hydrides and oxides of boron family
DESCRIPTION
hydrideTRANSCRIPT
Hydrides And Oxides of Boron Family HydridesBoranes are electron deficient compounds. It is important to note that although BX3 are well known, BH3is not known. This is due of the fact that hydrogen atoms in BH3 have no free electrons to form pπ–pπ back bonding and thus boron has incomplete octet and hence BH3 molecules dimerise to form B6H6 having covalent and three centre bonds.Al forms only one polymeric hydride (AlH3)n commonly known as alane It contains Al…..H……Al bridges. ether 4LiH + AlCl3 —————→ Li[AlH4] + 3 LiClOxidesB(OH)3 or H3BO3 is weak monobasic Lewis acid.Boric acid, B(OH)3 is soluble in water as it accepts lone pair of electron to act as Lewis acid. Rest all hydroxides of group 13 are insoluble in water and form a gelatinous precipitate. B(OH)3 + H2O → B(OH)4
1–+H+
Al2O3 being amphoteric dissolves in acid and alkalies both. Al2O3 + 3H2SO4 → Al2 (SO4)3 + 3H2O fuse Al2O3 + 2NaOH ————→ 2NaAlO3 + H2O Sodium meta aluminate
One of the crystalline form of alumina (Al2O3) is called corrundum. It is very hard and used as abrasive. It is prepared by heating amorphous form of Al2O3 to 2000 K.
Action of Acids and Alkalies on Boron FamilyAction of AcidsBoron does not react with non oxidizing acids, however, it dissolves in nitric acid to form boric acids.Action of AlkaliesBoron dissolves only in fused alkalies, 2B + 6NaOH (fused) → 2Na3BO3 + 3H2
Al and Ga dissolves in fused as well as in aqueous alkalies, 2Al + 2 NaOH + 2H2O → 2NaAl O2 + 3H2
Diagonal relationship between Boron and SiliconThe chlorides of both are liquid, fume in most air and readily hydrolysed by water. BCl3 + 3H2O → B(OH)3 + 3HCl SiCl4 + H2O → Si(OH)4 + 4HClBoth form weak acids like H3BO3 and H2SiO3.Both form binary compounds with several metals to give borides and silicide. These borides and silicide react with H3PO4 to give mixture of boranes and silanes. 3Mg+2B → Mg3B2; Mg3B2 + H3PO4 → Mixture of boranes (Magnesium boride)
2Mg + Si → Mg2Si ; Mg2Si + H3PO4 → Mixture of silanes (magnesium silicide) The carbides of both Boron and silicon (B4C and SiC) are very hard and used as abrasive.Oxides of both are acidic and can be reduced by limited amount of Mg In excess of Mg boride and silicide are formed. B2O3 + 3Mg → 3MgO + 2B ; SiO2 + 2Mg → 2MgO + SiBoth the metals and their oxides are readily soluble in alkalies. 2B + 6NaOH → 2Na3BO3 + 3H2 (borate)
Si + 2NaOH + H2O → Na2SiO3 + 2H2 (silicate)
B2O3 + 6NaOH → 2Na3BO3 + 3H2O SiO2 + 2NaOH → Na2SiO3 + H2O
Acids of both these elements form volatile esters on heating with alcohol in presence of conc. H2SO4. B(OH)3 + 3ROH → B(OR)3 + 3H2O Si(OH)4 + 4ROH → Si(OR)4 + 4H2OBoron HydridesBoron forms hydrides of the types BnHn+14 and BnHn+6 called boranes. Diborane is the simplest boron hydride which is a dimer of BH3 .Preparation 450K(a) 8BF3 + 6LiH ———→ B2H6 + 6LiBF4
(b) 4BCI3 + LiAIH4 ——→2B2H6 + 3ACI3 + 3LiCI(c) In the laboratory, it is prepared by the oxidation of sod. Borohydride with I2 . Polyther2NaBH4 + I2 ———→ B2H6 + 2NaI + H2 Properties : (a) Since Boron in boranes never complete its octet of electrons hence all boranes are called as electron-deficient compounds or Lewis acids.