UNIT 6 ELEMENTS OF GROUP 13

structure
6.1 Introduction I
Objectives
6.2 Oailcrence, Extraction and Uses
Occurrec -
Extraction
uses
i 6.3 General Characteristics
I

- 6.5 Halides of Bpron anel Aluminium

Halides of Boron
Halides of Aluminium
6.6 Oxides of Boron and Aluminium I
Boric Oxide
Aluminium Oxide
6.7 Oxoacids of Boron and Borates
6.8 Borazine
6.9 Complexation Behaviour
6.10 Anomalous Behaviour of Boron
6.11 Summary
6.12 Terminal Questions
6.13- Answers

' --
6.1 INTRODUCTION
In the previous two units, you studied the main features of the chemistry of Group
1and Group 2 elements, i.e. the alkali and the alkaline earth metals. In this unit you -
will study the elements of Group 13, namely, boron, aluminium, gallium, indium and,
thallium. While studying the alkali and alkaline earth metals, you have seen that all
Zhe elements of these two groups are highly reactive metals and the first element o f
each group shows some differences from the rest. In Group 13, the differences
between the first element and the remaining elements become so pronounced that
the first member of the group, i.e. boron is a nonmetal wheieas the rest of the
elements are distinctly metallic in nature. In a way, this is the first group of the
periodic table in which you observe a marked change in the hature of the elements
. down the group.

describe the chemistry of hydrides, halides and oxides of boron and aluminium,
elucidate the structures of hydrides of boron and aluminium,

EXTRACTION AND USES

6.2 OCCURRENC~,
Elements b f Group 13 are sufficiently reactive. Therefore, none of them occurs in
the native state. Some of these elements and their compounds find important uses in
diverse,areas of modern science and technology and even in every day life. Let us,
therefore, first study their occurrence, extraction and raes.
5
 6.2.1 Occurrence
Both boron and aluminium have a high affinity for oxygen, so neither of them is
found native. Boron occurs principally in the earth's crust as boric acid, H3B03and
as borates, such as, borax, Na2B407-10H20,kernite, Na2B407.4H20and
colemanite, Ca2B,0,,.5H20. Aluminium is the most abundant metal (8.13%) in the
earth's crust and is the third most abundant element, next only to oxygen (46.6%)
and silicon (27.7%)). It occurs widely in the complex alumino-silicates, such as, clay
from whicb, however, it cannot be extracted economically. The im ortant ores of
aluminium are bauxite, A1,0,-xH20, where x = 1-3, cryolite, Na3!lF, and
corundum, ~ 1 ~ 0 Gallium
;. (1.9 x lo--'%), indium\(2.1 x lod5%) and thallium (7.0
x lo"%) are much less abundant than aluminium. Gallium and indium are found in
aluminium and zinc ores. But even the richest sources contain less than 1% galliuni
and still less indium. Thallium is widely distributed in nature and occors in sulphide
ores of zinc, lead, copper and iron. \

6.2.2 Extraction
Boron is-obiamed by the red-uction of B2O3wittimagnesium or sodium. B203is first
prepared by strongly katingH,B03 which isobtained by the action of HCI or H2S04-
on a concentrated solution of borax:

Pure crystalline boron may be obtained in small quantities by the reduction of BBr,
with H2 on a heated tantalum metal filament at 1275-1475 K.
'\
Bauxite approximately contains Aluminium cannot be extrpcted economically from the silicate minerals. Therefore,
A~,o,=ss%. Fe203=159~, bauxite is the most important ore for the extraction of alumimum, but this has many
Ti02=2%, sio2=39" and problems. A1203is a very stable compound. It is not reduced by heating in hydrogen.
. H20=25%.
On strong heating with carbon, A120, gives the carbide, AI,C,. As A1203 does not
India is fortunate in having large melt below 2300 K, it cannot be electrolysed conveniently. However, electrolysis of
deposltsof bauxitein a reasonably a solution of A120, in fused cryolite occurs at a much lower temperature of 1100-1300
pure state. K. Thus, aluminium is extracted by electrolysis of purified alumina in fused cryolite.
Pure alumina is obtained from bauxite ore. Powdered bauxite mineral is heated in a
concentrated solution of sodium hydroxide under pressure when alumina and silica
get dissolved:
A1203 + 2NaOH + 3 H 2 0 2 NaAI(OH)4
Si02 + 2NaOH NazSi03 + H 2 0
Iron oxide and titanium dioxide do not dissolve in the alkali and are filtered off as a
sludge. The solution is cooled and most of the aluminium hydroxide is precipitated
either by the passage of carbon dioxide or by seeding
-
with some+reshlyprecipitated
aluminium hydroxide :
2NaAl(OH)4 + C 0 2 ~ A ~ ( O H4) , + Na2C03 + H 2 0

