Hydroboration of Alkenes- Hydroboration-Oxidation Reaction, Mechanism, Hydroboration Process, Oxidation Process, Practice Problems and Frequently Asked Questions

One of the most effective strategies to combat germs and avoid getting sick is to keep your hands clean. When you can't get to a sink to wash your hands with soap and water, hand sanitiser is a good alternative.
You can buy hand sanitiser prepackaged, but you can also manufacture your own at home.

There are a few things you'll need:

4 cups rubbing alcohol, commonly known as isopropyl alcohol (99 per cent)
14 cups hydrogen peroxide (household) (3 per cent)
4 teaspoons glycerin (also known as glycerol)
2 cups of Distilled water

So, alcohol plays an important role in the preparation of sanitiser and other solvents, and today we'll look at one of the methods for preparing alcohol by hydroboration-oxidation of alkenes.

Table of Contents

Hyboration-Oxidation Reaction
Mechanism of Alkene Hydroboration-Oxidation
The Hydroboration Process
The Oxidation Process
Practice Problem
Frequently Asked Questions-FAQs

Hyboration-Oxidation Reaction:

The hydroboration oxidation reaction is an organic chemical reaction that is used to convert alkenes into primary alcohols or alkynes into ketones or aldehyde. This is accomplished through a two-step procedure that includes a hydroboration step and an oxidation step. This is accomplished through a net addition of water (across the entire double bond) using an anti-Markovnikov Rule.

Characteristics of Hydroboration

Stereospecific-syn addition
Anti Markovnikov mechanism
Primary alcohol formation

Let us consider the example of Hyboration oxidation of alkene hex-1-ene.

Mechanism of Alkene Hydroboration-Oxidation

The mechanism of hydroboration oxidation can be thought of as an anti-Markovnikov reaction in which a hydroxyl group attaches itself to the less substituted carbon. Herbert Charles Brown, an English-born American chemist, reported the hydroboration oxidation reaction for the first time in the second half of the 1950s.

For this work, he was awarded the Nobel Prize in Chemistry in 1979.

The conversion of alkenes into primary alcohols takes place here. As explained below, the entire reaction can be broken down into two steps.

1. The Hydroboration Process

The first step is the addition of borane in the form of BH3 to the given double bond. This results in the transfer of a hydrogen atom to the carbon atom next to the carbon atom bonded with the boron atom. The hydroboration step is now repeated further twice, yielding three alkenes attached to the boron atom from the starting BH3.

2. The Oxidation Process

Now that the trialkyl borane has been produced, the second step in the hydroboration process can begin. The boron atom is attacked in this step by the hydroperoxide ion, which is nucleophilic in nature. The R group, along with its electron bond pair to the adjacent oxygen atom, is now rearranged.

The ion of hydroxide has now been removed. This process is repeated three times to produce trialkyl borate as the product. This trialkyl borate is now treated with water to produce the required primary alcohol. The steps of the mechanism are illustrated below.

Practice Problems

Q1. When methyl cyclohexene undergoes hydroboration-oxidation reaction, the expected product would be

1-Methyl Cyclohexan-1-ol
2-Methyl Cyclohexan-1-ol
2-Methyl Cyclohexan-2-ol
2-Methyl Cyclohexan-1-ol

Solution: The hydroboration oxidation reaction converts alkenes to neutral alcohols by an organic chemical reaction. A two-stage technique, which involves a hydroboration step and an oxidation step, is used to produce this. Using an anti-Markovnikov Rule, a net addition of water is achieved.

When methyl cyclohexene is reacted with BH3 followed by H₂O₂/OH^-, the expected product is according to the Anti Markovnikov rule is 2-Methyl cyclohexan-1-ol. Hence, the correct answer is option (B).

