Download GROUP 13 ELEMENTS -THE BORON FAMILY -

GROUP 13 ELEMENTS
-THE BORON FAMILY 
Boron - B

Aluminium - Al

Gallium - Ga

Indium - In

Thallium - Tl
• Members of Group13 are:
ELEMENT
Boron
Aluminium
Gallium
Indium
Thallium
SYMBOL
B
Al
Ga
In
Tl
ELECTRON
CONFIGURATION
[He]2s22p1
[Ne]3s23p1
Ar]3d104s2 4p1
[Kr]4d105s2 5p1
[Xe]4f145d106s2 6p1
2
General Properties
• Boron is the only group 13 element that is a nonmetal.
• The remaining members of this group are fairly
reactive metals, and are called p-block elements.
• Aluminium, Al, is the third most abundant element
in the Earth's crust and the rest elements are much
less common.
• All elements show a stable oxidation state of +3,
except for thallium.
3
• The small sizes of the ions, their high charge
and large values for their sum of the First
ionization energy suggest that the elements
are largely covalent.
• Boron is always covalent and many simple
compounds like AlCl3 and GaCl3 are covalent when
anhydrous. However, in solution, the large amount
of hydration energy evolved offsets the high
ionization potential (energy) and all ions exists in
hydrated states.
4
• Unlike the s-block elements, some of the elements
of this group display lower valency state in
addition to the group valency. The heavier
elements in this group show an increased tendency
to form univalent compounds, and univalent thallium
compounds are the most stable.
• This monovalency is due to the s-electrons in the
outer shell remaining paired, and therefore not
participating in bond formation because the energy
to unpair them is too great. This tendency occurs
among heavy elements in the p-block and is called
the INERT PAIR EFFECT.
5
• Inert Pair Effect is the resistance or
reluctance of s-electrons to get unpaired, get
lost, get promoted or take part in covalent
bonding. It is only p orbital electrons, which
are involved in bond formation.
6
Appearance
• Group 13 metals have silver luster and erratic
variation in melting points down the group. B(2300
ºC), Al(660.4 ºC), Ga(29.78 ºC), In(152.6 ºC) and
Tl(303 ºC),
• The low melting point of Gallium is reflected in the
unusual structure of the metal, which contains Ga2.
Ga, In and Tl ( thallium develops a bluish tinge on
oxidation.) are mechanically soft metals. Boron is a
non-metallic gray powder. The header elements of
each group in the periodic table often displays
properties anomalous to the rest of the group.
7
General Reactivity
• The general trend down Group 13 is
from non-metallic to metallic character.
– Boron is a non-metal with a covalent
network structure.
– The other elements are considerably larger
than boron and consequently are more ionic
and metallic in character.
8
– Aluminium has a close-packed metallic
structure but is on the borderline between
ionic and covalent character in its
compounds.
– The remainder of Group 13 elements are
generally considered to be metals,
although some compounds exhibit
covalent characteristics.
9
• It should be borne in mind that group 13 is the
first group of the periodic table to contain a
non metal (Boron). The remaining elements
of group 13 are metals (sometimes called
poor metals).
• Electropositive character /nature of the
element in this group increases from Boron to
Aluminium and then decreases from
Aluminium to Thallium (why? See next slide)
10
Reason
This increase of electropositivitity from Boron to
Aluminium is the the usual trend according or
associated with increasing size. However, B and Al
follows immediately after s block elements,
while Ga, In and Tl follows after
d block
elements. So the extra d-electrons in Ga, In and Tl
do not shield the nuclear very effectively, so that the
orbital electrons are more firmly or tightly held and
the metal are less electropositive. This is evidenced
by the increase of ionization energy between Al and
Ga even though the large atom would be expected to
have a lower value
11
Occurrence and Extraction
• Group 13 elements are not found free in nature, but
are all present in various minerals and ores.
Aluminiun is the most abundant metal in the Earth's
crust making up 8% of the Earth's crust and it is
occurring in igneous rocks such as:
– Feldspars (Group of rock forming minerals
which make up as much as 60% of the
earth's crust: (KAlSi3O8, NaAlSi3O8 and
CaAl2Si2O8 )
12
– Micas (group of sheet silicate mineral chemically
given by general formula X2Y4-6Z8O20(OH, F)4
where X = K, Na or Ca; Y = Al, Mg, Fe and Z = Si)
– Clays (naturally occurring material composed
primarily of fine grained minerals which show
plasticity through a variable range of water content
e.g. Kaolinite, Al2Si2O5(OH) and Pyrophyllite,
Al2Si4O10(OH)2 .
– Cryolite (Na3AlF6 – sodium aluminium fluoride)
13
– Spinel (MgAl2O4)
– Bauxite (Al2O3..H2O)
– Gemstone, which are impure form of the oxide of
Al2O3 containing small amount of transition metals
that give them colours. e.g.
 Ruby: Al2O3 + traces of Cr3+
 Blue Sapphire: Al2O3 + traces of Fe2+, Fe3+ and Ti+4
 White Sapphire: The germ from aluminium itself
14
Aluminum (Al), among Group 13 metals exists
as aluminosilicates in the Earth’s crust and is
more abundant than iron.
o
The most important mineral for metallurgy is
bauxite, Al2O3.H2O
o
Although the Al metal was as valuable as noble
metals the 19th century, the price fell
dramatically after it came to be manufactured in
large quantities by electrolysis of alumina, Al2O3,
melted in cryolite, Na3AlF6.
15
o However, because its production requires
consumption of a large amount of
electrical power, the metallurgy of
aluminum is economically feasible only in
countries where the price of electrical power
is low.
o The properties of aluminum are well
known as it is widely used and
encountered in every day life, for
example in coins, aluminum foil, cooking
pans, aluminum window sashes, etc.
