Diborane

Diborane is a chemical compound that consists of boron and hydrogen atoms and has a molecular formula B₂H₆. This substance is highly unstable at room temperature with a sweet odour. The compounds consisting of boron and hydrogen atoms are called boranes. Diborane is one of the simplest boron hydrides.

The boron hydrides mix well with the air forming explosive mixtures. This substance will ignite rapidly at room temperature. The other names for diborane are boro ethane and diboron hexahydride.

Table of Contents

• Structure of Diborane
• Properties of Diborane
• Preparation of Diborane
• Uses and Applications of Diborane

Structure of Diborane

The structure of the diborane molecule consists of four hydrogen atoms and of two boron atoms coming on the same plane. In between these planes, there are said to be two dividing atoms of hydrogen.

The boron atom is known to be sp3 hybridised and has four hybrid orbitals. From these four hybrid orbitals, three of the orbitals have one electron each, out of which one is an empty orbital. The two electrons of the hybrid orbitals in each of the boron atoms form 2 bonds with the 1s hydrogen atoms. The two atoms of boron left with that of each unpaired electron orbital and empty orbital form the two bridgings (B–H–B) bonds with that of the two 1s hydrogen atoms, which is also called the banana bond.

Properties of Diborane

Some of the physical and chemical properties of diborane are as given below:

• Diborane is said to be a colourless and highly flammable type of gas at room temperature. At high concentrations, it ignites rapidly in the presence of moist air at room temperature.
• It smells sweet.
• It is said to have a boiling point of about 180 K and is a toxic gas.
• It releases a huge amount of energy when burnt in the presence of oxygen.
• Diborane readily hydrolyses in water to give hydrogen gas and boric acid.
• Most of the diborane is known to be flammable in the air.

Preparation of Diborane

The action of a metal hydride with boron produced diborane. This approach is common in the industrial production of diborane.

The reaction of iodine with sodium borohydride in diglyme can also produce diborane in small quantities.

2NaBH₄ + I2  → B₂H₆  + 2NaI + H₂ 

On heating magnesium boride with HCl, a mixture of volatile boranes is obtained.

2Mg₃B₂  + 12HCl → 6MgCl₂ + B₄H₁₀ + H₂

B₄H₁₀ + H₂ → 2B₂H₆

Reactions

– Pure diborane does not react with air or oxygen at room temperature, but it gives B₂O₃ together with large amounts of energy in impure form.

B₂H₆ + 3O₂  → B₂O₃ + 3H₂O

ΔH = -2165 KJ mol-1

– Diborane reacts with methyl alcohol to give trimethyl borate.

B₂H₆ + 6CH₃OH  → 2B(OCH₃)₃ + 6H2

– Reaction with ionic hydrides

It forms metal borohydrides when treated with metal hydrides

– Reaction with ammonia

Diborane is given when treated with excess ammonia at low temperatures. It gives borazole when heated at higher temperatures.

– Hydroboration

The ether solvent, diborane, adds alkenes and alkynes at room temperature. This reaction is called hydroboration and is commonly used in synthetic organic chemistry, especially for the addition of anti-Markovnikov.

B₂H₆ + 6RCH=CHR → 2B(RCH-CH₂R)₃

Uses and Applications of Diborane

Diborane is a chemical substance that has many applications in various fields, and some are given below:

• Diborane is used as a rocket propellant.
• It is utilised in the manufacture of borophosphosilicate which is a form of glass.
• In most of the chemical reactions, it is employed as a reducing agent.
• Diborane is used as a catalyst and rubber vulcaniser in polymerisation reactions.
• It is even used as a doping agent in the manufacturing of semiconductor devices.