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Hydroboration is the process of the addition of hydrogen-boron bonds to carbon-carbon(C-C), carbon-nitrogen(C-N), and carbon-oxygen(C-O) double bonds. Hydroboration is a part of organic chemistry depicting the study of the linkage between h-b bonds to c-c bonds. They can also be added to triple carbon-carbon bonds. Hydroboration is basically the conversion of alkenes to neutral alcohols. It results in formation of organoborane compounds which react with a wide range of reagents.
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Key Terms: Hydroboration, Boron-carbon bond, Hydroboration oxidation, Organoborane compounds, anti-Markovnikov process, Alkenes, alkynes hydroboration
Boron-Carbon bond Properties
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Carbon-boron bond has a relatively low polarity as the electronegativity of carbon is 2.55 and the electronegativity of boron is 2.04. The polarity between the carbon-boron bond is low which makes the alkyl-boron compounds quite stable. Also, the alkyl-boron compounds oxidize easily. As the electronegativity of boron is less as a result it tends to form electron-deficient compounds.
Boron-Carbon bond Properties
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Hydroboration Oxidation
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Hydroboration is the process of converting alkyne into alcohol. The process involves two steps. One in which a double bond is created between hydrogen and hydroxyl groups. Hydroboration reaction involves the mechanism of anti-Markovnikov reaction which helps in adding carbon and hydrogen atoms together. Therefore, this reaction is mostly preferred over other hydration reactions.
For example, when Propene undergoes a process of hydroboration oxidation it gives propan-1-ol- and when propene reacts with diborane it creates a byproduct called Trialkyl Borane.
Hydroboration Oxidation
Properties of Hydroboration Oxidation
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- Hydroboration oxidation converts alkyne to alcohol.
- Hydroboration oxidation is denoted by a chemical notation of BH3.
- Hydroboration oxidation is the most preferred way to make alcohol.
- Hydroboration oxidation happens when hydrogen boron is added to carbon.
- In order to transform alkynes into neutral alcohol, a double bond is created between carbon components and hydrogen.
- Alkene formed after the process of hydroboration oxidation is symmetrical and the alcohol is produced.
- During the concentrated transition state C-H to C-B bonds are formed at the same time.
- Anti Markovnikov process helps in the addition of Hydrogen and Carbon.
- Hydroboration Is done in one step followed by oxidation in another step.
- There is no formation of Carbocation in Hydroboration oxidation.
Alkenes and Alkynes ydroboration
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The process of transforming alkenes into alcohols is termed as hydroboration which includes the addition of water into alkynes. Further oxygen is always attached to a less substituted carbon. Alkynes hydroboration begins with the process of addition of Borane and hydrogen across a double bond. Borane attaches itself to a less substituted carbon.
Second step involves the addition of chemicals such as hydrogen peroxide and base sodium hydroxide which results in the formation of a strong base of hydrogen peroxide. Then Boron is attacked by hydrogen peroxide and Carbon bond migrates with oxygen to break weak oxygen. OH comes back to create a bond with Boron thereby resulting in formation of an additional product called alcohol.
Alkynes further undergo the process of hydroboration, in which boron atom attacks the less substituted carbon atom. It is necessary to attach the alkyne to the borane stage in order to stop the reaction at the alkenyl stage. Borane also results in the formation of pi bonds of both alkynes if used on its own.
Oxidation process begins with the oxidation of alkyne borane into vinyl alcohol which has an alkene group and is also a part of the OH group. The reason why this reaction occurs is due to the presence of hydroperoxide reaction in the basic solution.
Methyl Propane, Methyl Propanol, Hydroxyborane, Hexanol, Methylcyclohexanol are a few chemicals that are prepared by hydroboration oxidation.
Things to Remember
- Hydroboration is the process of converting alkyne into alcohols.
- Carbocation is not formed in the Hydroboration oxidation.
- Hydroboration follows Anti Markovnikov process.
- Carbon-boron bonds have relatively low polarity.
- Borane results in the formation of pi bonds.
