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Ionization Energies of Group 13 Elements (kJ/mol) Boron Group Compounds Element IE1 IE2 IE3 B 800.6 2427 3660 Group 13 (3A, III, IIIA) B, Al, Ga, In, Tl Valence electron configuration: ns2np1 Al 577.5 1817 2745 Ga 578.8 1979 2963 In 558.3 1821 2704 Tl 589.4 1971 2878 2nd and 3rd ionization energies would be expected to continually decrease moving down the group Oxidation States • • • • Boron Compounds The lighter group 13 elements (B, Al) exist in compounds in a +3 oxidation state. Gallium is also found in a +3 oxidation state, but is sometimes found in a +1 oxidation state Indium is more commonly found in the +1 oxidation state, while thallium is only found in this state (e.g. TlBr) This behavior is also seen in other p-block groupings, and is explained by the inert pair effect (results from the ionization energies of the 2nd and 3rd electrons in period 4 and heavier pblock elements being higher than expected). Borax H2O2 ‘washing soda’ Boric acid - cleaning agents -insecticides -antiseptics ns2np1 Inert pair effect: the apparent stability of the valence s electrons in heavier p-block elements which stabilizes an oxidation number that is 2 less than the element’s (old) group number Hydrides of Group 13 Elements • Having three valence electrons, group 13 elements might be expected to form EH3-type hydrides (sp2-hybridized) • In fact, there is ample evidence for oligomerization of many of these hydrides, to yield bridged compounds 2e- 2c, bond (terminal) H H H B B H H H 3c-2e- bond (bridging) General Chemistry: Chapter 21 Copyright © 2011 Pearson Canada Inc. ElectronElectron-Deficient Boron Cluster Compounds • Boron hydride (and carborane) compounds represent a fascinating class of cage structures that were once viewed as viable fuel sources • Electron-deficient borane clusters are formed because boron compounds possess fewer valence electrons than are required for a localized bonding scheme terminal hydrogen the bridging H atom is labeled µ-H • Because of its electron-deficient nature, heavier boron hydrides are extensively bridged structures (need to be in order to obtain octets of electrons) bridging hydrogen 1 ElectronElectron-Deficient Boron Cluster Compounds • There are several basic cluster types: e.g. closo-, nido-, and arachno• Successively smaller clusters are named according to structures that are formed as successive vertices of the closed structures are removed (nido-, arachno-, hypho-). Naming Cluster Compounds • The name used to identify these electron-deficient cluster compounds involves several components. The proper name • indicates its shape (relative to a reference shape) • indicates the number of boron and hydrogen atoms • indicates the charge on anions anion with charge = -2 six hydrogens closo- structures are closed shapes closo-hexahydrohexaborate(2-) six borons “ate” indicates anion 6-boron atoms in parent, closed shape Naming Boron Hydrides Hydrides of Group 13 Elements Contrast the name of the anionic [B6H6]2- with that for a neutral boron hydride Number of hydrogens is instead indicated by a number at the end of the name (brackets) Name ends in “-ane” • Aluminum hydride exists as a three-dimensional network of octahedral aluminum centers (3c-2e- Al-H-Al bonds) H H H H H H Al H Al H Al H H H H H H • Ga2H6 has recently been characterized; structurally similar to B2H6 nido-pentaborane(9) • Hydrides of In and Tl have not been characterized Halides of Group 13 Elements Hydrides of Group 13 Elements • Boron trihalides are monomeric under ordinary conditions • Studies on the thermodynamics of formation of boron trihalide-Lewis base adducts indicate that stability increases in the order: L•BF3 < L•BCl3 < L•BBr3 Group 13 hydrides are susceptible to attack by Lewis bases The size of the Lewis base can impose different reaction pathways + N H H E NH3 NH3 + H H Asymmetric Cleavage E -rel. small base H NH3 H H H rel. large base H E E H H H E = B or Ga NPh3 2 H H H E NPh3 Symmetric Cleavage X B X N X X B X X • How does this trend agree with electronegativities of X? • In the trigonal planar BX3 structure, π-bonding exists, most X X important for smaller elements B Rem: correct Lewis F F B structure shows all bonds and electrons F F B F F F B F F X B-X distance X = F: 131 pm X = Cl: 174 pm X = Br: 189 pm X = I: 210 pm 2 Boron Nitrides • B-N unit is isoelectronic with C-C bond (same # of electrons) χPB = 2.0 χPN = 3.0 χPC = 2.6 • Borazine, (HBNH)3 is structurally similar to benzene H • The reactivity of these units, however, is very different as a result of different electronegativities of these elements • • • Boron Nitrides B-N bond: polar C-C bond: non-polar H H (borazine) B N N B H H B N H H H H H H benzene is aromatic and requires special conditions for chemical reaction H • However, they react very differently • Benzene is quite stable • (HBNH)3 is (comparatively) susceptible to nucleophilic and electrophilic attack Boron Nitrides • Hexagonal boron nitride exists as layers of fused rings, similar in structure to graphite • In these assemblies, boron atoms lie directly over nitrogen atoms Structurally, hexagonal boron nitride looks similar to graphite • London forces operate between planes (weak interactions) and weak dipole-dipole forces • This material is a solid lubricant (like graphite), but a poor conductor (unlike graphite) Aluminum Electrolysis cell for aluminum production by the Hall-Hérault process Third most abundant element, 8.3% by mass of crust. Lightweight alloys. 5 Mtonne/yr production Easily oxidized to Al3+ Principal raw source is bauxite Al2O3 Production of aluminum metal by electrolysis: oxidation reduction 3× × {C(s) + 2 O24× × {Al3+ + 3 e- 3 C(s) + 4 Al3+ + 6 O2- CO2(g) + 4 e-} Al(l)} 4 Al(l) + 3 CO2(g) High energy consumption, 15 kWh/kg Al. (cf Na about 5 kWh/kg) It takes only 5% of the energy to recycle compared to production from bauxite. 3 Aluminum Aluminum Oxide and Hydroxide Easily oxidized to Al3+ 2 Al(s) + 3/2 O2(g) → Al2O3(s) ΔH = -1676 kJ The Thermite reaction (used in on-site welding of large objects): Anodized aluminum 2 Al(s) + 3 H2O(l) → Al2O3(s) + 6 H+ + 6 e- Drinking cups made of anodized aluminum. 2 Al(s) + Fe2O3(s) → Al2O3(s) + Fe(s) Aluminum Halides Friedel Crafts Alkylation Bonding in Al2Cl6 C2H5Cl + AlCl3 Amphoteric hydroxide Al(OH)3(s) + 3 H3O+(aq) → 2 [Al(H2O)6]3+(aq) Al(OH)3(s) + OH-(aq) → 2 [Al(OH)4]-(aq) [C2H5]+ + [AlCl4]- Aluminum Sulfate and Alums Al2SO4•18H2O 1 Mtonne/yr production Water purification systems Sizing deposition agent for paper Alum production – mixed salts with various uses 4 Uses of Other Group 13 Metals Gallium Dopant in semiconductors Indium Makes low melting alloys. Low-temperature transistors and photoconductors. Thallium Extremely toxic. Few industrial uses. Tl2Ba2Ca2Cu3O8+x exhibits superconductivity up to 125K. 5