THE GROUP 13 ELEMENTS

1. 14 15 1 2 13 16 17 0 THE GROUP 13 ELEMENTS Include boron, aluminum, gallium, indium and thallium. Boron is the only nonmetal in the group many of their compounds of the elements are electron deficient and act as Lewis acids. Aluminum is a metalloid And gallium, indium and thallium are metals.

2. Properties of Elements Page 288 Sh&At

3. Trends from the table Electronic configuration – ns2 np1 Generally exhibit +3 oxidation state and changes to +1 as the group is descended. Covalent, metallic and ionic radii increases from B to Tl. Electronegativity: Ga is more electronegative than Al due to the alternation effect. (consequence of increased nuclear charges of the 4p elements due to the presence of their poorly shielding 3d electrons. • Distinct chemical properties from those of other elements in a group: • - Boron forms acidic oxides, B2O3, and aluminium forms amphoteric oxide, Al2O3 • - Boron forms polymeric oxide structure. • Boron form flammable, gaseous hydrides, aluminium hydride is a solid • Boron is electron deficient and making all its neutral compounds Lewis acids.

4. Not in the prescribed text

5. Occurrence and recovery Al – most abundant and Tl and In are least abundant. B:- Borax; Na2B4O5(OH)4.8H2O and kernite; Na2B4O5(OH)4.2H2O; Borax - Na2B4O5(OH)4.8H2O → boric acid, B(OH)3 → boron oxide, B2O3 →reduced with Mg, hydrofluoric acid, HF Pure boron is produced by reduction of BBr3 vapour with H2: 2 BBr3(g) + 3 H2(g) → 2 B(g) + 6 HBr(g) Al:- clays and aluminosilicate minerals and commercially as bauxite. Gallium oxide occurs naturally as an impurity in bauxite and indium is obtained in the Pb and Zn ores. Thallium compounds are found in the flue dust.

6. Aluminium alloys Al is the widely used nonferrous metal, it’s light, has high electrical and thermal Conductivity and high reflectance. Small percentages (less than 2 %) of other metal gives desirable weight to strength ratio. Al/Mn – most widely used and contains Fe and Si in traces to give added strength and hardness Al/Mg – have good ductility and corrosion resistant but less strong. Al/Si – have good fluidity and used in welding wires. Al/Li – have very low density and high elasticity. Al/Cu/Mg – used in making aircrafts. Al/Cu – used in satellites and space vehicles. Other uses of Al alloys include building and construction – walls, gutters, window frames, roofs and doors, aerosol cans, drink cans, sheets and foils and cooking utensils – toys, tools, refrigerators, cosmetic tubes and airconditioning units and in cars – body panels, engine blocks, wheels, bumpers and radiators.

7. Not in the prescribed text Production of Aluminium (as in Alusaf) First step: Purification of the ore – Bayer’s process. Bauxite is crushed and digested with caustic soda under pressure. Al2O3 dissolves to form sodium aluminate. • The solution is diluted thereby impurities like TiO2, sodium aluminium silicate and iron(III) oxide precipitate out. They are filtered out. • CO2 is bubbled through the liquor to reduce the pH to enable Al(OH)3 precipitate and then filtered and washed. • Al(OH)3 is calcined at ~1300°C to give pure Al2O3 + 3H2O.

8. Second step: Smelting process This is done by electrolysis using the Hall-Héroult process. Al2O3 is dissolved in molten cryolite, Na3[AlF6] and electrolysed in a graphite- lined steel tank which serves as the cathode and carbon as anode. Calcium fluoride (fluospar) is added to lower the m.p. Molten aluminium (m.p. 600) is drained from the bottom of the cell at intervals. Cryolite is made synthetically to augment the mined cryolite. 2Al(OH)3 + 3NaOH + 6HF Na3[AlF6] + 6H2O • (Cryolite improves the electrical conductivity of the cell as Al2O3 is a poor • conductor. It also serves as an added impurity and lowers the m.p. of the • mixture to about 900°C). • Typical electrolyte composition ranges are Na3[AlF6] (80-85%), CaF2 (5-7%), AlF3 (5-7%), Al2O3 (2-8% - intermittently recharged). • The following dissociations occur;