(b) All boranes catch fire in the presence of oxygen to liberated a lot of heat energy. Thus, they can also be used as high energy fuels.B2H6 + 3O2 ——→ 2B2O3 + 3H2O; H = - 1976 KJ / mole (c) Boranes are readily hydrolysed by water.B2H6 + 6H2O ——→ 2H3BO3 + 6H2 (d) With carbon monoxideB2H6 + 2CO ———→ ( BH3 ← CO)2 (e) Boranes are used for formation of hydroborates or borohydrides such as LiBH4 or NaBH4 , which are extensively used as reducing agents in organic synthesis. Diethyl ether 2LiH + B2H6 ———→ 2Li+[BH4]- Diethyl ether2NaH + B2H6 ———→ 2Na+[BH4]- Structure of diborane : B2H6 has a three centre electron pair bond also called a banana shape bond.(a) B - Ht : It is a normal covalent bond (two centre electron pair bond i.e., 2c - 2e )(b) B - Hb : This is a bond between three atoms, (three centre electron pair bond i.e., 3c - 2e )
or Structure of diborane ( B2H6 )
Borax ( Na2B4O7. 10 H2O )Ca2B6O11 + 2Na2CO3 ———→ Na2B4O7 + 2CaCO3 + 2NaBO2
Colemanite Borax
Properties :(a) Its aqueous solution is alkaline due to hydrolysis,Na2 B4O7 + 7H2O ——→ 2NaOH+4 H3BO3 .(b) On heating borax loses its water of crystallization and swells up to form a fluffy mass. On further heating, it melts to give a clear liquid which solidifies to a transparent glassy bead consisting of sodium metaborate (NaBO2 ) and boric anhydride ( B2O3 ),
Na2B4O7. 10H2O ———→ Na2B4O7 ———→ 2NaBO2 + B2O3 -10H2O Boraxbed
Borax bead test It is used for the detection of coloured basic radicals under the name borax bead test.Borax bead test : Borax bead is a mixture of NaBO2 and B2O3. B2O3 on heating combines readily with a number of coloured transition metal oxides such as Co, Ni, Cr, Cu, Mn, etc. to form the corresponding metaborates which possess characteristic colours, ∆CoSO4 ——→ CoO + SO3 ; CoO + B2O3 ———→ Co (BO2)2 Cobalt meta borate ( Blue) Colours of some important metaborates are : Cupric metaborate, Cu (BO2)2 is dark blue, chromium metaborate, Cr (BO2)2 is green, nickel metaborate, Ni(BO2)2 is brown and manganese metaborate,Mn(BO2)2 is pink violet.When heated with C2H5OH and conc. H2SO4 it gives volatile vapours of triethyl borate which burns with a green edged flame.Na2B4O7 + H2SO4 + 5H2O ——→ Na2SO4 + 4H3BO3 H3BO3 + 3C2H5OH ———→ B ( OC2H5)3 + 3H2O Triethyl borate This reaction is used as a test for borate radical in qualitative analysis.Uses : (a) In making optical and hard glasses. (b) In the laboratory for borax bead test. (c) In softening of water. (d) In the preparation of medicinal soaps due to its antiseptic character. Boric acid or orthoboric acid (H3BO3) It is obtained from borax by treating with dil. HCl or dil.H2SO4 ,Na2B4O7 + 2HCI + 5H2O ———→ 2NaCI + 4H3BO3 It can also be obtained from the mineral colemanite by passing SO2 through a mixture of powdered mineral in boiling water,Ca2B6 O11 + 4SO2 + 11H2O ——→ 2Ca ( HSO3)2 + 6H3BO3
Properties : (a) It is a very weak monobasic acid, does not act as a proton doner but behaves as a Lewis acid i.e. it accepts a pair of electrons from OH- ion of H2O , H3BO3 + H2O ——→ [B (OH )4 ]- + H+ It acts as a strong acid in presence of polyhydroxy compounds such as glycerol, mannitol etc. and can be titrated against strong alkali .(b) With NaOH it forms, sodium metaborate,H3BO3 + NaOH ——→ NaBO2 + 2H2O (c) With C2H5OH and conc. H2SO4 , it gives triethyl borate Conc. H2SO4
H3BO3 + 3C2H5OH ————→ B ( OC2H5 )3 + 3 H2O
Borazine or Borasole or Triborine triamine ( B3N3H6 ) It is a compound of B, N and H. It is a colourless liquid and is also called inorganic benzene. 180oC
2B2H6 + 6NH3 ———→ 2B3N3H6 + 12H2. It has a six membered ring of alternating B and N atoms, each is further linked to a H- atom.