NaAl(OH)4 ~ 4 6J ~+ NaOH
) ~
The silicates remain in solution, since silica is a more acidic oxide than alumina.
Aluminium hydroxide is filtered, washed and heated to give pure alumina.

'.4
Alumma is dissolved in fused cryolite to which calcium fluonfe is added to lower the
v l t ~ n point.
g The solution is then electrolysed at 1100-1300 K in an iron cell, lined
For each k g o f A1 produced. about Mth graphite, which acts as the cathode and carbon rods suspended in the electrolyte
kg Of A'2°3, 0.150 kg Of atting as the anode (Fig. 6.1). Electrolysis of the solution gives aluminium at the
NaOHpO.SO kgoECand 6'0 lo' cathode and oxygen at the anode. The discharged aluminium sinks to the bottom of
k J of electricity are consumed
the cell and is tapped off. Fresh alumina is added as required. The anode is slowly
attacked by liberated oxygen to form carbon monoxide. Therefore, anode has to be
continually replaced, adding substantially to the cost of the process. The temperature
of the cell is maintained by the passage of electric current. Following reactions take
place during electrolysis:
6 -
 302- . 3/20, .+ 6e, at anode
+
~ A I ~ + 6e 2A1, at cathode
Gallium, indium and thallium are usually obtained by electrolysing aqueous solutions
of their salts. This method is not applicable in the case of aluminium salts as they are
hyal G : , sed considerably by water.
-Carbon electrodes

~ n n n n ~ i y :

