaseous diborane.
7. Why does the hydroboration-oxidation reaction not involve any carbocation rearrangement?
This reaction proceeds without rearrangement because a discrete carbocation intermediate is never formed. The initial hydroboration step is a concerted mechanism, where the breaking of the C=C pi bond and the formation of new C-H and C-B bonds occur simultaneously through a four-membered cyclic transition state. The absence of a carbocation intermediate prevents the possibility of hydride or alkyl shifts that are common in other alkene addition reactions like acid-catalysed hydration.
8. How does the product of hydroboration-oxidation of an unsymmetrical alkene compare to the product of its acid-catalysed hydration?
The two reactions yield different constitutional isomers, providing complementary methods for alkene hydration:
- Hydroboration-Oxidation: Follows Anti-Markovnikov's rule to produce the less substituted alcohol. For example, propene gives propan-1-ol.
- Acid-Catalysed Hydration: Follows Markovnikov's rule, proceeding through a carbocation intermediate to produce the more substituted alcohol. For example, propene gives propan-2-ol.
