Hydroboration oxidation reaction is another important chemical reaction of organic chemistry which converts alkene into an alcohol. This reaction gives a more stereospecific and regiochemical alternative to other hydration reactions such as acid-catalyzed addition and oxymercuration reduction reactions. It is also known as HBO reaction.
Hydroboration oxidation reaction was discovered by American Chemist Herbert C. Brown in the 1950s. He won the Nobel prize in Chemistry in 1979.
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Hebert C. Brown
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 –
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Or it can be represented directly as follows –
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For unsymmetrical alkene –
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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).
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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 –
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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 –
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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 –
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Now two more reactions of BR3O with hydroperoxide takes place which give trialkyl borate. Reaction is given below –
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Now trialkyl borate reacts with aqueous NaOH to give the alcohol and sodium borate. Reaction is given below –
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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.
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