3. Some important compounds of boron

SOME COMPOUNDS OF BORON

Structure of boric acid

It has a layer structure in which planar BO₃ units are joined by hydrogen bonds as shown in Fig.

Physical properties of boric acid:
(i) It is a white crystalline solid.
(ii) It is soft soapy in touch.
(iii) It is sparingly soluble in cold water but fairly soluble in hot water.

Uses:
(i) In the manufacture of heat resistant borosilicate glazes.
(ii) As a preservative for milk and food stuffs.
(iii) In the manufacture of enamels and glazes in pottery.

(iii) Diborane, (B₂H₆): The series of compounds of boron with hydrogen is known as boranes.
Diborane is prepared by the reduction of boron trifluoride with LiAlH₄ in diethyl ether.
4BF₃ + 3LiAlH₄ ——> 2B₂H₆+ 3LiF + 3AlF₃

Laboratory method of preparation. In laboratory diborane is prepared by the oxidation of sodium borohydride with iodine.
2NaBH₄ + I₂ ——> B₂H₆ + 2NaI +H₂

Industrial method of preparation. On industrial scale, diborane is prepared by reduction of BF₃ with sodium hydride.

Physical Properties:
(i) Diborane is a colourless, highly toxic gas with a b.p. of 180 K.
(ii) Diborane catches fire spontaneously upon exposure to air.
(iii) Higher boranes are spontaneously flammable in air.

Chemical properties:
(i) Boranes are readily hydrolysed by water to form boric acid
B₂H₆(g) + 6H₂0(Z) ——> 2B(OH)₃(aq) + 6H₂(g)
(ii) It burns in oxygen evolving an enormous amount of heat
B₂H₆ + 30₂ —–> B₂0₃ + 3H₂0
(iii) Reaction with Lewis base:
Diborane on treatment with lewis bases undergo cleavage reactions to form borane which then reacts with Lewis bases to form adducts.
B₂H₆ + 2NMe₃ ——> 2BH₃. NMe₃
B₂H₆ + 2CO ———> 2BH₃ .CO