JEE Chemistry The P Block Elements Notes Set 12

Group 13: Boron Family

Members are B (non-metal), Al, Ga, In and Tl (metals). General configuration is [noble gas] ns2 np1. Boron is the only non-metal, and metallic character grows down the group as ionisation enthalpy falls.

Boron and Aluminium: Reactions

1. Boron exists in several allotropic forms, all built from B12 icosahedra (20 faces, 12 corners). Al, In and Tl have close packed metallic structures.
2. Boron is isolated by treating borax or colemanite with acid to give boric acid, heating to B2O3 and reducing with Na, K, Mg or Al: \( \ce{B2O3 + 6Na -> 2B + 3Na2O} \).
3. Al is covered by a thin protective oxide film, so it resists air and water: \( \ce{2Al + 3/2 O2 -> Al2O3} \) (heat released is very large, about 1670 kJ per mole).
4. Al is amphoteric. It dissolves in dilute acids and in aqueous alkali: \( \ce{2Al + 2NaOH + 6H2O -> 2Na[Al(OH)4] + 3H2} \). Ga is also amphoteric.
5. Boron does not react with cold or hot water or with HCl and dilute H2SO4. Hot conc. H2SO4 and HNO3 oxidise it to boric acid: \( \ce{2B + 3H2SO4 -> 2H3BO3 + 3SO2} \) and \( \ce{B + 3HNO3 -> H3BO3 + 3NO2} \).
6. With fused alkali boron gives borate: \( \ce{2B + 2NaOH + 2H2O -> 2NaBO2 + 3H2} \). With N2 and carbon at high temperature it forms BN and B4C, and Al forms AlN and Al4C3.
7. Conc. HNO3 makes Al passive by building a firm oxide layer.

Diborane

1. Laboratory route: \( \ce{3LiAlH4 + 4BF3 -> 3LiF + 3AlF3 + 2B2H6} \), or hydrolysis of magnesium boride with acid, which gives a mixture of boranes.
2. It burns in air, \( \ce{B2H6 + 3O2 -> B2O3 + 3H2O} \), and is hydrolysed completely by cold water, \( \ce{B2H6 + 6H2O -> 2H3BO3 + 6H2} \).
3. Diborane is electron deficient. The two bridging hydrogens are joined by three-centre two-electron bonds.

Boric Acid, Borax and Aluminium Compounds

1. Boric acid, B(OH)3, is a weak monobasic Lewis acid. It accepts OH- instead of giving H+: \( \ce{B(OH)3 + 2H2O <=> [B(OH)4]- + H3O+} \).
2. It cannot be titrated sharply with NaOH. Adding glycerol, mannitol or sugar (a cis-diol) forms a stable chelate with borate, so it behaves as a strong monobasic acid and phenolphthalein gives a sharp end point.
3. On heating: H3BO3 gives HBO2 (metaboric acid) near 100 C, then H2B4O7 (tetraboric acid) near 140 C, then glassy B2O3 at red heat.
4. Borax, \( \ce{Na2B4O7.10H2O} \), is made from colemanite by boiling with sodium carbonate; the filtrate is concentrated and crystallised. It is used in glass, enamel and pottery glaze and in medicinal soaps. Perborate formed from boric acid and H2O2 is used in washing powders.
5. Al2O3 is made by heating Al(OH)3 near 300 C and is used for refractory bricks, abrasives and high alumina cement.
6. Anhydrous AlCl3 is deliquescent, fumes in air and sublimes near 180 C. It is covalent and exists as the dimer Al2Cl6, even in benzene or ether. It is a Friedel-Crafts catalyst.
7. Alums have the formula M2SO4.M'2(SO4)3.24H2O with M a univalent ion (Na, K, NH4, Rb, Cs, Tl) and M' a trivalent ion (Al, Cr, Fe). Potash alum is K2SO4.Al2(SO4)3.24H2O. They are used as coagulants for water purification, in tanning and as dyeing mordants.