Q2. The product obtained and its stereochemistry when Pent-2-ene undergoes a hydroboration-oxidation reaction would be

Pentan-2-ol, Optically active compound
Pentan-2-one, Optically inactive compound
Pentan-3-ol, Optically active compound
Pentan-3-one, Optically inactive compound

Solution: The hydroboration oxidation reaction converts alkenes to alcohol by an organic chemical reaction. A two-stage technique, which involves a hydroboration step and an oxidation step, is used to produce this. Using an anti-Markovnikov Rule, a net addition of water is achieved.

When Pent-2-ene is reacted with BH3 followed by H₂O₂/OH^-, the expected product is according to the Anti Markovnikov rule is Pentan-2-ol. The product here obtained contains a chiral centre which contains one hydroxyl group, one methyl, one propyl and one hydrogen. Hence, the product is optically active. Hence, the correct answer is an option (A).

Q3. In which case will hydroboration oxidation and acid hydration produce different products?

(A)
(B)
(C) CH₂= CH₂
(D) CH3-CH=CH-CH3

Solution: The hydroboration oxidation reaction converts alkenes into alcohol by an organic chemical reaction. A two-stage technique, which involves a hydroboration step and an oxidation step, is used to produce this. Using an anti-Markovnikov Rule, a net addition of water is achieved.

Acid catalysed hydration is a chemical reaction in which water is added to an unsaturated substrate while an acid catalyst is present. The hydration of ethene is one example.

Cyclohexene (option B), CH₂= CH₂ (option C) and CH3 - CH = CH - CH3 (option D) all are symmetrical alkene. Hence, the Markovnikov Rule is not applicable in this case. As products formed from Markovnikov or anti-Markovnikov are the same compound.

In option (A), the unsymmetrical alkene will yield a different product concerning the reagent and can be shown as

Hence, the correct answer is option (A).

Q4. The nature of Product A is

Optically Active primary alcohol
Optically Active secondary alcohol
Optically Inactive primary alcohol
Optically Inactive secondary alcohol

Solution: The hydroboration oxidation reaction, which employs an anti-Markovnikov Rule, converts alkenes into alcohol via an organic chemical reaction. This is made using a two-stage technique that includes a hydroboration step and an oxidation step.

Because carbon contains two hydrogens and one alkyl group, the alcohol produced by this reaction is a primary alcohol.

Because carbon-containing two identical groups (2 hydrogens) is achiral, a compound is optically inactive if it lacks chiral carbon.

Hence, product A is the primary optically inactive alcohol. Hence, the correct answer is (C).

Frequently Asked Questions-FAQs

Question 1. What exactly do you mean by "hydroboration"?
Answer. The process of adding the hydrogen boron bond to a double bond between carbon and carbon or carbon and nitrogen is known as hydroboration. It is also possible to perform it on a carbon-carbon triple bond. Some organic compounds can be synthesized using hydroboration.

Question 2. What are the uses of the Hydroboration reaction?
Answer. The main application of the Hydroboration Oxidation Reaction is stereospecific and regioselective alcohols, as opposed to other oxidation reactions used in the formation of alcohol.

Question 3. In organic chemistry, what is a stereospecific reaction?
Answer. Stereospecific: A reaction in which the stereochemistry of the reactants determines the outcome. In general, one stereoisomer of a given reactant yields one stereoisomer of a given product, whereas a different stereoisomer of the same reactant yields a different stereoisomer of the same product.

Question 4. In organic chemistry, what is a regiospecific reaction?
Answer. A regiospecific reaction is a chemical reaction that produces only one structural isomer when other isomers are theoretically possible.

Question 5. What is the primary distinction between regioselectivity and stereoselectivity?
Answer. The primary distinction between regioselectivity and stereoselectivity is that the former refers to the formation of one positional isomer over another. Stereoselectivity, on the other hand, refers to the formation of one stereoisomer over another.

Related topics

Physical and Chemical Properties of Alkynes	Reductive and Oxidative Ozonolysis of Alkynes
Reaction of Alkynes	Oxidation of Alkynes
Catalytic hydrogenation of alkenes and alkynes	Halogenation of Alkyne