Ionization Energies of Group 13 Elements (kj/mol) Boron Group Compounds Group 13 (3A, III, IIIA) B, Al, Ga, In, Tl Valence electron configuration: ns 2 np 1 Element B Al Ga In Tl IE 1 800.6 577.5 578.8 558.3 589.4 IE 2 2427 1817 1979 1821 1971 IE 3 3660 2745 2963 2704 2878 2 nd and 3 rd ionization energies would be expected to continually decrease moving down the group Oxidation States 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 2 nd and 3 rd electrons in period 4 and heavier p- block elements being higher than expected). ns 2 np 1 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 Borax washing soda Boron Compounds 2 O 2 General Chemistry: Chapter 21 Boric acid - cleaning agents -insecticides -antiseptics Copyright 2011 Pearson Canada Inc. ydrides of Group 13 Elements aving three valence electrons, group 13 elements might be expected to form E 3 -type hydrides (sp 2 -hybridized) In fact, there is ample evidence for oligomerization of many of these hydrides, to yield bridged compounds 2c, 2e - bond (terminal) B B 3c-2e - bond (bridging) the bridging atom is labeled µ- Because of its electron-deficient nature, heavier boron hydrides are extensively bridged structures (need to be in order to obtain octets of electrons) Electron-Deficient Boron Cluster Compounds terminal hydrogen 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 bridging hydrogen 1
Electron-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-). closo- structures are closed shapes 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 six hydrogens 6-boron atoms in parent, closed shape anion with charge = -2 closo-hexahydrohexaborate(2-) six borons ate indicates anion Naming Boron ydrides Contrast the name of the anionic [B 6 6 ] 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 nido-pentaborane(9) ydrides of Group 13 Elements Aluminum hydride exists as a three-dimensional network of octahedral aluminum centers (3c-2e - Al--Al bonds) Al Al Al Ga 2 6 has recently been characterized; structurally similar to B 2 6 ydrides of In and Tl have not been characterized ydrides of Group 13 Elements Group 13 hydrides are susceptible to attack by Lewis bases The size of the Lewis base can impose different reaction pathways + - rel. small base rel. large base N 3 N E E 3 NPh E E 3 2 N 3 Asymmetric Cleavage E = B or Ga E NPh 3 Symmetric Cleavage alides 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 B 3 < L BCl 3 < L BBr 3 N ow does this trend agree with electronegativities of? In the trigonal planar B 3 structure, π-bonding exists, most important for smaller elements Rem: correct Lewis structure shows all bonds and electrons B + B B N B B B B- distance = : 131 pm = Cl: 174 pm = Br: 189 pm = I: 210 pm 2
Boron Nitrides B-N unit is isoelectronic with C-C bond The reactivity of these units, however, is very different as a result of different electronegativities of these elements χ P B = 2.0 χ P N = 3.0 χ P C = 2.6 (same # of electrons) B-N bond: polar C-C bond: non-polar Boron Nitrides Borazine, (BN) 3 is structurally similar to benzene (borazine) N B B N N B owever, they react very differently Benzene is quite stable (BN) 3 is (comparatively) susceptible to nucleophilic and electrophilic attack benzene is aromatic and requires special conditions for chemical reaction Boron Nitrides exagonal 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 Third most abundant element, 8.3% by mass of crust. Lightweight alloys. 5 Mtonne/yr production Easily oxidized to Al 3+ Principal raw source is bauxite Al 2 O 3 Electrolysis cell for aluminum production by the all-érault process Production of aluminum metal by electrolysis: oxidation 3 {C(s) + 2 O 2- CO 2 (g) + 4 e - } reduction 4 {Al 3+ + 3 e - Al(l)} 3 C(s) + 4 Al 3+ + 6 O 2-4 Al(l) + 3 CO 2 (g) igh 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 Easily oxidized to Al 3+ 2 Al(s) + 3/2 O 2 (g) Al 2 O 3 (s) Δ = -1676 kj The Thermite reaction (used in on-site welding of large objects): Aluminum Oxide and ydroxide Anodized aluminum 2 Al(s) + 3 2 O(l) Al 2 O 3 (s) + 6 + + 6 e - Drinking cups made of anodized aluminum. 2 Al(s) + e 2 O 3 (s) Al 2 O 3 (s) + e(s) Aluminum alides riedel Crafts Alkylation Bonding in Al 2 Cl 6 C 2 5 Cl + AlCl 3 [C 2 5 ] + + [AlCl 4 ] - Amphoteric hydroxide Aluminum Sulfate and Alums Al(O) 3 (s) + 3 3 O + (aq) 2 [Al( 2 O) 6 ] 3+ (aq) Al(O) 3 (s) + O - (aq) 2 [Al(O) 4 ] - (aq) Al 2 SO 4 18 2 O 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. ew industrial uses. Tl 2 Ba 2 Ca 2 Cu 3 O 8+x exhibits superconductivity up to 125K. 5