16
ECTRACTION OF ALUMINIUM
17
Introduction
Aluminum is the most important metal in the Boron
Family. It is a metal with the chemical symbol Al and
atomic number 13. It has the electron configuration
[Ne]2s22p1 and mainly have the oxidation state of +3.
This element isthe most abundant metal in the Earth's
crust (7.5-8.4%). Even though it is very abundant,
before 1886 aluminum was a semiprecious metal.
Aluminum was hard to isolate due to it's high melting
point. However in 1886, simultaneously two scientists
discovered a method to isolate the element through
electrolysis.
18
Aluminum is a soft malleable metal with a silver or
gray color. It is a very reactive element so it is found in
nature combined with other elements. One would
think aluminum will react with water but in reality
aluminum is protected by a layer of Al2O3, which is
known as anodizing. The thickness of the layer can
vary through galvanic reactions. Another important
feature of aluminum is that it is a good reducing
agent. Aluminum extracts oxygen from any metal
oxide. This reaction is known as the thermite
reaction:
Fe2O3(s) + 2Al(s)  Al2O3(s) + 2Fe(l)
.
19
• 45% Al produced in the U.S. is from recycling
aluminum scraps.
• Impure Al2O3 are found as gemstones such as
ruby and sapphire
EXTRACTION PROCESS OF ALUMINIUM
20
EXTRACTION OF ALUMINIUM
• The most important aluminium-containing minerals
are bauxite (Al2O3..H2O) and cryolite (Na3AlF6).
• Aluminium is the most widely used element in this
Group.
• It is obtained by the electrolysis of aluminium
oxide, which is purified from bauxite.
• The melting point of the aluminium oxide is too
high for electrolysis of the melt, so instead it is
dissolved in molten cryolite, Na3AlF6 (Sodium
hexafluoroalumiminate)
21
• Aluminium is quite an electropositive and
reactive and cannot be extracted chemically
using carbon as a reducing agent from its ore,
bauxite because it forms carbide.
• It is produced on an industrial scale by the
electrolysis of bauxite. The electrolytic
production today is achieved by the Bayer –
Hall Herout
processes (major industrial
process for production of aluminium)
22
Bayer – Hall Herout processes
23
2Al2O3(l) + 3C ¾¾® 4Al (l) + 3CO2 (g)
2Al2O3(l) + 3C ¾¾® 4Al (l) + 3CO2 (g)
o The Bayer - Hall Heroult process produces
primary aluminium from bauxite ore. Initially,
the Bayer process produces pure alumina
(Al2O3) by dissolving the raw bauxite ore in
aqueous alkali solution. This is carried out at
high pressure and temperature. Pure Al(OH)3
is precipitated from the resulting solution,
which allows separation of insoluble
impurities. It is then calcined to pure alumina
24
o Next, the Hall-Héroult process involves an
electrochemical cell and pure Al2O3 as the feed
material. It is dissolved in a molten Cryolite,
Na3AlF6, itself modified by the addition of AlF3,
CaF2 and others, which brings the melting point
to 950–1000°C).
The addition of the cryolite
therefore saves energy operational cost.
o At the cathode, the Al2O3 is reduced to molten
Al. At the anode, oxygen from the alumina reacts
with the C electrode to form CO2(g). The overall
cell reaction is written as:
2Al2O3(l) + 3C  4Al (l) + 3CO2 (g)
25
Note that in this process, molten Bauxite
(Al2O3.nH2O) is the electrolyte. The melting point of
Bauxite is high above 2000°C. This mean that large
amount of energy is needed. To minimize the energy
operational cost, some cryolite (Na3AlF6 ) reduces the
melting point to about 1000°C. The aluminium metal
is produced at the cathode and oxygen is liberated at
the anode where it reacts with carbon electrode to
form CO2 gas.
Aluminium is a reactive metal, but is always found
with a protective, coherent oxide layer, which renders it
inert in acids, although it is attacked by alkalies
26
• The electrical conductivity of Al is about 60%
that of copper. Its use in electrical
transmission is favoured by its lightness and
prices
27
EXTRACTION OF BORON
• The primary sources for boron are hydrated sodium
borates such as the mineral borax ( NaB4O7.10H2O)
28
• Boron is found in ores widely distributed in
Earth's crust.
• Largely, boron is found in concentrated
deposits, as borax, that is hydrated borates,
Na2B4O7.10H2O and similar tri, tetra and
pentaborates of calcium and sodium.
• On reduction scale, Boron is formed by
magnesium or sodium reduction of the oxides
(B2O3) or using H2 in the BCl3 in the presence
of Tungsten (W) filament
29
• Preparation of some boron compound from
Borax is as outlined below
30
Boron
o Boron is the top element of group 13 and is
the only non-metal of this group and the its
crystalline form is very hard, inert and nonconducting.
o the amorphous form of Boron (which is more
common than crystalline) is much more
reactive.
o reacts directly with other metals to give hard,
inert binary compound of various formulae
called borides, which resembles the carbides
and nitrides, that is:
B + M → M2B, MB, M3B4, MB2, MB4, MB6, MB12. 31
• It has the electron configuration [He]2s22p1 and
mainly the oxidation state of +3.
• This element does not exist alone. It forms
compounds which can be widely found in the Earth's
crust.
• Boron is an essential nutrient for plants. Also there
are a few locations where boron ores, known as
borax, are found in great concentrations
32
• Boron forms hydrides and the simplest boron
hydride found is diborane, B2H6. Boron hydrides
are used to synthesize organic compounds.
• One main compounds used to form other boron
compounds is boric acid. Boric acid is a weak
acid and may be formed by the following
reaction
B2O3 (s) + 3 H2O (l)  2B(OH)3 (aq)
Boric acid
B(OH)3 (aq) + 2H2O(l)  H3O+(aq) + B(OH)4-(aq)
33
• Boric acid can be found in products that we
use in our daily lives, such as disinfectants,
insecticide, glass, bleaches, and dyes.
