Download Boron Group Compounds Oxidation States Boron

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