What is meant by hydroboration-oxidation reaction? Illustrate it with an example.
Hydroboration-oxidation reaction is a method for preparing alcohols from alkenes. In this reaction, diborane (BH₃)₂ reacts with alkenes to give trialkyl boranes as addition product, which is then oxidised to alcohol by hydrogen peroxide in the presence of aqueous sodium hydroxide.
The addition of borane to the double bond takes place in such a manner that the boron atom gets attached to the sp² carbon carrying greater number of hydrogen atoms. The alcohol so formed looks as if it has been formed by the addition of water to the alkene in a way opposite to the Markovnikov's rule.
Example: Propene undergoes hydroboration-oxidation to give propan-1-ol: CH₃-CH=CH₂ + (H-BH₂)₂ → (CH₃-CH₂-CH₂)₃B (CH₃-CH₂-CH₂)₃B + 3H₂O₂ + OH⁻ → 3CH₃-CH₂-CH₂-OH + B(OH)₃
Explanation
This question is from NCERT Class 12 Chemistry, Chapter 7 (Alcohols, Phenols and Ethers). The textbook explains hydroboration-oxidation as a two-step process: first, diborane adds to the alkene to form trialkyl borane; second, oxidation with H₂O₂/OH⁻ yields alcohol. The key feature is anti-Markovnikov addition where boron attaches to the less substituted carbon. The example of propene to propan-1-ol is directly taken from the textbook context.