Chemistry of Group 13 and Group 14 Elements - Reactions of Diborane

  • Diborane is a compound composed of boron and hydrogen with the formula B2H6.
  • It is a colorless gas with a pungent odor.
  • Diborane is highly reactive and has several important reactions.

1. Thermal Decomposition

  • Diborane can undergo thermal decomposition at high temperatures.
  • The reaction can be represented as: B2H6 → 2BH3 + H2.
  • This decomposition reaction is highly exothermic and releases a large amount of heat.

2. Reaction with Water

  • Diborane reacts vigorously with water to produce boric acid (H3BO3) and hydrogen gas.
  • The reaction can be represented as: B2H6 + 6H2O → 2H3BO3 + 6H2.

3. Reaction with Oxygen

  • Diborane reacts with oxygen to give boron oxide (B2O3) and water.
  • The reaction can be represented as: 2B2H6 + 3O2 → 2B2O3 + 6H2O.

4. Reaction with Alcohols

  • Diborane reacts with alcohols to form boron esters.
  • The reaction can be represented as: B2H6 + 2ROH → B(OR)3 + 3H2.

5. Reaction with Ammonia

  • Diborane reacts with ammonia to give borazine (B3N3H6).
  • The reaction can be represented as: B2H6 + 6NH3 → 2B3N3H6 + 6H2.

6. Hydrolysis

  • Diborane undergoes hydrolysis in the presence of water to give boric acid and hydrogen gas.
  • The reaction can be represented as: B2H6 + 6H2O → 2H3BO3 + 6H2.

7. Oxidation

  • Diborane can be oxidized to form boron oxide.
  • The reaction can be represented as: 2B2H6 + 3O2 → 2B2O3 + 6H2O.

8. Reduction

  • Diborane can act as a reducing agent and is used in various reduction reactions.
  • It can reduce metal ions to form borides.
  • The reduction reaction can be represented as: B2H6 + 2M2+ → B2M + 2H2 + 4H+.

9. Reaction with Acids

  • Diborane reacts with acids to form boranes, which are compounds containing boron and hydrogen.
  • The reaction can be represented as: B2H6 + 2HCl → 2BH3 + 2H2 + Cl2.