- Triple bond is also added in hydroboration.
- Organoborane compounds are formed in hydroboration.
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Sample Questions
Ques. Explain the Process of Creating 2-methylcyclopentanol from 1- methylcyclopentanol by using the Method of Hydroboration Oxidation. [3 Marks]
Ans. The steps to create 2-methylcyclopentanol from 1- methylcyclopentanol are as follows:
- 2 methylcyclopentanol can be created from 1-methylcyclopentanol through the process of hydroboration oxidation.
- 2 methylcyclopentanol is formed by pure synthesis and it involves a two-step process. This chemical is an isomer which is known as 2 methylcyclopentanol. The most substituted form of alkene is 1-methylcyclopentene which is formed when methylcyclopentanol undergoes a process of dehydration. Here, dehydration of alcohol will give the correct form of an alkene.
Ques. Why is Hydroboration a Syn Addition? {2 Marks}
Ans. As we know that the process of hydroboration is an anti-Markovnikov mechanism, in which both hydrogenation and electrophilic addition occurs which helps in creating two new bonds when the compounds are broken down. Here Syn addition means that hydroboration is taking place on the same side of the double bond. Therefore, hydroboration is a syn addition.
Ques. What is the Role of THF in Hydroboration? {2 Marks}
Ans. Since Borane is a Lewis acid and is highly reactive in nature it can catch fire easily in the air and also reacts with water. As we know that THF being a Lewis base can be used as a solvent as it can donate oxygen atoms to the lone pair of Borane which further helps in stabilizing the Borane atom.
Ques. Give a reason for the higher boiling point of ethanol in comparison to methoxymethane? {2 Marks}
Ans. Ethanol has a higher boiling point in comparison with methoxymethane because ethanol undergoes the process of intermolecular hydrogen bonding due to the presence of the negative hydroxyl group. Thereby, resulting in the requirement of extra energy by ethanol in order to break the hydrogen bonds. On the other hand, methoxymethane does not produce hydrogen bonds.
Ques. Why is ortho nitrophenol more acidic than ortho methoxy phenol? {2 Marks}
Ans. As nitro group is electron-withdrawing in nature therefore it will increase +ve charge on the oxygen atom to make it more acidic whereas OCH3 is electron releasing in nature thereby it decreases the positive charge on the oxygen atom, thus making it less acidic. This is the reason why ortho nitrophenol is more acidic than ortho methoxy phenol.
Ques. Is carbocation formed in the process of hydroboration? {1 Mark}
Ans. No, Carbocation is not formed in the process of hydroboration. Since hydroboration is the process of the addition of hydrogen-boron bonds to carbon-carbon double bonds no carbocation is formed in the entire process.
Ques. Define hydroboration. {2 Marks}
Ans. Hydroboration is the process of the addition of hydrogen boron bonds to the double or triple bonds of carbon-carbon or carbon-nitrogen bonds. Hydroboration results in the formation of organoborane compounds. Hydroboration also involves the addition of hydrogen-boron bonds to the carbon-oxygen double bonds.
Ques. Alcohols are comparatively more soluble in water than hydrocarbons of comparable molecular masses. Explain. {1 Marks}
Ans. Since alcohol is a polar solvent they form Hydrogen Bond with water molecules on the other hand hydrocarbons of comparable molecular masses do not form Hydrogen Bond as they are non-polar in nature. Thereby making alcohol more soluble in water than any other hydrocarbons of the same molecular masses.
Ques. What is meant by the hydroboration-oxidation reaction? {2 Marks}
Ans. Hydroboration–oxidation is a reaction that occurs in two steps and undergoes a hydration reaction which converts an alkene into alcohol. The process further results in the syn addition of hydrogen and hydroxyl groups to the double bond.
Ques. Name the reagent used in the bromination of phenol to 2,4,6-tribromophenol. {1 Marks}
Ans. The reagent used in the bromination of phenol to 2,3,6-tribromophenol is Bromine called water. The formation of 2,4,6-tribromophenol takes place with the help of a reagent called bromine water.
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