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Ch 13. Group 13
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Element properties
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Elemental forms Boron has 3 polytypes, all contain B12 icosahedra.
Two major sources of B are in the Mohave desert, Turkey. Al to Tl All are cp metal structures (except Ga - mp 30°C, bp 2403°C)
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Al metal Ellingham diagram
Al3+ is abundant in the crust, but Al is very electropositive Al(0H)4¯ / Al E0 = −2.3 V Reduction with C or CO requires t > 2000 °C Ellingham diagram
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Al (m) production by electrolysis
Price $ / kg Al Hall Herault process (1888) Al2O Al (m) +e¯ , ~5V Na3AlF6(liq)
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Halides BX3 : Lewis acidity BBr3 > BCl3 > BF3 This is the opposite of expected inductive effect B(s) /2 F2(g) BF3(g) −1109 3F(g) 3/2 F2(g) −3/2 D(F-F) = −233 B(g) B(s) −406 B(g) F(g) BF3(g) −1748 So Davg (B-F) = note for comparison that D(C-F) ~ 490 kJ/mol But ΔH (BF3(g) + F− (g) BF4− (g)) = −380 The lower LA is attributed to loss of π-bonding upon D3h Td geometry change Only BF3 is stable in aqueous solution: BCl3 + 3H2O B(OH)3 + 3HCl Similarly, BF4− is stable in aqu acid, but not BCl4−,BBr4− AlF4− Td AlF63 − Oh (only period 3 and higher can be hypervalent
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Borates B(OH)3 + 2H2O → H3O+ + B(OH)4− pK ~ 9 (weak BA and LA)
B3O3(OH)4− 2 B with CN=3, 1 B with CN=4 Borate esters B(OR)3 ex: B(OCH3)3 a weak LA
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Graphite / BN structures
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BN compounds Cl3B = :NH3 Cl3B−NH3 (s)
1/3 B3N3H3Cl3 (s) HCl (g) (trichloroborazine) BN (s) + HCl (g) Note: there are also cubic BN and BN analogs to fullerenes, nanotubes
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Heavier congeners
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Borides CaB6 AlB2
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Borohydride clusters
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Wade’s rules
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11B NMR for nido [B11H14]-
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Geometric isomers
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Cluster chemistry
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Stack compounds
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Organoaluminum compounds
Al2R6 dimeric except for large R groups Al2R6 ↔ 2 AlR3 K ~ for R = CH3 K ~ for R = C4H9 K > for R = C6H3(CH3)3 Al2Me HCl Al2Cl2Me CH4 + 2 ROH Al2(OR)2Me CH4 preference for bridging sites is OR- ~ X- > H- > Ph- > R- Need good e- donor and small size to bridge
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Organometallic compounds
Δ Al(iso-C4H9)3 Al2H2(iso-C4H9) H2C=C(CH3)2 alkene elimination 3 Al2Cl2Me Na 2 Al2Me Al NaCl Tl(CH3)3 Tl(CH3)2+ is linear, stable in aqu base, isoelectronic w/ Hg(CH3)2 (η5-C5H5)Tl is air stable monomeric in gas phase, chain structure in solid state
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Ziegler-Natta catalysis
½ Al2R6 + C2H4 R2Al(CH2CH2)R R2Al(CH2CH2)R + C2H4 R2Al(CH2CH2)2R etc.. 3n C2H Al[(CH2CH2)n(CH2CH3] NaO(CH2CH2)nCH3 n C2H (C2H4)n (HDPE) n CH2=C(H)CH3 (CH2C(H)CH3)n PP (isotactic) HDPE = 50 x 106 tons/yr ≈10 lbs / person ≈500 x 106 barrels oil / yr (US consumes 720 x 106 barrels / day) Al2R6 H2O, NaOH Al2Et6, TiCl4 heptane n=7-10 detergents, soaps
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