Ch 13. Group 13

Element properties

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)

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

Al (m) production by electrolysis Price $ / kg Al 1852 1200 1886 17 1890 5 1950 0.4 2004 1.6 Hall Herault process (1888) Al2O3  Al (m) +e¯ , ~5V Na3AlF6(liq)

Halides BX3 : Lewis acidity BBr3 > BCl3 > BF3 This is the opposite of expected inductive effect B(s) + 3/2 F2(g)  BF3(g) −1109 3F(g)  3/2 F2(g) −3/2 D(F-F) = −233 B(g)  B(s) −406 B(g) + 3 F(g)  BF3(g) −1748 So Davg (B-F) = 583 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

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

Graphite / BN structures

BN compounds Cl3B = :NH3  Cl3B−NH3 (s)  1/3 B3N3H3Cl3 (s) + 2 HCl (g) (trichloroborazine)   BN (s) + HCl (g) Note: there are also cubic BN and BN analogs to fullerenes, nanotubes

Heavier congeners

Borides CaB6 AlB2

Borohydride clusters

Wade’s rules

11B NMR for nido [B11H14]-

Geometric isomers

Cluster chemistry

Stack compounds

Organoaluminum compounds Al2R6 dimeric except for large R groups Al2R6 ↔ 2 AlR3 K ~ 10-8 for R = CH3 K ~ 10-4 for R = C4H9 K > 1 for R = C6H3(CH3)3 Al2Me6 + 2 HCl  Al2Cl2Me4 + 2 CH4 + 2 ROH  Al2(OR)2Me4 + 2 CH4 preference for bridging sites is OR- ~ X- > H- > Ph- > R- Need good e- donor and small size to bridge

Organometallic compounds Δ Al(iso-C4H9)3  Al2H2(iso-C4H9)4 + 2 H2C=C(CH3)2 alkene elimination 3 Al2Cl2Me4 + 6 Na  2 Al2Me6 + 2 Al + 6 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

Ziegler-Natta catalysis ½ Al2R6 + C2H4  R2Al(CH2CH2)R R2Al(CH2CH2)R + C2H4  R2Al(CH2CH2)2R etc.. 3n C2H4  Al[(CH2CH2)n(CH2CH3]3  NaO(CH2CH2)nCH3 n C2H4  (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