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.
Source: Inorganic Chemistry by R.D. Madan

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 0oC , 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
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