Diborane: Synthesis, Production, Physical and Chemical Properties

Definition:

It is a chemical compound that consists of B and H-atoms.

1. Has a molecular formula B₂H₆, which is the simplest boron hydrides.
2. B₂H₆ is unstable at the room temperature with a sweet smell
3. B₂H₆ will ignite rapidly at room temperature
4. B₂H₆ mix well with the air forming explosive mixtures

Production and Synthesis ( Source: Wikipedia):

1. Industrial synthesis of diborane: Diborane is produced on a large scale by the reduction of BF₃ by sodium hydride, lithium hydride or LiAlH₄

8BF₃ + 6LiH = B₂H₆ + 6LiBF₄

2. Laboratory method: By the action of BCl₃ with lithium aluminium hydride,LiAlH₄ or from BF₃ ether solution with sodium borohydride. Both methods result in as much as a 30% yield:

4 BCl₃ + 3 LiAlH₄ → 2 B₂H₆ + 3 LiAlCl₄

4 BF₃ + 3 NaBH₄ → 2 B₂H₆ + 3 NaBF₄

3. Older methods: entail the direct reaction of borohydride salts with a non-oxidizing acid, such as phosphoric acid or dilute sulfuric acid.

2 BH₄⁻ + 2 H⁺ → 2 H₂ + B₂H

4. Oxidation of borohydride salts : Convenient for small scale preparations. For example, using iodine as an oxidizer:

2 NaBH₄ + I₂ → 2 NaI + B₂H₆ + H₂

5. By treating an alkali metal borohydride with HCl:

2Li[BH₄] + 2HCl = B₂H₆ + 2 LiCl + 2H₂ (g)

6. By reducing vapours of BCl₃ by molecular H₂ :

BCl₃ (vapours) + 6H₂ = B₂H₆ + 6HCl

Properties: ( Physical properties):

1. It is an inflammable gas with a sweet odour
2. It has a boiling point of about 180 K
3. It is an extremely reactive gas and very toxic

Chemical Properties:

1. Stability: It is stable at low temperatures only in the absence of moisture. At higher temperatures, diborane gets decomposed into a number of higher boranes.

2. Hydrolysis:

It is readily hydrolysed by water to form H₃BO₃ and H₂.

B₂H₆ + 6 H₂O = 2H₃BO₃ + 6H₂

With CH₃OH, trimethoxyborate is obtained.

B₂H₆ + 6CH₃OH= B(OCH₃)₃ + 6H₂

With aqueous alkalies: when diborane is passed in a con. aqueous solution of KOH at 0^oC , a reaction occurs and the solution obtained on evaporation in vacuo deposits potassium hypoborate, K₂(B₂H₆O₂).

B₂H₆ + 2KOH —–> K₂(B₂H₆O₂)

If KOH solution is diluted, then potassium hypoborate is converted into potassium metaborate, KBO₂.

K₂(B₂H₆O₂) + 2 H₂0 = 2KBO₂ + 5H₂

(The evolution of H₂ by this process makes diborane as a useful reducing agent)

( Remaining important reactions will be given by turns)

3. Combustibility:

Though pure B₂H₆ undergoes no change when mixed with dry air or O₂ at room temperature, it may ignite or burn if impure.

B₂H₆ + 3O₂ = B₂O₃ + 3 H₂O + 1976 KJ/mol

( The production of large amount of heat in the above reaction makes diborane a useful rocket fuel)

4. Reaction with halo-acids:

B₂H₆ reacts with halo-acids to give halodiboranes evolving H₂ gas.

The reactivity of halo-acids follow the order: HI> HBr> HCl

HBr and HCl react with B₂H₆ in presence of catalyst ( such as AlCl₃ and AlBr₃ ) whereas HI reacts with B₂H₆ without catalyst.

5. Action of Halogens: Like halo acids ( HCl, HBr, and HI), halogens (Cl₂, Br₂, and I₂) also react with B₂H₆ to form halodiboranes.

Reactivity: Cl₂> Br₂> I₂

Cl₂ = Reacts explosively/ vigorously at room temperature

Br₂ = Reacts rapidly at 373 K

I₂= Generally no reaction occurs but at higher temperature reacts slowly

6. Formation of Adducts:

On treatment with Lewis bases, B₂H₆ first undergoes cleavage to form borane which then reacts with Lewis bases to form adducts.

In these reactions, B₂H₆ acts as an electron acceptor (Lewis acid) whereas CO, dimethyl ether act as donors (Lewis bases).

7. Reaction with ammonia at various conditions:

a). When B₂H₆ reacts with excess of NH₃ at low temperature (-120°C), it forms an addition product called diammoniate of diborane, B₂H₆.2NH₃.

B₂H₆.2NH₃ is a white non-volatile solid and is soluble in water. It is an ionic compound and hence is represented as [BH₂(NH₃)₂]⁺[BH₄]^–.

b) With excess of NH₃ and high temperature, boron nitride (BN)_x is formed.

c) When B₂H₆ and NH₃ are reacted at high temperature in a 1:2 ratio, borazole (also called borazine or inorganic benzene), B₃N₃H₆ is formed.

Source:

1. Inorganic Chemistry By R.D Madan
2. Google Books
3. Wikipedia
4. Online Open Sources