• Boron produces a green flame.
• Research indicates Boron may be beneficial
for the treatment of arthritis.
34
Gallium, Indium and Thallium
• The elements Gallium, Indium and Thallium
are only found in the form of miner
components of various minerals and the
elements are produced or extracted by
electrolytic reduction in aqueous solution
• Gallium, Indium and Thallium are relatively
soft and reactive, which readily dissolve in
acids.
35
Gallium, Ga
Gallium has the chemical symbol Ga and atomic number
31. It has the electron configuration [Ar]2s2 2p1 and +3
oxidation state. The melting point is 29.8º C and therefore
melts by increasing room temperature by a little. Gallium is
important because it forms gallium arsenide (GaAs), which
can convert light directly into electricity. Also due to
thermite reaction, aluminum can extract oxygen from water
and hydrogen is released. However, as mentioned above,
aluminum forms a protective coat in the presence of water.
By combining gallium and aluminum, this protective layer
does not form and aluminum will reduce water to
hydrogen. This alloy can provide a great hydrogen source.
36
• Gallium is one of three elements that naturally occur
as a liquid at or close to room temperature, the other
two being mercury and Cesium. Ga has a melting
point of 29.76 °C and a boiling point of 2204 °C
• Gallium easily forms alloys with most metals,
and is used to create low melting alloys and
used in in low temperature solders
• Gallium has a liquid range of 2174°C, one of the
largest liquid ranges of any metal, so it has found
use in high temperature thermometers.
37
• Gallium is an important element in the
electronics industry and has applications as
doping material in semiconductors. One of the
most important of these materials is gallium
arsenide, GaAs, which can produce laser light
directly from electricity and is used in diode
lasers and Light Emitting Diodes (LEDs)
• The fluorescent compound MgGa2O4 is used
in photocopiers.
• Due to it's melting point, gallium melts in our
hands.
38
Indium, In
Indium has the chemical symbol In and atomic
number 49. It has the electron configuration [Kr]
2s22p1 and may have +1 or +3 oxidation state.
However the +3 oxidation state is more
common. It is a soft malleable metal and similar
to gallium, indium can form InAs which is found
in photoconductors in optical instruments.
39
• Indium is also extracted by electrolytic
process and like gallium is also an important
element in the electronics industry.
• Many indium compounds find applications in
semi-conducting materials for transistors,
thermistors and photo-active devices, e.g. A
thin layer of indium tin oxide can be found on
LCD displays, such as through the monitor in
which you are reading (see next slide)
40
Screen on the keyboard containing a thin layer of
indium tin oxide
41
Thallium, Tl
Thallium has the chemical symbol Tl and atomic number
81. It has the electron configuration [Xe] 2s22p1 and has the
+3 or +1 oxidation state. Since thallium is heavier, it has a
greater stability in the +1 oxidation state (inert pair effect).
Hence, it is found more commonly in its +1 oxidation state.
Thallium is soft and malleable. It is very poisonous but
nevertheless it is still used, such as for high-temperature
superconductors. For its toxicity, thallium was widely used
in insecticide and rat poison but in 1975, its uses was
prohibited. Currently its usage is limited and must be
handled with care.
42
• Thallium is extracted from the fine dusts from the
sulphide ores of other elements by dissolution in
warm dilute acids. It is then separated from
impurities and purified by electrolysis and deposition
• Thallium and all its compounds are extremely toxic.
The element itself has no major uses; however the
thallium(I) compound Tl2SO4 was once used as
rodenticide and ants killer. Its use for these purposes
has since been banned in most countries due to its
odourless and tasteless nature making the risk of
accidental poisoning unacceptably high
43
• The electrical conductivity of thallium sulphide
changes with exposure to infrared light, and
so this compound is used in photocells.
• Thallium oxide has been used to produce
glasses with a high refractive index, and is
also used in the manufacture of photocells.
• At one time, thallium sulphate was used in
medicine as a depilatory agent, and thallium
carbonate was used to treat mildew in textiles.
44
• Thallium bromide-iodide crystals have been used as
infrared optical materials. Thallium has been used
in treating ringworm and other skin infections;
however, its use has been limited because of the
narrow margin between toxicity and therapeutic
benefits.
• Thallium also has two stable states, Tl (I) and Tl (III),
and Tl (II) is a mixed valence compound of
monovalent and trivalent Tl.
• Since the element is very poisonous the metal and
its compounds should be handled carefully.
45
• Thallium is a little more reactive and is
oxidized by air to get a tinge oxide
• Thallium changes color when exposed to air
(metallic gray color  Bluish-grey tint)
• Thallium has no taste or odor warning its
presence.
46
Physical Properties of Group 13 metals
47
Property
B
Al
Ga
In
Tl
5
13
31
49
81
Outer electron configuration
2s22p1
3s23p1
4s23d104p1
Atomic rradii (pm)
80-90
143
122
167
170
Ionic radii (pm)
20
54
62
80
89
Electronegativity
2.37
1.50
1.60
1.70
1.80
Melting point (°C)
Boiling point (°C)
2300
3650
660
2467
29.7
2403
156
2080
304
1357
Density (g/cm3)
2.37
2.696
1.607
7.310
1.80
Atomic number (Z)
Ionization energies - 1st
Ionization energies - 2nd
Ionization energies - 3rd
M(s)  M3+(aq) + 3e-
Standard Reduction Potentials
(V, at 25°C)
M2+(aq) + 2 e-  M(s)
800.6 577.6
2427 1816
3659 2744
-0.87 -1.66
578.8
1979
2962
-0.53
5s24d105p1 6s24f145d106p1
558
1820
2704
-0.34
589.3
1970
2877
-0.72
48
Physical properties continues
Hardness
Electroconductivity
-
2.75
1.5
1.2
1.25
59.7
9.1
19.0
8.82
49
• The influence of the non-metallic character in
this Group is reflected by the softness of the
metals.
