Diborane

Diborane, with the molecular formula B₂H₆, is a chemical compound composed of boron and hydrogen atoms. It falls within the category of boranes, which are compounds containing boron and hydrogen. Notably, diborane is recognized as one of the simplest boron hydrides. Despite its significance in chemical synthesis, it poses challenges due to its high instability at room temperature, accompanied by a distinct sweet odor.

Diborane is classified as a borane, and boranes, in general, exhibit the tendency to form explosive mixtures with air. This property makes diborane prone to rapid ignition at room temperature. Among its alternate names are boro ethane and diboron hexahydride.

1.0Structure of Diborane

• Boron-Boron Bridge:

Diborane consists of two boron atoms connected by a bridge made of two bridging hydrogen atoms.

• Terminal Hydrogen Atoms:

Each boron atom is bonded to two terminal hydrogen atoms, resulting in a total of three hydrogen atoms around each boron.

• Bridging Hydrogen Atoms:

The two hydrogen atoms forming the bridge between boron atoms are shared, creating a distinctive banana-shaped bond.

• Molecular Geometry:

Diborane exhibits a trigonal planar molecular geometry around each boron atom, contributing to its three-dimensional structure.

• Steric Number and Hybridization:

Each boron atom has a steric number of three, corresponding to the three atoms bonded to it, and shows sp3 hybridization.

• Electron Count:

Diborane has a total of 12 valence electrons, with each boron contributing three valence electrons and each hydrogen contributing one.

• Banana Bonding:

The unique bonding in diborane involves delocalized electron density in the B-B bond region, forming the characteristic banana bond.

• Molecular Orbital Theory:

Molecular orbital theory is often used to explain the bonding in diborane, highlighting the overlap of atomic orbitals to form molecular orbitals.

• Distinctive Chemical Reactivity:

The specific molecular structure of diborane influences its chemical reactivity, making it a powerful reducing agent and participant in various organic synthesis reactions.

• Electron-Deficient Nature:

Diborane is considered electron-deficient due to its unique electron arrangement, impacting its interactions in chemical reactions.

2.0Physical Properties of Diborane

3.0Chemical Properties of Diborane

1. Reducing Agent:

Diborane is a powerful reducing agent and can reduce various compounds, including aldehydes, ketones, and acids, by donating electrons.

2. Combustibility:

Diborane is highly flammable and can ignite spontaneously in the  presence of air, forming boron oxides and water as combustion products.

           B₂H₆ (g)        +           3O₂ (g)     →        B₂O₃ (s)      +       3H₂O (g)

3. Addition Reactions:

Diborane undergoes addition reactions with unsaturated organic compounds, such as alkenes and alkynes, leading to the formation of boron-containing organic compounds.

4. Complex Formation:

Diborane can form complexes with Lewis bases, expanding its reactivity. These complexes are crucial in various chemical applications.

Diborane undergoes cleavage reactions with Lewis bases(L) to give borane adducts, BH₃.L

    B₂H₆ + 2NMe₃   →        2BH₃.NMe₃

    B₂H₆ + 2CO      →        2BH₃.CO

The reaction with ammonia depends on conditions.

Borazine is much more reactive than benzene. Borazine readily undergoes addition reactions which do not occur with benzene. Borazine also decomposes slowly and may be hydrolysed to NH₃ and boric acid at elevated temperature. If heated with water, B₃N₃H₆ hydrolyses slowly.

B₃N₃H₆ + 9H₂O      →    3NH₃ + 3H₃BO₃ + 3H₂O

5. Hydrolysis:

Diborane reacts with water through hydrolysis, yielding boric acid (H3BO3) and hydrogen gas.

Boranes are readily hydrolysed by water to give boric acid.

B₂H₆  (g)  +  6H₂O   (l)  →  2B(OH)₃(aq)  +  6H₂(g)

${\mathrm{B}}_2{\mathrm{H}}_6+\mathrm{HCl}\text{ (dry) }\underset{{\mathrm{Alcl}}_3}{\overset{\text{ anh }}{\to }}{\mathrm{B}}_2{\mathrm{H}}_5\mathrm{Cl}+{\mathrm{H}}_2$

4.0Preparation of Diborane

• It is prepared by treating boron trifluoride with LiAlH₄ in diethyl ether.

     3LiAlH₄     +   4BF₃       →    3LiF   +   3AlF₃   + 2B₂H₆

• Laboratory method : For the preparation of diborane involves the oxidation of sodium borohydride with iodine.

         2NaBH₄   +   I₂                 →    B₂H₆   +   2NaI   + H₂

• Industrial scale : By the reaction of BF₃ with sodium hydride.

         2BF₃           +    6NaH     →      B₂H₆   + 6NaF

5.0Applications and Uses of Diborane

Diborane (B₂H₆) has several important uses and applications in various fields, owing to its unique chemical properties. Here are some key applications of diborane:

Reduction Agent in Organic Synthesis:

• Diborane is a powerful reducing agent and is used in organic synthesis to reduce various functional groups, such as aldehydes, ketones, and carboxylic acids. It provides a selective and mild reduction method.

Hydroboration Reactions:

• Diborane is employed in hydroboration reactions, where it adds boron and hydrogen across carbon-carbon double bonds. This is a useful method for introducing boron into organic molecules.

Boron Source in Semiconductor Industry:

• Diborane is used as a source of boron in the semiconductor industry for the production of boron-doped silicon, which is crucial for the fabrication of semiconductors and integrated circuits.

Fuel Additive in Rocket Propellants:

• Historically, diborane has been considered as a potential fuel additive in rocket propellants due to its high energy content. However, safety concerns and other factors have limited its widespread use in this application.

Boron Source in Chemical Vapor Deposition (CVD):

• Diborane is utilized in chemical vapor deposition processes to deposit thin films of boron-containing materials, such as boron carbide and boron nitride, on various substrates.

Flame Retardant in Polymers:

• Diborane is used as a flame retardant in some polymer materials. Its ability to release boron compounds under specific conditions contributes to flame inhibition.

Laboratory Research and Synthesis:

• Diborane is used in laboratories for experimental and synthetic purposes. Its unique reactivity and ability to introduce boron into organic compounds make it valuable in research settings.

Hydrogen Storage Material:

• Diborane has been investigated for its potential use as a hydrogen storage material in fuel cells and other hydrogen-based energy storage systems. Its high hydrogen content makes it an interesting candidate for such applications.

It's important to note that diborane is a highly reactive and potentially hazardous substance, and proper precautions must be taken when handling it. Safety considerations and environmental concerns limit some of its applications in certain industries.