Approaching Chemistry: Hydroboration Oxidation of Alkynes Mechanism

Hebert C. Brown won the Nobel Prize in Chemistry in 1979 for his discovery of the hydroboration oxidation reaction. This chemical reaction is common in chemical experiments and is often used to prepare aldehydes and ketones. What is the hydroboration oxidation of alkynes mechanism? Let’s explore together.

Introduction of Alkynes and Hydroboration Oxidation

Alkyne is an unsaturated aliphatic hydrocarbon, which is a general term for carbon-hydrogen compounds containing carbon-carbon triple bonds in their molecules.  The molecular formula of straight-chain alkynes is C_nH_2n-2 (where n is a non-1 positive integer). Simple alkynes include acetylene (C₂H₂), propynes (C₃H₄), etc. Among them, acetylene is the most important alkyne, which can be used in industry for lighting, welding, and cutting metals, and is also a basic raw material for manufacturing acetic acid, benzene, synthetic rubber, etc.

The melting point, boiling point, and density of simple alkynes are higher than those of alkanes or alkenes with the same number of carbon atoms. They are not easily soluble in water but are easily soluble in organic solvents such as ether, benzene, and carbon tetrachloride. Alkynes can undergo addition reactions with halogens, hydrogen, hydrogen halides, and water, as well as polymerization reactions. Because acetylene releases a large amount of heat during combustion, it is often used as a raw material for welding.

Alkynes have low melting and boiling points, low density, are difficult to dissolve in water, and are easily soluble in organic solvents. Generally, they also change gradually with the increase of the number of carbon atoms in the molecule. The solubility of alkynes in water is slightly higher than that of alkanes and alkenes. Acetylene and propynes are weakly polar, slightly soluble in water, and easily soluble in non-polar solutions. It with the same carbon framework has a low polarity at the chain end. Alkynes have a dipole moment, and with more alkyl branches are more stable.

Caution! It is extremely easy to burn and explode. It can mix with air to form explosive mixtures, which can cause combustion and explosion in the presence of open flames and high heat. It reacts violently with oxidants. Contact with fluorine, chlorine, etc. can cause severe chemical reactions. It can react with compounds such as copper, silver, mercury, etc. to produce explosive substances.

What is the hydroboration oxidation? Please read on.

The hydroboration oxidation reaction is a two-step organic chemical reaction in which an alkene reacts with a borane to form an alcohol, followed by oxidation. This is a Markovnikov reaction, in which the hydroxyl group is added to the carbon with fewer substituents. It is a common reaction in organic synthesis.

Hydroboration Oxidation of Alkynes Mechanism

The reaction of alkynes with borane to form alkylboranes is called the hydroboration oxidation of alkynes. Hydroboration oxidation of alkynes mechanism is as follows: First, the boron atom in borane lacks electrons and is close to the C atom with a higher electron cloud density in alkynes. At this time, the boron atom with some electrons has an increased tendency to release hydrogen, forming a cyclic four-center transition state. Then further reaction produces an alkyl borane. The reaction mechanism shows that for asymmetric alkynes, the regioselectivity of addition is anti-Markovnikov, that is, hydrogen is added to the double-bond carbon atom with less hydrogen. From the perspective of stereochemistry, this is a stereospecific cis addition without passing through a carbocation intermediate, so the substituents of each carbon atom remain in their original relative positions.

In most cases, the oxidation of alkynes with potassium permanganate results in the cleavage of the carbon-carbon triple bond, generating two carboxylic acids. The hydroboration oxidation of alkynes mechanism can be used to prepare aldehydes and ketones, and the replacement of hydrogen peroxide with other organic reagents can also be used to prepare amines, haloalkanes, etc. Transition metals can catalyze the addition of alkenes and alkynes, which can significantly increase the reaction rate.

Conclusion

To sum up, hydroboration oxidation of alkynes mechanism is a relatively common reaction in chemical experiments and can be used to prepare aldehydes and ketones. The production of acetylene using coal or petroleum as raw materials is the two main ways to produce acetylene. With the development of the natural gas chemical industry, natural gas will soon become the main source of acetylene. It must be very careful in preservation to avoid explosion and combustion.