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Diborane is a chemical compound containing hydrogen and boron, with the molecular formula B2H6. It is a colorless gas and has a sweet odor. It is highly unstable at room temperature as it mixes well with air forming up explosive mixtures. Most of the Diborane is known to be inflammable in the air. Diborane is also known as boron hydride, bromoethane and die-borohydride. It is one of the simplest boron hydrides. Compounds that combine boron and hydrogen atoms are called boranes.
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Structure Of Diborane
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The structure of the Diborane molecule consists of 4 hydrogen atoms and 2 boron atoms coming on the same plane. It adopts a D2h structure which consists of four terminals and two bridging hydrogen atoms. The B-H terminal weakness and the B-H bond are reflected by their vibration structures at an infrared spectrum of approximately 2100 and 2500 cm.
The model determined by molecular orbital theory shows the bonds between boron and the hydrogen atoms are a 2-center, 2-electron covalent bond. The relation between boron atoms and the binding hydrogen atoms are different from molecules like hydrocarbons.
Structure of Diborane
After using two electrons in bonding to the terminal hydrogen atoms, each boron has one electron of extra valency left to combine. Combined hydrogen atoms provide one electron each. So, the B2H2 ring is contained together with four electrons, for example, 3-center 2-electron bonding. This kind of bond is also called the 'banana bond'.
Properties Of Diborane
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Given below are some physical and chemical properties of diborane:
- Diborane is a colorless and highly flammable gas at room temperature.
- It has a sweet smell.
- Diborane readily hydrolyzes in water to form hydrogen gas and boric acid.
- It is a toxic gas and is said to have a boiling point of about 180 K.
- At higher temperatures, it burns faster when there is moist air at room temperature.
- Most Diborane is flammable in the air.
- It releases a large amount of energy when burned in the presence of oxygen.
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Preparation Of Diborane
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Diborane can be prepared by the action of boron with metal hydride. This method is commonly used in the industrial production of diborane.
Diborane can be obtained in small quantities by reaction of iodine with sodium borohydride in diglyme.
2NaBH4 + I2 → B2H6 + 2NaI + H2
When we heat Magnesium boride with HCl, a mixture of flexible boranes is obtained.
2Mg3B2 + 12HCl → 6MgCl2 + B4H10 + H2
After obtaining B4H10,
B4H10 + H2 → 2B2H6
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Reactions of Diborane
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- Action of Air
At room temperature, diborane gives B2O3 along with a large amount of heat.
B2H6 + 3O2 → B2O3 + 3H2O
ΔH = -2165 KJ mol-1
Diborane and methyl alcohol react to give trimethyl Borate.
B2H6 + 6CH3 OH → 2B(OCH3)3+ 6H2
- Action of Water
Diborane and water react to give Boric acid.
B2H6 + 6H2O → 2H3BO3 + 6H2
- Action of Alkali
Diborane reacts with water and alkali to give boric acid and metaborates.
B2H6 + 2NaOH +2H2O → 2NaBO2 + 6H2
- Hydroboration
Hydroboration is highly used in synthetic organic chemistry, mostly for anti-Markovnikov addition.
B2H6 + 6RCH=CHR → 2B(RCH-CH2R)3
- Reaction with ionic hydrides
Diborane forms metal borohydrides when treated with metal hydrides.
B2H6 + 2LiH → 2LiBH4
B2H6 + 2NaH → 2NaBH4
- Reaction with ammonia
Diborane gives diborane-di-ammonate when treated with excess ammonia at low temperatures. On heating at higher temperatures, it gives borazine.
Applications of Diborane
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Given below are some applications and uses of Diborane:
- Diborane is used to make borophosphosilicate which is a type of glass.
- Diborane is used in welding torches.
- It is used as a reducing agent in various chemical reactions.
- Used as a rocket propellant.
- It is also used as a doping agent in the production of semiconductor devices.
- It is used as a catalyst and rubber vulcanizer in polymerization reactions.
Things to Remember
- Diborane is a chemical compound that contains boron and hydrogen.
- It is colourless with a sweet odour.
- It is highly unstable and inflammable in the air.
- It has 4 hydrogen atoms and 2 boron atoms.
- It is a toxic gas.
- Diborane can be prepared by reaction of boron with metal hydride.
- It is used in a variety of industrial applications.
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Sample Questions
Ques: How many bridges and terminal H are present in Diborane? What is its chemical formula? (1 mark)
Ans: 2 bridged H and 4 terminal H are present in Diborane. The chemical formula of Diborane is B2H6.
Ques: What type of hybridization is seen in Boron when it is a Diborane compound? (1 mark)
Ans: sp3 hybridization is seen in Boron when it is a Diborane compound.
Ques: Diborane can be used in the reduction of which compound? (1 mark)
Ans: Diborane can be used in the reduction of Esters.
Ques: Name the two H−B−H angles in Diborane. (1 mark)
Ans: The two H−B−H angles are 95 degrees and 120 degrees respectively.
Ques: What is the hybridization state of B and N in inorganic benzene respectively? (1 mark)
Ans: The hybridization state in inorganic benzene is sp2 for both B and N.
Ques: What is the deficiency of Diborane electrons? (2 marks)
Ans: Diborane is an electron-deficient structure because Boron needs 5 electrons to complete its octet. But in this molecule, each Boron atom is composed of two-terminal hydrogen atoms each. Also, both Boron atoms are held together by two hydrogen atoms. Therefore, it is called an electron deficient molecule.
Ques: What are the applications of Diborane? (2 marks)
Ans: There are numerous uses of diborane. It is used as a reducing agent in various chemical reactions, as a rocket propellant, as a catalyst for hydrocarbon polymerization, as a rubber vulcanizer, a flame speed accelerator, used in welding torches, and as a doping agent in the production of semiconductor devices. It is also used in electronics to provide electrical properties to pure crystals.
Ques: What kind of bond exists in Diborane? (2 marks)
Ans: B2H6 forms strong banana bonds and is also known as three centers and two electron bonds. In the diborane atom, two types of hydrogen atoms are present. Hydrogen terminal atoms form covalent bonds with boron which usually consist of two centers and two electron bonds. The hydrogen-bound atom forms a banana bond of three centers and two electron bonds.
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