– The melting points of all the elements are
high, but the melting point of boron is much
higher than that of beryllium in Group 2,
whereas the melting point of aluminium is
similar to that of magnesium in Group 2
(diagonal relationship).
– The densities of all the Group 13 elements
are higher than those of Group 2 elements.
50
• The ionic radii are much smaller than the
atomic radii.
– This is because the atom contains three
electrons in a quantum level relatively far
from the nucleus, and when they are
removed to form the ion the remaining
electrons are in levels closer to the nucleus.
In addition, the increased effective nuclear
charge attracts the electrons towards the
nucleus and decreases the size of the ion.
51
CHEMICAL PROPERTIES
GROUP 13 ELEMENTS
52
The chemical properties of Group 13 elements reflect
the increasingly metallic character down the group.
Here only boron and aluminium will be considered.
• Boron is chemically unreactive except at high
temperatures.
• Finely divided boron is usually impure and burns in
air to form oxide and nitride:
4B(s) + 3O2(air)  2B2O3 (Oxide) ;
2B(s) + N2(air)  2BN (Nitride).
Accordingly in halogen Boron form trihalides
2B(s) + 2X3 (g)  2BX3.
53
• Boron reacts with nitric acid and sulphuric acid and
liberates hydrogen with NaOH
• Pure crystalline boron is unreactive except at high
temperature or with reagent such as hot conc. H2SO4
or Na2O2 (sodium peroxide).
• Boron forms an intensive and interesting series of
hydrides, the boranes (BH3) the simplest B2H6
(diborane). Higher borane also exist.
• Boranes are highly unstable due to their extreme
electron deficiency. Their highly exothermic reaction
with oxygen lead to their consideration as rocket
fuels by the space program
54
• Aluminium is a highly reactive metal which is readily
oxidised in air. This oxide coating is resistant to acids
but is moderately soluble in alkalis.
• Aluminium can therefore reduce strong alkalis, a
product being the tetrahydroxoaluminate ion,
[ Al(OH)4 ]• Aluminium also reacts violently with iron(Ill) oxide to
produce iron in the Thermit reaction
2Al(s) + Fe2O3(s)  2Fe(l) + Al2O3(s)
55
GROUP 13 COMPOUNDS
• Because of their electron-deficient nature,
Group 13 compounds containing the element
(M) in the (+3) oxidation state have a formally
vacant npz orbital and usually act as Lewis
acids (electron acceptors).
56
Oxides (M2O3) -SESQUIOXIDE
• The M2O3 of all the elements can be made by
heating the elements in oxygen:
4M (s) + 3O2(g  2M2O3 (s)
though B2O3 is more usually made by dehydrating
boric acid (refer the reaction profile provided below)
Note
Sesquioxide is an oxide containing three atoms of
oxygen with two atoms (or radicals of another
element. For example, aluminium oxide (Al2O3) is a
sesquioxide.
57
B(OH)3
Orthoboric acid
100 oC
-H2O
+H2O
Red hot
-H2O
HBO2
Metaboric acid,
+H2O
B2O3
Boron oxide
which exists in
three crystalline
forms of which
contains the cyclic
unit
OH
H
O
B
B
OH
O
O
H
B
OH
58
• H3BO3 is a weak acid. Its weakly acidic nature is
due to its electron deficient tendency. The electron
deficiency B(OH)3 accepts an OH- ion from the self
ionization of water forming a complex ion:
B(OH)3 + 2H2O  [B(OH)4] + H3O+
• The hydroxide boric acid B(OH)3 is formed by the
hydrolysis of many boron compounds. It has a
layer structure made up of planar molecules linked
by hydrogen bonding (ref slide overleaf). It is a Lewis
acid that acts as a Brønsted acid in protic solvents.
In water the equilibrium
59
• So a source of H3O+ in B(OH)3 (aq) is water
itself. The ionization and the fact that B(OH)3
is a monoprotic (not triprotic) acid imply that
the reasonable formula of boric acid is B(OH)3
and not H2BO3.
• Orthoboric acid (H2BO3) behaves as a weak
monobasic acid
H2BO3 (aq)
H+ + HBO3-
60
Upon titration with NaOH solution, H2BO3
gives salt of sodium and water:
H2BO3 + NaOH(aq)  NaBO2 +
2H2O
sodium metaborate
(salt of weak acid)
On hydrolysis, borates salts produces basic
solution, which accounts for their use in
cleansing agent. Boric acid can be used as an
insecticide for killing roaches and as an
antiseptic in eyewash solutions
61
Oxides of oxidation state +3 of the Group 13 Elements
Oxide
B2O3
Property


Al2O3 and Ga2O3
In2O and Tl2O3



Weak acid
many metal oxides gives glasses
with B2O3 as in the borax bead test
Amphoteric
Weak basic
Tl2O3 gives O2 and Tl2O on heating
to 100 °C
62
• Boric oxide and borates find extensive
application. Borax (NaB4O7.10H2O) and other
borates find use in water treatment and in
preserving timber from insect. Large amount
of sodium or calcium borates and of boric
acid or oxides are used in glass manufacture.
• Borosilicates glass have a lower coefficient of
thermal expansion than the more conventional
ones; therefore more robust under heating.
Sodium perborate (NaBO3.4H2O) is widely
used as a bleach
63
• Hydration of oxides gives a variety of hydrates and
hydroxy species
MO3
H2O
M(H2O)63+
M(H2O)5(OH)2+
hydrates
hydroxy species
• Boron oxide, B2O3, is an acidic oxide and an
insoluble white solid with a very high boiling point
(over 2000K) because of its extended covalentlybonded network structure. Aluminium oxide, Al2O3 is
also amphoteric.