10. Polymerization

  • Diborane can undergo polymerization reactions to form polyboranes.
  • These reactions involve the combination of multiple units of diborane to form larger molecules.
  1. Physical Properties of Diborane
  • Diborane is a colorless gas with a pungent odor.
  • It is highly flammable and can form explosive mixtures with air.
  • The boiling point of diborane is -92.6°C, and the melting point is -165°C.
  • It is insoluble in water but soluble in organic solvents.
  • Diborane is a denser-than-air gas, which means it can accumulate in low-lying areas.
  1. Preparation of Diborane
  • Diborane can be prepared by the reaction of boron trichloride (BCl3) with lithium aluminum hydride (LiAlH4).
  • The reaction can be represented as: BCl3 + 3LiAlH4 → B2H6 + 3LiCl + 3AlCl3.
  • Another method of preparation involves the reaction of boron trifluoride (BF3) with lithium borohydride (LiBH4).
  • The reaction can be represented as: BF3 + LiBH4 → B2H6 + 3LiF.
  1. Uses of Diborane
  • Diborane is used as a reducing agent in organic synthesis.
  • It is used for the production of boron compounds, such as boron halides and boron nitride.
  • Diborane is used in the manufacture of semiconductor devices.
  • It is used as a rocket propellant and in pyrotechnics.
  • Diborane is also used in the production of high-purity boron.
  1. Safety Precautions with Diborane
  • Diborane is highly flammable and can form explosive mixtures with air.
  • It poses a fire and explosion hazard and should be handled with extreme caution.
  • It is toxic and can cause severe respiratory and eye irritation.
  • Diborane should be stored and handled in a well-ventilated area.
  • Protective equipment, such as gloves and goggles, should be worn when working with diborane.
  1. Applications of Diborane in Organic Synthesis
  • Diborane is commonly used as a reducing agent in organic synthesis.
  • It can reduce a wide range of functional groups, including carbonyl groups, double bonds, and nitro groups.
  • Diborane reduces carbonyl groups to alcohols through a process known as hydroboration.
  • Example: Reduction of an aldehyde to an alcohol - RCHO + B2H6 → RCH2OH + BH3.
  1. Hydroboration
  • Hydroboration is the addition of boron-hydrogen bonds across a carbon-carbon or carbon-oxygen double bond.
  • Diborane acts as a source of BH3, which adds to the double bond to form a boron-hydrogen intermediate.
  • The intermediate can then be further oxidized or treated with water to give the corresponding alcohol.
  • Hydroboration offers a regioselective and stereospecific method for the synthesis of alcohols.
  1. Hydroboration Examples
  • Hydroboration is commonly used in the synthesis of alcohols.
  • Example 1: Hydroboration of an alkene - RCH=CH2 + B2H6 → RCH2CH2BH2.
  • Example 2: Hydroboration of an alkyne - RC≡CH + B2H6 → RCH2CH2BH2.
  • Example 3: Hydroboration of a carbonyl compound - R2C=O + B2H6 → R2CHOH + BH3.
  1. Hydroboration-Oxidation Reaction
  • After hydroboration, the boron-hydrogen intermediate can be oxidized to yield the corresponding alcohol.
  • Oxidation is typically achieved using basic hydrogen peroxide (H2O2) or sodium hydroxide (NaOH).
  • The reaction proceeds via the replacement of the boron atom with an oxygen atom.
  • Hydroboration-oxidation is a versatile method for the synthesis of various alcohols.
  1. Hydroboration-Oxidation Examples
  • Hydroboration-oxidation is a powerful synthetic tool for alcohol synthesis.
  • Example 1: Hydroboration-oxidation of an alkene - RCH=CH2 + B2H6 → RCH2CH2OH.
  • Example 2: Hydroboration-oxidation of an alkyne - RC≡CH + B2H6 → RCH2CHOH.
  • Example 3: Hydroboration-oxidation of a carbonyl compound - R2C=O + B2H6 → R2CH2OH.
  1. Conclusion
  • Diborane is a highly reactive compound with several important reactions.
  • It undergoes thermal decomposition, reacts with water, oxygen, alcohols, and ammonia.
  • Diborane can act as a reducing agent, undergo hydrolysis, and undergo polymerization reactions.
  • It is used in various applications, such as organic synthesis and the production of boron compounds.
  • Safety precautions should be followed when working with diborane due to its flammability and toxicity.
  1. Preparation of Diborane
  • Diborane can also be prepared by the reaction of boron oxide (B2O3) with magnesium hydride (MgH2).
  • The reaction can be represented as: 2B2O3 + 6MgH2 → 2B2H6 + 3MgO.
  • Another method involves the reaction of boron trichloride (BCl3) with lithium hydride (LiH).
  • The reaction can be represented as: BCl3 + 6LiH → B2H6 + 6LiCl.
  1. Reactions with Alkenes
  • Diborane can add to alkenes through a process known as hydroboration.
  • Hydroboration of an alkene involves the addition of boron and hydrogen across the double bond.
  • The reaction proceeds via the formation of a boron-hydrogen intermediate.
  • Hydroboration of alkenes can lead to the formation of alkylboranes.
  • Further oxidation of alkylboranes can yield alcohols or other functional groups.
  1. Hydroboration of Alkenes
  • Hydroboration of an alkene with diborane proceeds in an anti-Markovnikov fashion.
  • The boron atom adds to the less substituted carbon of the double bond.
  • The hydrogen atom adds to the more substituted carbon of the double bond.
  • This regioselectivity is different from traditional electrophilic additions to alkenes.
  1. Hydroboration of Alkynes
  • Diborane can also undergo hydroboration with alkynes.
  • The reaction leads to the formation of vinylboranes or alkenylboranes.
  • The boron atom adds to the less substituted carbon of the triple bond.
  • The hydrogen atom adds to the more substituted carbon of the triple bond.
  1. Hydroboration of Carbonyl Compounds
  • Diborane can react with carbonyl compounds, such as aldehydes and ketones.
  • The reaction proceeds via the addition of boron and hydrogen to the carbonyl group.
  • The boron atom adds to the carbonyl carbon, and the hydrogen atom adds to the oxygen atom.
  • The resulting intermediate can be further oxidized or treated with water to form alcohols.
  1. Reduction of Nitro Groups
  • Diborane can reduce nitro groups (-NO2) to amines (-NH2).
  • The reaction involves the addition of boron and hydrogen to the nitro group.
  • The resulting intermediate can be further hydrolyzed to yield the corresponding amine.
  1. Reduction of Nitriles
  • Diborane can also reduce nitriles (-CN) to primary amines (-NH2).
  • The reaction proceeds via the addition of boron and hydrogen to the nitrile group.
  • The resulting iminium ion can be further reduced to yield the primary amine.
  1. Reduction of Oximes
  • Diborane can reduce oximes (-C=N-OH) to primary amines (-NH2).
  • The reaction involves the addition of boron and hydrogen to the oxime group.
  • The resulting iminium ion can be reduced to yield the primary amine.
  1. Reduction of Epoxides
  • Diborane can reduce epoxides to alcohols.
  • The reaction proceeds via the addition of boron and hydrogen to the epoxide ring.
  • The resulting intermediate can be further hydrolyzed or treated with water to form alcohols.
  1. Summary and Applications
  • Diborane is a versatile reducing agent used in various organic synthesis reactions.
  • It can add to alkenes, alkynes, and carbonyl compounds via hydroboration.
  • Diborane can also reduce nitro groups, nitriles, oximes, and epoxides.
  • These reactions offer regio- and stereo-selective methods for the synthesis of important organic compounds.
  • The application of diborane in organic synthesis demonstrates its significance in the field of chemistry.