I=-II -
-- olten c r v o l t t e - A L ~mixture-:
Molten Al

Fig. 6.1 : man^^ d A l

6.2.3 Uses
Boron is used to increase the hardness of steels. Crystalline boron is used in
transistors. Boron is a good neutron absorber and is used as shields and control rods
in nuclear reactors. Boranes are used as high energy fuels, for example, in rockets.
Boric acid is used as' an antiseptic. Borax is used to make heat resistant borosilicate
glass. It is also used for vitreous enamelling of baths, domestic appliances and for
glazing tiles and pottery. Another use of borax is to make peroxyborates, e.g.,
NaBO2.H2O2-3H20,which are useful cleaning and bleaching agents. In the
laboratory, borax is used for standardising acids and in the borax bead test in
qualitative analysis.
~ l u & % u mexhibits useful properties of low density, high thermai and electricat
conductivity, good corrosion resistance with non-toxic nature of the metal and its
compounds. Due to these properties, it is the most widely used non-ferrous metal.
Aluminium is used for making electrical conductors, cooking utensils and wrapping When an alum~niurna91cle is
made the anode in the electrolysis
materials. You must have seen milk bottles capped with aluminium foil. Aluminium of dil. sulphuric or chromic acid, a
is extensively used for structural purposes, either alone or alloyed, in aircraft, ship thick hard film of AI2O3is formed
and buildiig industries. Large amounts of aluminium are converted into alloys, such on the surface of the article. This
as, duralumin and magnalium containing a few per cent of copper or magnesium. process is called anodising. A1203
These alloys are harder and stronger than pure aluminium but possess almost the layer adsorbs many dyes and takes
a very high polish. You must have
same properties of lightness and corrosion resistance which can be further increased seen many bright, colourful
by anodising process. Aluminium beryllium alloys are harder and Iighter than other articles made of anodised
alloys of aluminium and are extensively used in space-craft. However, toxic nature aluminium.
of beryllium makes their handling difficult. Due to its strong affinity for oxygen,
aluminium is a good reducing agent and is used in aluminothermic process for
extraction of metals and in welding. Suspension of finely powdered aluminium in oil
is used as paint.
Anhydrous is an important catalyst, used in organic synthesis and in the
cracking of petroleum. A12(S04)3as such or as potash alum, K2S04.A12(S04)3.24H20
is used for sizing paper, for tanning leather, for waterproofing cloth and as a mordant
for dying cotton. It is extensively used for purification of water and in sewage
treatment.
Because of its extreme hardness, high m.p., non-volatility, chemical inertness and
good electrical insulating properties, corundum (A120,) finds many applications in
abrasives, refractories and ceramics. Large crystals of a-A1203 when coloured with
metal-ion impurities are prized as gemstones, e.g., ruby (cr3+, red), sapphire
(Fe2+"+ni4+ , blue), oriental emerald (cr3+/V3+, green), oriental topaz ( ~ e ~ + ,
yellow), etc. Aluminates are important constituents of Portland cement. Gallium has
the longest liquid range (303-2343 K) of any known substance and so finds use as a
high temperature thermometer liquid.
 Gallium is mainly used in semicondyctor technology. It is used for doping othcr
semicondl~ctorsand in solid state devices such as transistors.
Compounds of Ga with P and As; such as, Gap and GaAs have semiconductor
, properties similar to those of elemental Si and Ge. These are used as light emitting
diodes (LEDs) familiar in pocket calculators, wrist watches, etc. They are also used
in infrared emitting diodes, infrared detectors, photocathodes and photomultiplier
tubes.
The most important applications of indium are in protection of bearings against wear
and erosion, in low-melting alloys and in electronic Thallous salts, being
toxic, are used as fungicides, for the treatment of
ring-worms. Thallium compounds find optical glass on
account of their high refractive power.
We will discuss the general characteristics of these elements in the m x t section but
before that try this ~ A Q .

SAQ 1
a) Write the n a m o r m " 1 a e of two important ores of the most abuAdant
metallic element in the earth's crust.

b) Match the following properties of A1 with the uses to which the metal or its
compounds are put:
i) Good thermal conductivity a) Building ships and aircrafts
ii) Good electrical conductivity b) Utensils
iii) Low density and resistance c) Electric wires
to corrosion
iv) Non-toxicity d) Adsorbent
v) Gel nature of Al(OH)3 e) Food packaging

6.3 GENERAL CHARACTERISTICS

All the elements of Group 13 have similar valence shell electronic configuration of
m2np1,but the underlying core varies. For B and Al, it is the preceding noble gas
core, for Ga and In, it is noble gas plus dl0 and for TI, noble gas plus 4f45d''. This
variation in the electronic configuration of the core is also reflected in the ionisation
. energies of these elements and has a strong influence on their properties.

6.3.1 Physical Properties -

Elements of Group 13 are lesnnetallic than $hose of ~ i o u 1~and s 2. Within the
group, there is a variation in metallic charactey. Boron, the smallest element in this
group is a nonmetal. The other elements in this group are fairly reactive metals. This
is the first group in which 'change from non-metallic to metallic nature occurs. Some
physical properties of Group 13 elements are listed in Table 6.1.
The elements of Group 13 have smaller atomic radii and higher electronegativities as
compared to s-block elembps of the same period. However, these properties do not
vary in a regular way down the group, in contrast to the brapertiesof the element of,
- Groups 1 and 2. You know that the size and the ionisat~orpenergies of atoms depend
on effective nuclear charge of the-atom. B and Al follow immediately the s-block
elements Be and Mg, respectively. Their size and ionisation energies are as expected.
But between the s- and p-block elements of the fourth and successive rows, the
d-block elements, i.e., the transition elements get inserted. Thk insertion of transition
elements results in higher effective nuclear charge of the foutth row elements Ga,
Ge, etc., than expekted by simple extrapolation from the second and third row
elements. In other words, the nuclei of these fourth row elements attract electrons
more strongly than expected and t v s affects their properties. Thus, the atomic size
of gallium is smaller; its electronegativity and ionisation energies higher than
8 expected. Atoms with dl0 . - shell, in general, are smaller and have higher
. inner
ionisation energies. In a similiir way, the inc~usidnof fourteeh electrons in 4f orbital
further affects the size and ionisation energy of TI, As a result of this, irregulaqies
in atomic
-*
radii, electronegativit'y and ionisation energies are seen from B to Tl.
Table 6.1 shows that densities of these elements show a gradual increase while
melting and boiling points fall in value. The exceptionally low melting point of
gallium (303 K). however. has, so far, no simple explanation.