64
• B2O3, (s) is a non-metallic oxide which is acid in its
properties. On heating with metal oxide it gives
metaborates which often have characteristic colours.
This is the basis of the borax beads test
CoO (s) + B2O3
D
 Co(BO2)2
65
Oxides of Aluminium
• The only oxide of aluminium is alumina, Al2O3,
which comes in a variety of hydrated and anhydrous
forms, and also occurs in minerals
•
It is made by dehydrating Al(OH)3 or from the
elements . They are all white or transparent. α-Al2O3
and g-Al2O3 are the two anhydrous forms and
differ in the arrangement of the Al and O atoms
– α-Al2O3 is a hard substance that is stable at high
temperatures and resists hydration by water and
reaction with acids.
66
– g-Al2O3 on the other hand will readily take up water
and dissolve in acidic solutions. The hydroxide Al(OH)3
doesn't exist in minerals but can be produced as a
precipitate by bubbling carbon dioxide through basic
solutions of Al(OH )4Al2O3 (aluminium oxide) has several common names:
 Alumina when in crystalline form
 Corrundum when pure
 Gemstone: Germ forms of Alumina are
 Ruby: Al2O3 + traces of Cr3+
 Blue sapphire: Al2O3 + traces of Fe2+, Fe3+ and Ti4+
67
2Al(s) + 3H2O ® Al2O3 + 6H+ + 6e
Al2O3 is:
o an amphoteric and ionic compound. This is due
to small Al3+ ions and small O2- ion making a
strong ionic bond.
o made by dehydrating Al(OH)3 or from elements
2Al(s) + 3H2O  Al2O3 + 6H+ + 6e
• Al has a strong affinity for oxygen, and the
heat of formation of Al2O3 is over 400 C kcal
per mole 1680 kJ per mole.
68
• Accordingly Al is used in the thermite
reduction of metal oxides
3Mn3O4 + 8Al  4Al2O3 + 9Mn
• Al2O3 reacts with acid and base (amphoteric)
Al2O3(s) + H3O+ (aq)  3H2O + 2[Al(OH2)6]3+
Al2O3(s) + OH- (aq)  3H2O + 2[Al(OH)4]- (aq)
69
• Al(OH)3 is an amphoteric and when
– (i) acts as a base, gives salts with acids which
contain the [ Al(H2O)6 ]3+ ion
– (ii) acts as acid give rise to salts called aluminates,
which contain AlO2- or AlO33-.
Al(OH)3  H+(aq) + AlO2- + H2O
Al(OH)3  3H+(aq) + AlO33Recall that amphoterism is observed for
higher elements of Group 2 and 13 as BeO,
Al2O3 and Ga2O3. Hence Al2O3 is
70
 very hard due to cubic closest structure (ionic
compound) material, is often used as an abrasive
material,
 resistant to heat ( M.P = 2020 °C) and for this
reason, it is used for high temperature furnaces,
and as a catalyst support in industrial process,
 relatively unreactive except at high temperatures,
 stable at high temperature and therefore utilized as
refractory material.
71
HALIDES OF GROUP 13 ELEMENTS
All elements form trihalides. They are nonpolar trigonal
planar molecules in which the boron atom is sp2
hybridized
• The halides of boron are BX3 are all volatile, highly
reactive, covalently bonded molecular compounds
and are gases
• Boron halides are all hydrolyzed by water
• The Boron fluoride (BF3) form fluoroborates, and the
other halides giving boric acids
BF3 + H2O  [BF3OH] H
BCl3 + H2O  H3BO3 + 3HCl
72
• Boron (an electron deficient atom) in BX3 has
6 electrons in its outer shell. It can readily
accept a lone pair of electrons from a donor
atom (O, N, P or S ). This tendency makes
BF3 a useful organic catalyst for Friedel Craft
reaction such as:
 Alkylations
 Acylation
 Estirification
 Polymerization of olefines
73
• The fluorides of: Al, Ga, In, and Tl are
ionic having high melting points. The other
halides are covalent when anhydrous. AlCl3 ,
AlBr3 and GaCl3 exist as dimers thus attaining
an octet of electrons
74
This dimeric formula is
retained or prevails when
the halides dissolves in
non-polar solvent (e.g.;
Benzene). But because of
high heat of hydration
when halides dissolves in
water, the covalent dimer
Figure. 1
is broken into [M.6H2O ]3+
and 3X 75
• Aluminium, Gallium and Indium all form
trifluorides, MF3 which are ionic in nature and
have high melting points (~ 1000 °C).
– The chlorides, bromides and iodides of
these metals are covalent and much more
volatile, having much lower melting points.
They exist as dimeric molecules with the
formula M2X6 using two halide atoms to
bridge the metals which have tetrahedral
configuration as shown in Figure 1 above.
– They are also soluble in non-polar organic
solvents.
76
o The most important halide of boron is boron
trifluoride, which is a gas.
o Aluminium chloride, AlCl3, is a volatile solid
which sublimes at 458K. The vapour formed
on sublimation consists of an equilibrium
mixture of monomers (AlCl3) and dimers
(Al2Cl6). It is used to prepare the powerful and
versatile
reducing
agent
lithium
tetrahydridealuminate, LiAlH4.
77
o Both boron chloride and aluminium chloride act
as Lewis acids to a wide range of electron-pair
donors, and this has led to their widespread
use as catalysts. Aluminium chloride is used in
the important Friedel-Crafts reaction
o AlX3 (Aluminium halides) are very reactive
lewis acids – they accepts a pair of electrons
forming an acid base compound called adducts
AlCl3
Lewis Acid
+
C2H5 – O – C2H5
dimethyl ether ( Lewis base )
78
AlCl3
Lewis Acid
+
C2H5 – O – C2H5
dimethyl ether ( Lewis base )
79
o AlX3 is used in a number of reactions in which
by means of adduct formation they act as
catalyst. For example when BENZENE is
treated with Acyl halide (ClCOR ) in the presence
of anhydrous AlCl3 as catalyst 
ketone
Aromatic
80
SELF ASSESSMENT QUESTION ( SAQ)
a. By making use of chemical equation from a summary diagram
of the reaction chemistry, write chemical equation for the
successive conversions of borax to:
(i) B2O3
(ii) Boric acid
(iii) Impure boron metal
b.