9. Al is preferentially discharged at the cathode and oxygen at the anode. Carbon anode burns to CO2, so the anode is replaced periodically. Some aluminium fog dispersed in the electrolyte, reduces CO2 to CO. Exit gases contain about 30% CO. The overall anode reaction seems to be Al2O2F42- + 2AlF63- + C 4AlF4- + CO2 + 4e and at the cathode AlF63- + 3e Al + 6F- Overall cell reaction: 3Al2O2F42- + 10AlF63- + C 12AlF4- + 3CO2 + 4Al + 24F-

11. BORON COMPOUNDS HYDRIDES Figure 12.12 page 304 Boron hydride compounds – Borane, the simplest form is diborane – B2H6 its structure is described as 2c,2e and 3c,2e bonds; can be prepared by metathesis of boron halides with either LiAlH4 or LiBH4 in ether. 3 LiBH4(et) + 4 BF3 (et) = 2 B2H6 (g) + 3 LiBF4(et) All the boranes are electron deficient, colourless and diamagnetic. Higher boranes are classified according to their electron count: Type Formula skeletal electron pairs Examples Closo BnHn2- n + 1 B5H52- to B12H122- Nido BnHn+4 n + 2 B2H6, B5H9, B6H10 Arachno BnHn + 6 n + 3 B4H10, B5H11 Hypho BnHn + 8 n + 4 none

12. Wade’s Rules: established by Kenneth Wade in the 1970s based on correlation between the number of electrons, the formula and the shape of the molecules. This apply to a class of polyhedra called deltahedra because they are made up of triangular faces resembling Δ. For molecular and anionic boranes - predict shapes of molecule or anion from its formula. B-H bonds – 2c-2e B-H-B bonds – 3c-2e Structure of diborane Diborane, like all boranes, is electron-deficient. There are 12 electrons (6 from H and 3 each from B). The four B-H bonds use 8 electrons, leaving 2 electrons each for the B-H-B bonds. The B-H-B bonds are therefore electron –deficient (short of 4 electrons)

13. Characteristics reactions of boranes and borohydrides • Cleavage of BH2 unit from diborane or tetraborane by NH3. • Deprotonation of large boron hydrides by bases. • Reaction of boron hydrides with borohydride ions to produce larger borohydride anions. • Fridel-Crafts type substitution for hydrogen in pentaborane and some larger boron hydrides

14. B(OH)3 B(OR)3 B(NH2)3 H2O ROH RNH2 Protolysis BX3 PR3 NR3 Complex formation SR2 X3BPR3 X3BNR3 X3BSR2 BORON TRIHALIDES Are useful Lewis acids as a result of incomplete octet, with BCl3 stronger than BF3. They consist of trigonal planar BX3 molecules. BCl3, BF3 are gases; BBr3 is a volatile liquid and BI3 is a solid. REACTIONS OF BX3 COMPOUNDS

15. BORON OXYGEN COMPOUNDS Important oxides – B2O3, polyborates and borosilicate glasses. 2 B(OH)3(s) Δ→ B2O3(s) + 3 H2O(l) An example of cyclic polyborate anion, [B3O6]3- - three coordinate B atom and [B(OH)4]- four coordinate B atom. Polyborates form by sharing O atom with the neighboring B atom. The rapid cooling of molten B2O3 lead to the formation of borate glasses. Sodium perborate, commonly written as NaBO3.4H2O contains peroxide ion – O22-, and hence the accurate formula is Na2[B2(O2)2(OH)4].6H2O. It is used as a bleach in laundry powders, automatic dishwashing powders and whitening toothpaste.

16. LEWIS ACIDITY Diborane and other hydrides act as Lewis acids and cleaved by reaction with Lewis bases; B2H6 + 2 N(CH3)3 → 2 H3B-N(CH3)3 Symmetric cleavage B2H6 + 2NH3 → [BH4]- + [BH2(NH3)2]+ Unsymmetric cleavage HYDROBORATION, METAL BORIDES – SUPERCONDUCTING ABILITY OF MgB2 METALLABORANES, CARBORANES SELF STUDY

