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
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
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
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
When treated with metal hydrides it forms metal borohydrides
When treated with excess ammonia at low temperatures diborane gives diboranediammonate. On heating at higher temperatures it gives borazole.
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.
· 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
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