Using the summary diagram of the reaction chemistry, write chemical
equation for the sequence of the reaction by which borax is converted to
diborane and BF3
c.
Predict the probable products of the following reaction and write the
balanced chemical equations
i.
BF3 and excess NaF in acid aqueous solution
ii.
BCl3 and excess NaCl in acidic aqueous solution
iii.
BBr3 and excess NH(CH3)2 a hydrocarbon solvent
81
Compounds of Boron with Electronegative
Elements
• BX3 (Boron halides) are very useful reagents and
Lewis acid catalysts.
• All the BX3 except BI3 may be prepared by direct
reaction of the halogens with Boron element
• The preferred method for BF3 preparation is the
reaction of Be2O3 with CaF2 in sulphuric acid
(H2SO4)
Be2O3(s) + 3CaF2(s) + 6H2SO4(aq)

------ (1)
2BF3(g) + 3[ H3O][HSO4]soln + 3CaSO4(s)
,
82
This reaction (1) is driven by the strong affinity
of concentrated sulfuric acid for the water
obtained by protonation of the solid boric oxide
• BX3 (Boron trihalides) consists of trigonal
planar molecule and unlike the halides of the
other elements in the group BX3 are
monomeric in the gas, liquid and solid states:
BF3 and BCl3 are gases, BI3 is a volatile
liquid and BI3 is a solid
83
Properties of Boron trihalides
BX3, X =
Halide
MP(°C)
BP (°C)
DGf (kJ/mol)
F
Cl
Br
I
-127
-107
-46
49
-100
12
91
210
-1112
-339
-232
+21
The trend in volatility is consistent with the
increase in strength of disperse/London force
(intermolecular
forces
associated
with
instantaneous and induced dipoles) with the
number of electrons in the molecules.
84
• Their structures are trigonal planar and
monomeric (they do not dimerized in the way
the BH3 does.) As an example, the structure of
BBr3 is shown below.
85
o The B-X bond distances are shorter than
might be expected, and the B-X bond
energies are correspondingly higher.
o The B-F bond energy (646 KJmol-1) is the
highest known for a single bond.
o All this has lead to the proposal that
some π-bonding may occur between the
unhybridized 2p orbital of the boron and
the filled np orbitals of the halides.
86
The structure of BX3
molecules. The boron
atom is sp2 hybridized.
Some evidence exists
that some π- bonding
may occur between
the unhybridized 2p
orbital of the boron and
the filled (shaded) np
orbitals.
87
• BX3 are Lewis acids and the order of their
strength in this role of Lewis acidity (i.e. the
sequence of acidic strength is
BF3 < BCl3 < BBr3
in contrary to the order of electronegativity of
the attached halogens (F > Cl > Br).
This is usually explained by postulating the strongest
B–X π-bonds to occur between the two secondperiod elements B and F, with weaker π-effects
occurring for the heavier halogens.
88
Furthemore the presence of these partial
double bonds is sought to explain when the
monomers are stable, and dimers of these
boron halides are now know
More simplified explanation
This (i.e. of acidicty) trend is considered to
stem from the greater X-B p bonding for the
lighter, smaller halogens giving rise to the
occupation of the p-orbital on the Boron atom
by the electrons supplied by the halogen
atoms. This is illustrated as follows:
89
B(Z = 5): 1s22s22p1, F ( Z = 9 ): 1s22s22p5
↑
↑↓
2Px 2py 2pz
↑
2px 2py 2pz
z
z
p bond
x
↑↓

y
x
p bond
90
• BX3 (all) forms simple Lewis complexes with
suitable bases, e.g.;
BF3(g) + NH3(g) → F3B-NH3
BF3(g) + N(CH3)3 → F3B-N(CH3)3
91
• The boron halides, like boric acid, are Lewis
acids; that is they can readily accept a pair of
electrons into their vacant, unhybridized p
orbitals. Fox example

BF3 + F  BF4-
 BF3 + R-O-R  F3BOR2
• The chlorides, bromides and Iodides of Boron are
susceptible to protolysis by mild proton sources
such as: H2O, alcohol (ROH), Amine (RNH2). For
example on hydrolysis of BCl3:
92
BCl3(g) + 3H2O(l) → B(OH)3 aq + 3HCl(aq)
The reaction is so rapid with the formation of
complex Cl3B-OH2, which then eliminates HCl
and reacts further with H2O
• The boron halides are extremely versatile
reagents. Beside undergoing many complexformation reactions, they also undergo
protolysis with protic reagents to produce the
esters of boric acid. A display of typical
reactions is provided in the diagram following
diagram (Fig. 2)
93
Fig. 2: Reaction of BX3
B(OH)3
H2O
B(NH2)3
B(OR)3
This “reactivity chart ”
does not apply to
BF3, which resists
protolyis
reactions
under mild conditions.
PROT OLYSIS
RNH2
ROH
BX3
PR3
SR2
NR3
COMPLEX FORMATION
X3BPR3
X3BNR3
X3BSR2
Figure. 2: Reaction of BX3
94
Exercise 1
• AlF3 is almost insoluble in anhydrous HF, but
dissolves if KF is present. Passage of BF3
through the resulting solution causes AlF3 to
precipitate. How can you explain this
observation?