Table 6.1: Some properties of Croup 13 elenvats

Roperty B Al CP In rn
Atomic number 5 13 31 49 81
Atomic weight 10.81 26.98 69.72 114.82 204.38
Electronic structure [ ~ e ] 2 r ~ 2 p[' ~ e ] 3 s ~ 3 p[' ~ r ] 3 d " ' 4 ~ ~ 4 p(~r]4d'"5s~5p'
' [~e]4f~5d'"fis~6~'
Metallic radius (pm) 98 143 141 166 171
Covalent radius (pm) 82 125 125 144 155
Ion~cradius M ~ (pm)
+ 20 50 62 ' 81 95
Melting point (K) 2573 933 303 429 576
Boiling point (K) 2823 2740 2343 2273 1730
Density ( l d x kg m") 2.34 2.7 5 91 7.3 11.8
Ionisatidn 1st 800 577 579 558 589
energy 2nd 2427 1816 1979 1820 1970
(kJ mol-I) 3rd 3658 2745 2%2 2703 2879
Electronegativity (AIR) 2.0 1.45 1.8 1.5 1.45
Electron affinity -
(kJ mol-') -27 -44 -30 -30 -30
Common oxidation state (2)'. 3 3 (1)*,3 1,3 1, (3).
Common co-ordination
numbers 3, 4 3, 4, 6 3. 6 3. 6 3. 6
Natural abundance (%) 1.0 x 1 v 3 8.13 1.9 x l c 3 2.1 x 10-5 7.0 x 10-5

* Corqparatively less stable.

From Table 6.1 you will observe that these elements form small ions of high charge
density and the value of the sum of their first, second and third ionisation energies
is very high. These properties lead us to the conclusion that these elements will prefer
to form covalent rather than ionic compounds. Boron is always covalent and does not
form,^^+ ions, because' the energy required to remove three electrons is very high.
Many simple compounds of A1 and G a like AlC13 and GaC13 are also covalent when
anhydrous. However, in solution, the large amount of hydrrttion energy evolved
compensates the high ionisation energy and all the'metal ions exist in a hydrated state.
This can be explained with the help of Born-Haber cycle as given below. However,
this is a simplified appkoach given only on the bas; of enthalpy changes, whereas the
direction and extent of any reaction depend on the free energy changes which take
into account changes both in enthalpy and entropy.

AH,,, = 326 kJ 4 1 AHd,,=3x122W

Al(g) + 3Cl(g)
I,+I2+13=5138W 1 1 EA=-3~348k.l
AH"","
= -295 kJ ~ l ~ + ( g+) 3Cr(g)
I
I

AH^^^ ' - 4 6 k k ~ 1 1 AHhN=-3X38SW

AI"(~) +' XI-(aq)

AH,,, = 704 + 326 + 366 + 5138 -I1044 - 4630 - 1155

.

Thus, despite tbqlarge ionisation energies of aluminium, the enthalpy of'solution of

AlC13 has a large negative value. Therefore, in aqueous solution, AIC13 exists as
A I ~ + ( and
~ ~Cl-(aq).
) It might be thought that the same argument would apply to
BCI;. But to make the enthalpy of solution of BC13 negafive, the enthalpy of .
 hydration of B" should be - 6009 liJ which is unlikely for the small B ~ cation.
+
Therefore, BC13 hydrolyses in aqueous sohtion to give boric acid and not ~ ~ ' ( a ~ ) :
BC13 + 3H20 H3B03 + 3HCI .

