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
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.
(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)
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.