(vi) Boron nitride (BN)It is prepared by treating BCI3 with an excess of NH3 and pyrolysing the resulting mixture in an atmosphere of NH3 at 750°C, 750oC
BCI3 + NH3 ——→ [ H3N ——→ BCI3 ] ——→ BN + 3HCI. Excess NH3
It is a colourless, good insulator, diamagnetic and almost unreactive solidAluminum (1) Ores of Aluminum : Bauxite ( AI2O3.2H2O) Cryolite ( Na3AIF6, Felspar (KAISi3O8 ), Kaolinite (AI2O3. 2SiO2.2H2O ), Mica ( K2O.3AI2O3.6SiO2.2H2O), Corundum ( AI2O3) , Diaspore (AI2O3.H2O) Alunite or alum stone [ K2SO4. AI2(SO4)3. 4AI(OH)3] (2) Extraction : Aluminum is obtained by the electrolysis of the oxide (alumina) dissolved in fused cryolite. This involves following steps, Purification of ore (i) Baeyer's process Roasted + Caustic soda solution Bauxite ———→ Roasted ore ——————→ Finely powdered FeO→Fe2o3 high pressure (150oc, 80atm) (red) filtered, Fe2O3 asresidue Filtered Heat
FiItrate ———→ Pure AI2O3 ——→ AI(OH)3 (Sod. Aluminate) CO2 (ii) Hall's process +Na2CO3 CO2
Bauxite ——————→ Solution ———→(Finely powdered) Fused, extracted with 50 - 60oC and filtered. (red) water.Residue Fe2O3 Filtrate (Na2CO3) Heat
Precipitate Al (OH)3 ————→ Pure Al2O3 (iii) Serpek's process + Coke +N2
Bauxite ————→ Silicareduced to + Alumina form AIN (Finely powdered) Heated to Si which volatalises aluminium nitride (white) 1800oC
Hydrolysis Heated————→ Pure AI2O3 ————AI(OH)3
Compounds Of Aluminum (i) Aluminum oxide or Alumina ( AI2O3) It occurs in nature as colorless corundum and several colored minerals like ruby (red), topaz (yellow), Sapphire (blue), amethyst (violet) and emerald (green). These minerals are used as precious stones (gems). (ii) Aluminum chloride ( AI2CI6 ) : It is prepared by passing dry chlorine over aluminum powder. AI2O3 +3C+3CI2 —→2AICI3 +3CO(g) (anhydrous) It exists as dimer AI2CI6 in inert organic solvents and in vapor state. It sublimes at 100oc under vacuum. Dimeric structure disappears when AICI3 is dissolved in water. It is hygroscopic in nature and absorbs moisture when exposed to air. (iii) Thermite : A mixture of aluminum powder and Fe2O3 in the ratio 1:3. It is used for welding of iron. The reaction between AI and Fe2O3 is highly exothermic, AI + Fe2O3→ AI2O3 + Fe Heat (iv) Aluminium sulphate [Al2(SO4)3] : It is used for the preparation of alums e.g., potash alum AI2 ( SO4 )3 . K2SO4 . 24H2O . It is also used for making fire proof clothes. Alums In general, the term alum is given to double sulphates of the type M2SO4.M'2 ( SO4 )3 . 24H2O where M is a univalent cation like Na+, K+ and NH4
+, M' is a trivalent cation like AI3+, Fe3+ and Cr3+ . Some important points to be noted about the alums are (a) General formula is M2SO4 .M'2 ( SO4 )3 . 24H2O M = Monovalent metal; M' = Trivalent metal In alum crystals, 6 water molecules are held by monovalent ion, 6 water molecules are held by trivalent ion, 12 water molecules are held in the crystal structure.(b) All alums are isomorphous. Aqueous solutions of alums are acidic due to cationic hydrolysis of trivalent cation.(c) Double sulphates of divalent ions and trivalent ions with 24 water molecules in their crystals are known as Pseudoalums. General formula is MSO4 .M'2 ( SO4 )3 .24H2OM = Bivalent metal; M' = Trivalent metal(d) Pseudoalums are not isomorphous with alums.(e) Feather alum or ‘Hair-salt’ AI2SO4 .18H2O is a native form of aluminium sulphate.(f) Potash alum is used for tanning of leather, as mordant in dyeing and calico printing, for sizing paper, as a syptic to stop bleeding and purification of water. Some important alums are Potash alum K2SO4 .AI2 (SO4)3 .24H2O Sodium alum Na2SO4 .AI2 ( SO4 )3 .24H2O Ammonium alum (NH4 ) SO4 .AI2 ( SO4 )3 .24H2O Chrome alum K2SO4 .Cr2 ( SO4 )3 .24H2O