Exercise 2
• If Aluminium reacts with air and dissolves in
both acid and basic solution, explain why it
does not dissolve in pH-neutral water?
95
Sulphate of Group 13
• Aluminium (III) sulphate is sometimes added
to water supplies in order to remove fine
particles, colour and bacteria.
• As aluminium sulphate has an acidic pH,
water companies usually adjust the water to a
pH of between 7 and 8 and, under these
alkaline conditions aluminium hydroxide
precipitates out as fine solid particles which
can then be removed by means of sand filters.
96
• Thallium (I) sulphate Tl2SO4 adopts the same
structure as potassium sulphate, K2SO4.
• Thallium (I) sulphate is soluble in water and is highly
toxic since the thallium (I) cation is very similar to
potassium and sodium cations, which are essential
for life.
• Once the thallium ion enters the cell, many of the
processes that transport potassium and sodium are
disrupted. Once it has entered the body, thallium
sulphate concentrates in the kidneys, liver, brain,
and other body tissues.
97
• Thallium (I) sulphate has been used as a
rodenticide, Note that Thallium (I) sulphate
was used by Saddam Hussein (and others)
as choice of poison for dealing with Iraqi
dissidents.
98
Hydroxides
• Al(OH)3 is amphoteric and reacts with acids in a
manner as metal hydroxides do.
Al(OH)3 (s) + 3H3O+ → [ Al( H2O )6]3+ (aq)
• Al(OH)3 also reacts with a base in the reaction
represented as the formation of a hydro-complex
Al(OH)3 (s) + OH-  [ Al( OH )4]- (aq)
99
• When Al(OH)3 dissolves in a base, hydroxide
ion and water bonds to Al ion forming a
complex ion [ Al( H2O )2 (OH)4 ]- . The reaction
is as follows
Al(OH)3(s) + OH- (aq) + 2H2O(l)  [Al(H2O )2 (OH)4 ]-
• Al(OH)3 is used in the purification of water
because its gelatinous character enables it to
carry down any suspended material in the
water including most of the bacteria.
100
Exercise 3
• Insoluble hydroxide of Mg and Al can be
prepared by addition of a solution of a soluble
hydroxide (NaOH) to a solution of salts of Mg
or Al chlorides, nitrates or sulphates. Explain
why an excess of hydroxide solution is
avoided in the preparation of Al(OH)3 ?
Exercise 4
• Explain why a lot of energy is consumed in the
electrolytic production of aluminium?
101
Hydrides
• Special compounds that start of predominantly
covalent and become more ionic as we go down the
group.
• Most of the group 13 elements react directly with
hydrogen, and large number of interesting hydrides
are known.
• Boron forms an extensive and interesting series of
hydrides, the boranes. The simplest of these is not
BH3 as expected, but its dimer B2H6.
102
• The 8 well characterized boranes which fall
into two series BnHn+4 and less stable series
BnHn+6 are
1.
2.
3.
4.
5.
6.
7.
8.
B2H6
B4H10
B5H9
B5H11
B6H110
B9H15
B10H14
B10H16
Diborane
Tetraborane
Pentaborane (stable)
Pentaborane (unstable)
Hexaborane
Nonaborane (enneaborane)
decaborane
decaborane
103
• Many boron compounds lack an octet of
electrons about the central boron atom. This
feature make:
– boron compounds electron deficient, and therefore
make them strong Lewis acids.
– The electron deficiency of some boron compounds
leads to bonding type that have not previously
encountered especially in boron hydride:
104
Definitions
• The electron deficient compound is the one with
two few electrons for a Lewis structure to be written
• The electron precise compound is the one with
correct number of electrons pair on the central atom
• An electron enrich compound is the one with more
electron pair that are needed for the bond formation,
the extra electron pairs being present as nonbonding pair on the central atom
105
• No BH3 as it does not exist as separate molecules
• The simplest hydride is B2H6 and is an example of
electron deficient molecules or compound
Boron Hydride
• The molecule (borane, BH3) may exists as a reaction
intermediate, but it has not been isolated as a stable
compound.
• The B-atom in BH3 lacks the complete octet (i.e. it
has only 6 electrons in the valence shell). The
simplest boron hydride that have been isolated is
diborane (B2H6).
106
The Structure of B2H6 (Diborane) – Multicentre Bonds
The question of interest is what
holds the diborane together?
Explanation of the Structure of B2H6 .
• There are 12 valence electrons at our
disposal for chemical bonding (B has 3,
and H has 1, so 2  B + 6  H = 12)
• Each terminal B  H bond is
standard vanilla two electron
bond, and there are four of then,
thus accounting for a total of 8
electrons (Fig 3)
107
• This leaves a total of four
electrons to be shared between
the two bridging H atoms and the
two B-atoms. For this reason, two
B  H  B bridging bonds are
formed, each of which consists of
two electrons forming what are
called three centre – two
electron bond (3C, 2e) meaning
that 3 atoms share 2 electrons
(This sometimes called banana
bonds because they non-linear
but curved. (Fig. 4 )
108
Figure 4. The Structure of B2H6 (diborane) – multicentre bonds.
Contains a 3-centre-2-electron bond (called a banana bond)
109
• Bridging B-H have a bond order of ½ and this
accounts for the 1600 cm-1 B-H str in the IR.
• 4H’s terminal, 2H’s bridging accounts for the
1H-NMR spectrum.
