Diborane

Diborane

Preparation:

As discussed earlier diborane can be prepared by the action of metal hydride with boron.

This method is used for the industrial production.

Diborane can also be obtained in small quantities by the reaction of iodine with sodium borohydride in diglyme.

2NaBH₄ + I₂  → B₂H₆ + 2NaI + H₂

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

2Mg₃B₂ + 12HCl → 6MgCl₂ + B₄H₁₀ + H₂

B₄H₁₀ + H₂ → 2B₂H₆

Properties:

Boranes are colourless diamagnetic compounds with low thermal stability. Diborane is agas at room temperature with sweet smell and it is extremely toxic. It is also highly reactive.

At high temperatures it forms higher boranes liberating hydrogen.

Diboranes reacts with water and alkali to give boric acid and metaborates respectively.

B₂H₆ + 6H₂O → 2H₃BO₃ + 6H₂

B₂H₆ + 2NaOH +2H₂O  →  2NaBO₂ + 6H₂

Action of air:

At room temperature pure diborane does not react with air or oxygen but in impure form it gives B₂O₃ along with large amount of heat.

B₂H₆ + 3O₂  → B₂O₃ + 3H₂O

ΔH = -2165 KJ mol^-1

Diborane reacts with methyl alcohol to give trimethyl Borate.

B₂H₆ + 6CH₃ OH  → 2B(OCH₃)₃+ 6H₂

Hydroboration:

Diborane adds on to alkenes and alkynes in ether solvent at room temperature. This reaction is called hydroboration and is highly used in synthetic organic chemistry, especially for anti Markovnikov addition.

B₂H₆ + 6RCH=CHR → 2B(RCH-CH2R)₃

Reaction with ionic hydrides

When treated with metal hydrides it forms metal borohydrides

Reaction with ammonia:

When treated with excess ammonia at low temperatures diborane gives diboranediammonate. On heating at higher temperatures it gives borazole.

Structure of diborane:

In diborane two BH2 units are linked by two bridged hydrogens. Therefore, it has eight B-H bonds. However, diborane has only 12 valance electrons and are not sufficient to form normal covalent bonds. The four terminal B-H bonds are normal covalent bonds (two centre - two electron bond or 2c-2e bond).

The remaining four electrons have to be used for the bridged bonds. i.e. two three centred B-H-B bonds utilise two electrons each. Hence, these bonds are three centre- two electron bonds (3c-2e). The bridging hydrogen atoms are in a plane as shown in the figure 2.3. In diborne, the boron is sp³ hybridised.

Three of the four sp³ hybridised orbitals contains single electron and the fourth orbital is empty. Two of the half filled hybridised orbitals of each boron overlap with the two hydrogens to form four terminal 2c-2e bonds, leaving one empty and one half filled hybridised orbitals on each boron. The Three centre - two electron bonds), B-H-B bond formation involves overlapping the half filled hybridised orbital of one boron, the empty hybridised orbital of the other boron and the half filled 1s orbital of hydrogen.

Uses of diborane:

·           Diborane is used as a high energy fuel for propellant

·           It is used as a reducing agent in organic chemistry

·           It is used in welding torches