Unlike the s-block elements which show only one stable oxidation state, the elements
of this group show more than one stable oxidation state. While the trivalent state is
important for all the five elements, the univalent state becomes progressively more
stable on descending the group. Thus, for B, A1 and Ga, the +3 state is more stable
than +1 state; for In, both are equally stable, and for thallium the univalent state is
more stable than the trivalent state. Oxide, sulphide, carbonate, sulphate and halides,
etc. of TI(1) are well characterised, and are more stable in aqueous solution than
TI(II1) compounds. This is due to the s electrons in the outer shell of thallium tending
to remain paired and not participating in bonding because the energy required to
unpair them is rather high. This is called the inert pair effect. This effect is noticeable
particularly among the heavier elements in the p-block. In addition to the above,
some compounds are known, in which the elements show +2 oxidation state, e.g.,
B2F4and B2C14. However, these compounds are less stable.
SAQ 2
Covalent radius, ionisation energy and electronegativity of gallium are different from
those expected by simple extrapolation of these properties horn those of boron and
aluminium. Discuss this anomaly briefly.

6.3.2 Chemical Properties

Elements of Group 13 are comparatively less reactive than the alkali and alkaline
earth metals of Groups 1 and 2. However, the +activity of the elements increasts on
descending the group. The cry'stalline form of boron is black. very hard and inert. It
reacts with other elements only at L.gh temperatures. The more common amorphous
form is brown and more reactive than the black crystalline form. Aluminium is a light
and hard white metal. It reacts with atmaspheric oxygen with the formation of a
protective layer of aluminium oxide which prevents further attack. Amalgamation
with mercury removes the oxide coating and a rapid reaction occurs with oxygen,
water or dilute acids. Gallium, indium and thallium are relatively soft and more
reactive than boron and aluminium.
All the elementq except thallium when heated with halogens, oxygen or chalcogens
form halides (EXh), oxides (E203), chalcogenides (E2S3, etc.). Thallium on the
other hand forms TIX, T120, T12S, etc.
The reaction of aluminium with
Aluminium hasa very high affinity for oxygen; this is reflected in high exothermic
Fe203produces.temperatures heat of formation of A1203 (-1676 kJ mol-'). This allows aluminium to extract
approaching 3300 K - enough to oxygen from other metal oxides, and forms the basis of Goldschmidt's
mett the i b n metal produad. This aluminothermic process for Ca, Sr, Ba, Mn, Cr, Mo, etc.:
is called tde (bcraldc nrtioland
is used to weld cricks in iron and 3Mn02 + 4Al + 2AI,03 + 3Mn
steel articles. Fe203 + 2A1 Al,O, + 2Fe(l)
Bbron and aluminium when heated wlth nitrogen 1016 the nitrides BN and AIN.
Nitrides of Ga and In are formed by heating the elements with ammonia. Boron
nitride can also be made by the action of ammonia on boron at 1300 K, or by
passing nitrogen over a mixture of boron trioxide and carbon at a slightly higher
temperature:
2B + 2NH3

B,O, + 3C + N2
-- 1300 K 2BN +
3H2
> 1UW)K ? 2BN + 3 C 0
BN is isoelectronic with C2, and like carbon, it exists in diamond and graphite forms.
The latter is a useful lubricant with additional advantage of being inert. Boron nitride
is a white solid. It is chemically rather inert, but is hydrolysed to NH, and B(OH)3
by the action of steam or hot acids:
i
Boron and aluminium on heating react with carborl to form the carbides B,,C, and
A14C3, respectively. Aluminium carbide is a colourless, hi& melting ionic solid
and is decomposed by water to liberate methane. Therefore, it can be termed as
aluminium methanide also:
A14C3 + 12H20 4Al(OH)3 + 3CH4