• Other boron hydrides have similar bonding;
B4H10, B5H9, B5H11, B10H14 etc…
110
1s
2s
2p
• Each B atom is, approximately, sp3 hybridized
(hybridization is just a mathematical tool, for
mixing orbitals to place them at the
degenerate state)
1s

2s



2p
Hybridization


 
1s
2s
2p
Hybridized orbital (sp3)
111
Now consider one of the B atoms
• Two of the four sp3 hybrid orbitals from s
bonds to the terminal H atoms (1s orbitals)
• This leaves two B-sp3 hybrid orbitals, one of
which contains an electron, one of which is
empty
112
• For each bridge therefore one sp3 orbital from
each of the B – atoms combines with the 1s
orbital of the bridging H-atom to form three
new molecular orbitals [recall that n(AO) forms
n(MO)]. One B atom gives its remaining
valence electron to one bridge and the other B
atom gives to the other. Each bridge,
therefore, has two electrons, which fill our new
MO scheme starting with the lowest energy
bonding MO (Figure 4)
113
Figure 4. Molecular Orbital Energy diagram for
Diborane
114
This MO diagram are two sp3 hybrids from each B
and the two 1s AO’s for the bridging H atoms.
115
• The boranes are volatile and decomposes to
B and hydrogen (H2) at red heat
• The boranes are volatile and decomposed by
water or aqueous alkali
B2H6 + 6H2O  H3BO3 +
6H2 (g)
Boric acid
• All boranes reacts with NH3 and the products
depends on the condition imposed. For
example
116
B 2H 6
Excess NH3
 B2H6 .2NH3
Low temperature
B 2H 6
Excess NH3
 ( BN )x ( Boron nitride)
High temperature
2NH3 : 1 B2H6
B2H6  B3N3H6 ( Borazole or Borazine )
Low temperature
117
D
B2H6·2NH3

B3N3H6 (Borazole or
Borazine formely called inorganic benzene)
Borazine
The empty pz-orbital of boron can be part of
delocalized,
aromatic
systems.
Particularly
interesting is the case of "borazine" a compound that
is isoelectronic with benzene
118
• Boranes are highly unstable due to their
extreme electron deficiency. Their highly
exothermic reaction with O2 make them very
useful propellant in rocket or space program
shuttles
119
The Chemistry of B2H6
• NaH + B2H6  NaBH4
sodium borohydride an excellent
reducing agent used in organic
chemistry
• B2H6 – spontaneously flammable in air
• B2H6 + O2  B2O3 + H2O VERY EXOTHERMIC
• B2H6 reacts with many Lewis bases, even
weak ones:
• B2H6 + 2CO  2H3B-CO
also R3N: and R2O etc…..
120
Boric Acid B(OH)3
• Boron oxide is an acid anhydride that reacts with
water to afford boric acid.
B2O3 + 3H2O  2B(OH)3
• Boric acid which in aq. solution is a mild antiseptic
eye and mouthwash, has a planar structure.
• The boron atom assumes sp2-hybridization whereas
the oxygens are sp3. The
• VSEPR theory predicts that the O-B-O bond angle is
120, whereas the B-O-H angle is somewhat less
than 109.5, due to the presence of the two lone
pairs on each oxygen
121
Complexes
• However group 13 elements form complexes much
more readily than the s-block elements on account of
their small size and increased charge density. In
addition to the tetrahedral hydride and halide
complexes, LiAlH4 and H[BF4], many octahedral
complexes such as [GaCl6]3-, [ InCl6 ]3- and [ TlCl6 ]3are known.
• However, the most important octahedral complexes
are those with chelate group such as acetylacetone,
oxalate and 8-hydroxquionoline (Figure 5), which
are commonly used in gravimetric determination of
Aluminium (Al)
122
CH3
O
O
C
CH
M
C
O
O
C
O
M
M
C
O
O
CH3
3
Acetylacetone complex
N
3
Oxalate complex
3
8 hydroxyquinoline
Figure 5
123
Oxidation States and Ionisation Energies
• Boron and aluminium occur only with oxidation
number +3 in their compounds, and with a few
exceptions their compounds are best
described as ionic.
• The electron configuration shows three
electrons outside a noble gas configuration,
two in an s shell and one in a p shell.
124
• The outermost p electron is easy to remove as
it is furthest from the nucleus and well
shielded from the effective nuclear charge.
The next two s electrons are also relativeIy
easy to remove. Removal of any further
electrons disturbs a filled quantum shell so is
difficult. This is reflected in the ionization
energies. The first three ionization energies
are low, and the fourth very much higher.
125
Industrial Information / application
• Boron has limited uses, but is used in flares to
provide a highly visible green colour. Boron filaments
are now used extensively in the aerospace industry
as a lightweight yet strong material. Boric acic acid is
used as a mild antiseptic, and borax as a water
softener in washing powders. Borosilicate glass
contains boron compounds.
126
• Aluminium is one of the most industrially
important materials.
– It is light, non-toxic, has a high thermal conductivity,
– can be easily worked and does not corrode due to its
oxide coating, which is very effective although only
10nm thick.
– It has several domestic uses such as cooking
utensils, aluminium foil and bottle tops, and is widely
used in the building industry where a strong, light,
easily-constructed material is required. These
properties also make it invaluable in the building of
aeroplanes and spacecraft
127
• Boron compounds are used in variable
products such as: adhesive, cement,
disinfectants,
fertilizers,
fire
retardants
(extinguisher), glass, herbicides, metallurgical
fluxes and textile bleaches and dyes.
128
Conceptual Problems
1. In contrast, BX3, AlX3 are dimers in the gas phase.
For example Aluminium chloride has a molecular
formula Al2Cl6 in the vapour phase:
(a)
(b)
(c)
Draw the structure of Al2Cl6
Explain why the molecule is a self acid-base complex
What is meant by dimer and what is the dimer of AlCl3
(give formula)
2. In the Down process, the starting material is Mg2+ in
sea water and the final product is Mg metal. This
process seems to violet the principle of
conservation of charge. Does it? Explain
129
3. Arrange the following compounds in the
expected order of increasing solubility in
water and give the basis for your choice or
arrangement: Li2CO3, Na2CO3 and MgCO3.
4. Explain why a lot of energy is consumed in
the electrolytic production of aluminium
130