Some useful compounds of boron are borax, orthoboric acid and diborane. We will briefly study their chemistry.
It is the most important compound of boron. It is a white crystalline solid of formula Na2B4O7.10H2O.
In fact it contains the tetranuclear units [B4O5(OH)4]2- and correct formula; therefore, is Na2[B4O5 (OH)4].8H2O. Borax dissolves in water to give an alkaline solution.
Na2B4O7 + 7H2O → 2NaOH + 4H3BO3
On heating, borax first loses water molecules and swells up. On further heating it turns into a transparent liquid, which solidifies into glass like material known as borax bead.
Na2B4O7.10H2O → Na2B4O7 → 2NaBO2 + B2O3
Sodium metaborate Boric anhydride
The metaborates of many transition metals have characteristic colours and, therefore, borax bead test can be used to identify them in the laboratory.
For example, when borax is heated in a Bunsen burner flame with CoO on a loop of platinum wire, a blue coloured Co(BO2)2 bead is formed.
11.3.2 Orthoboric acid
Orthoboric acid, H3BO3 is a white crystalline solid, with soapy touch. It is sparingly soluble in water but highly soluble in hot water. It can be prepared by acidifying an aqueous solution of borax.
Na2B4O7 + 2HCl + 5H2O → 2NaCl + 4B(OH)3
It is also formed by the hydrolysis (reaction with water or dilute acid) of most boron compounds (halides, hydrides, etc.). It has a layer structure in which planar BO3 units are joined by hydrogen bonds as shown in Figure.
Boric acid is a weak monobasic acid. It is not a protonic acid but acts as a Lewis acid by accepting electrons from a hydroxyl ion:
B(OH)3 + 2HOH → [B(OH)4]– + H3O+
On heating, orthoboric acid above 370K forms metaboric acid, HBO2 which on further heating yields boric oxide, B2O3.
H3BO3 → HBO2 → B2O3
11.3.3 Diborane, B2H6
The simplest boron hydride known, is diborane. It is prepared by treating boron trifluoride with LiAlH4 in diethyl ether.
3BF3 + 3 LiAlH4 → 2B2H6 + 3LiF + 3AlF3
A convenient laboratory method for the preparation of diborane involves the oxidation of sodium borohydride with iodine.
2NaBH4 + I2 → B2H6 + 2NaI + H2
Diborane is produced on an industrial scale by the reaction of BF3 with sodium hydride.
2BF3 + 6NaH → B2H6 + 6NaF
Diborane is a colourless, highly toxic gas with a b.p. of 180 K. Diborane catches fire spontaneously upon exposure to air. It burns in oxygen releasing an enormous amount of energy.
B2H6 + 3O2 → B2O3 + 3H2O ∆cH– = -1976 KJ mol-1
Most of the higher boranes are also spontaneously flammable in air. Boranes are readily hydrolysed by water to give boric acid.
B2H6 (g)+ 6H2O(l) → 2B(OH)3 (aq)+ 6H2(g)
Diborane undergoes cleavage reactions with Lewis bases(L) to give borane adducts, BH3.L
B2H6 + NMe3 → 2BH3.NMe3
B2H6 + 2CO → 2BH3.CO
Reaction of ammonia with diborane gives initially B2H6.2NH3 which is formulated as [BH2(NH3)2]+ [BH4]– ; further heating gives borazine, B3N3H6 known as “inorganic benzene” in view of its ring structure with alternate BH and NH groups.
B2H6 + 6NH3 → 3[BH2(NH3)2]+ [BH4]– → 2B3N3H6 + 12H2
The structure of diborane is shown in Figure A. The four terminal hydrogen atoms and the two boron atoms lie in one plane. Above and below this plane, there are two bridging hydrogen atoms.
The four terminal B-H bonds are regular two centre-two electron bonds while the two bridge (B-H-B) bonds are different and can be described in terms of three centre–two electron bonds shown in figure B.
Boron also forms a series of hydridoborates; the most important one is the tetrahedral [BH4]– ion. Tetrahydridoborates of several metals are known.
Lithium and sodium tetrahydridoborates, also known as borohydrides, are prepared by the reaction of metal hydrides with B2H6 in diethyl ether.
Both LiBH4 and NaBH4 are used as reducing agents in organic synthesis. They are useful starting materials for preparing other metal borohydrides.