17. COMPOUNDS WITH OTHER ELEMENTS – Al, Ga, In AND Tl HYDRIDES • LiAlH4 and LiGaH4 are good precursors for metal hydride complexes, MH3L2 and • sources for H- ion for the metalloid hydrides – SiH4. Aluminium hydride, AlH3 is a • solid similar to the s-block metal hydrides although not readily available. Pure • Ga2H6 has been prepared recently and the hydrides of Tl and In are very • unstable. • Some of the reactions involving the hydrides: • 4 LiH + ECl3 → (Δ, ether) LiEH4 + LiCl (E = Al, Ga) • LiAlH4 + SiCl4 → (THF) LiAlCl4 + SiH4 • LiEH4 + [(CH3)3NH]Cl → (CH3)3N-EH3 + LiCl + H2 (E = Al, Ga) • (CH3)3N-EH3 + N(CH3)3 → ((CH3)3N)2EH3 + LiCl + H2 (E = Al, Ga)

18. HALIDES AND TRIHALIDES All elements form trihalides in their +3 oxidation state, however +1 becomes Common with Tl forming a stable monohalide. Trihalides of Al, Ga and In are Lewis acids. Generally prepared by reaction of the electropositive metal with hydrogen halides such as HCl, HBr gases. 2 Al (s) + 6 HCl (g) → (100 °C) AlCl3 (s) + 3 H2 (g) Trifluorides are mechanically harder than others and they have high melting points and sublimation enthalpies and have low solubility. Their reactivity towards most donors is low (not Lewis acids) and despite that AlF3 and GaF3 form slats such as Na3AlF6 (Cryolite) – used as a solvent for bauxite in the production of aluminium) and Na3GaF6.

19. Thallium trihalides are less stable than those of light congeners. The triiodide, TlI3 is a compound of +1 and +3 as it contains the I3- not the I- ion. Confirmed by the standard electrode potentials which indicates that Tl(III) is rapidly reduced to Tl(I) by iodide: Tl3+ (aq) + 2 e- → Tl+ (aq) EӨ = +1.26 V I3- (aq) + 2 e- → 3 I- (aq) EӨ = +0.55 V However, in excess iodide Tl(III) is stabilized by the formation of the complex; TlI3 (s) + I- → TlI4- (aq) Generally the +1 oxidation state becomes more stable progressively from Al to Tl. (Read more in page 310)

20. OXYGEN COMPOUNDS Al and Ga form α and β forms of oxide in which the elements are in their +3 oxidation state; Tl forms an oxide in which it’s in +1 oxidation state and a peroxide. α-alumina, Al2O3 is the most stable form and is very hard and a refractory material. In mineral form called corundum and as a gemstone is called sapphire, ruby, emerald or amethyst depending o the amount of metal ion impurities. Dehydration of Al(OH)3 at temperatures below 900 °C result in the formation of γ-alumina, which is metastable polycrystalline form with a defect spinel structure. The α and γ forms of Ga2O3 have the same structureas their Al analoques. The metastable form is β-Ga2O3, which has a ccp structure with Ga(III) in distorted octahedral and tetrahedral sites. In forms In2O3 and Tl forms Tl(I) oxide and peroxide, Ti2O and Tl2O2. The elements all form a series of salts called alums – MAl(SO4)2.12H2O where M is univalent cation such as Na+; K+; Rb+; Cs+; Tl+ or NH4+. Alums are thought of double salts containing the hydrated trivalent cation [Al(H2O)6]3+. The remaining water molecules form hydrogen bonds with the sulfate and the cations.

21. Al2O3 and Al(OH)3 are amphoteric • Ga2O3 and Ga(OH)3 are amphoteric and dissolve in alkali to form gallates. • In2O3 and Tl2O3 are completely basic and the metals form neither hydrates nor hydroxides. • TlOH is a strong base and is soluble in water, like Group 1A hydroxide. • Tl+ compounds are extremely poisonous and in large doses, can cause death. Not in the prescribed text

22. SULFIDE COMPOUNDS The only sulfide of Al, Al2S3, which is prepared by direct reaction of the elements at elevated temperatures: 2 Al(s) + 3 S(s) → (Δ) Al2S3(s). It is rapidly hydrolyzed in aqueous solution; Al2S3(s) + 6H2O(l) → 2Al(OH)3(s) + 3H2S(g) Al2S3, exists in three different forms – α, β and γ forms. The structure of α and β forms are based on wurtzite structure in α–Al2S3 the S2- ions are hexagonal close packed and the Al3+ ions occupy two-thirds of the tetrahedral sites randomly. The γ-form adopts the same structure as γ-Al2O3. The sulfides of Ga, Tl and In are numerous and varied than those of Al and adopt many different structural types. Al, In and Ga react with P, As and Sb to form materials that are semiconductors