Compounds of Boron

COMPOUNDS OF BORON

Borax

Preparation

It occurs naturally as tineal in dried up lakes. It is obtained by boiling of mineral colemanite with a solution of Na₂Co₃.

NaBO₂can be removed by passing CO₂through it.

4NaBO₂+ CO₂→ Na₂CO₃+ Na₂B_<4O₇

Properties

1. Its aqueous solution is basic in nature.

Na₂B₄O₇+ 7H₂O → 2NaOH + 4H₃BO₃

2. On heating with ethyl alcohol and cone.H₂SO₄it gives volatile vapours of triethyl borate which burn with a green flame.

3. Action of heat

Borax bead is used for the detection of coloured basic radicals under the name borax bead test e.g.,

Orthoboric acid

PREPARATION

• By treating borax with dil HCl or dil H₂SO₄

Na₂B₄O₇ + 2HCl + 5H₂O 2NaCl +4H₃BO₃

• By passing SO₂ through a mixture of powdered mineral colemanite in boiling water.

Ca₂B₆O₁₁ + 4SO₂ + 11H₂O2Ca(HSO₃)₂ +6H₃BO₃

PROPERTIES

It is a very weak monobasic acid. It does not act as a proton donor but accepts a hydroxyl ion i.e., it behaves as a lewis acid.

H₃BO₃ + H₂O[B(OH)₄]^– + H ⁺

With C₂H₅OH and conc. H₂SO₄, it gives triethylborate.

H₃BO₃ + 3C₂H₅OHB(OC₂H₅)₃ +3H₂O

With NaOH, it gives sodium metaborate

H₃BO₃ + NaOHNaBO₂ + 2H₂O

Heating effect

H₃BO₃ HBO_{2 2}B₄O₇₂O₃

USES

As an antiseptic and eye lotion under the name Boric lotion, and as a food preservative.

STRUCTURE

It has a layer structure in which planar units are linked by H- bonding, as shown in fig.

Diborane

The simplest boron hydride known, is diborane. It is prepared by treating boron trifluoride with LiAlH₄in diethyl ether.

4BF₃+ 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 BF₃with sodium hydride.

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.

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+ 2 NMe₃→2BH₃⋅NMe₃

B₂H₆+ 2 CO→2BH₃⋅CO

Reaction of ammonia with diborane gives initially B₂H6.2NH₃which is formulated as [BH₂(NH₃)₂]⁺[BH₄]^–; further heating gives borazine, B₃N₃H₆known as “inorganic benzene” in view of its ring structure with alternate BH and NH groups.

The structure of diborane is shown in Fig. 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–twoelectron bonds shown in Fig.

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 tetra-hydridoborates, also known asborohydrides, are prepared by the reaction of metal hydrides with B2H6in diethyl ether.

2MH + B₂H₆→2 M⁺[BH₄]^–(M = Li or Na)

Bonding in diborane. Each B atom uses sp3hybrids for bonding. Out
of the four sp3hybrids on each B atom, one is without an electron shown in broken lines. The terminal B-H bonds are normal 2-centre-2-electron bonds but the two bridge bonds are 3-centre-2-electron bonds. The 3-centre-2-electron bridge bonds are also referred to as banana bonds.

Both LiBH₄and NaBH₄ are used as reducing agents in organic synthesis. They are useful starting materials for preparing other metal borohydrides.

Boron halides

PREPARATION

By the reaction of boron and halogens at high temperature.

2B + 3X₂ 2BX₃

PROPERTIES

· BF₃ and BCl₃ are gases, BBr₃ is a volatile liquid and BI₃ is a solid at room temperature.

· These are covalent in nature and act as lewis acids. The decreasing order of acid strength is. BI₃ > BBr₃ > BCl₃ > BF₃

Boron nitride

It is a colourless liquid having a six membered ring of alternating B and N atoms. It is also called inorganic benzene. It is prepared by B₂H₆ as follows:

3B₂H₆ + 6NH₃2B₃N₃H₆ + 12H₂

The p electrons in borazine are only partially delocalised. It is much more reactive than benzene, because there is a retention of partial negative charge by nitrogen atoms in latter case. It is isosteric (presence of same number of atoms and electrons) with benzene.

Borazine