I On the other hand, the isolable form of boron carbide has the.molecular
composition B12C3.It is a black, extremely hard, high melting and chemically i+rt
covalent compound. It is used as an abrasive for polishing and tool sharpening.
Boron on heating reacts with many metals to f&m binary compounds called
borides, e.g., MgB2, VB and Fe2B, whereas other elements of Group 13 form
'

alloys. Metal bqrides are extremely hard, chemically inert, non-volatile, refractory
materials. They have high melting points and high thermal and electrical
conductivities. The diborides of Ti, Zr, Hf, Nb and Ta all have melting points
higher than 3200 K . The thermal and electrical conductivities of TiB2 and ZrB,
are about ten times greater than those of Ti and Zr metals.
Boron does not react with non-oxidising acids. Even hot concentrated oxidising
acids react with boron only slowly to form boric acid:
2B + 3H2SO4 - 2H3B03 + 3S02
B +. 3HN03 H3B03 + 3N02
Al, Ga, In and TI react with warm dilute HCI and H2S04to replace hydrogen and
form kf3+except TI which forms TI+:
2M
2Tl
+
6HCI
+ 2HCI - - 2 ~ +~6C1-+ + 3H2

With h9t and c o x . H2S04, SO, is liberated:

2TI+ + 2C1- + H2
% +I6H2S04 M2(S04), + 3S02 + 6 f i 0
Hot and conc. HNO, renders A1 and Ga passive. The initial attack of the acid
covers the metal with an impervious, coherent, unreactive layer of oxide which
prevents further attack. In and TI react with conc. H N 0 3 to form metal trinitrates
liberating NO,:
M + 6HN03 > M(N03), 3N02 +3H20 +
Boron reacts with fused alkalies forming the borates:
2B + 6NaOH 9 2Na3B03 + 3H2
Aluminium and gallimn dissolve in aqueous alkalis to form aluminate and gallate
ions, respectively:
M + 4NaOH + NaM(OH)4 + 2H2
Thus, you see that Lheelements of Group 13 aie fairly reactive and form many useful
compounds such as hydrides, halides, nitrides, oxides, oxoacids and their salts, t t c .
Let us now study somk of the compounds of B and A1 in detail. Compounds of Ga,
In and TI will be described only in brief, where appropriate, for the sake of
cdmparison only.

a) Explain in the space given below why the action of conc. nitric acid renders
aluminium passive.
........................................................................................................
..........................................................................................................
,
.........................................................................................................
1
.........................................................................................................
b) Write the formulae of the compounds you would expect B and A1 to form with
halogens, bxygen, sulphur, nitrogen and carbon.
........ ............................................................................................... 11
 Boron forms a series of volatile hydrides which resemble in some respects, e.g.,
volatility and covalent nature, the hydrides of carbon and silicon. The boron hydrides
are generally called boranes by analogy with the alkanes and silanes. In view of its
trivalency, boron is expected to form a simple hydride BH,, but it is unstable. The
simplest stable hydride of boron is dib.orane, B2H6. There are about a dozen well
characterised boranes which correspond to the following two stoichiometries:
i) BnH,+4 : B2H6, B5H9, B6H10, BSHIZ,B10H14, B18H22and iso-B1,H2,

Boranes are usually named by indicating the number of B atoms with a prefix and
the number of hydrogen atoms by an arabic number in parenthesis. For example,
B,H,, is named as tetraborane(l0) and B,H,, as nonaborane(l5). The hydride,
B2H6is simply called diborane as the compound of stoichiometry B2H8is not known.
Diborane is of special interest because it is the starting material for the preparation
of various other boron hydrides and because of its synthetic uses. So, let us first
discuss BZH6in detail.

6.4.1 Diborane (B,H,)

The chemistry of diborane has aroused considerable interest because of its usefulness
BZH, has only 12 valence in many synthetic reactions and also because the elucidation of its structure helped
electrons; for ethane type - to clarify the basic concepts about the structure of electron deficient compounds. We
structure 14 valence electrons are shall briefly discuss the chemistry of diborane below. ',

needed
a) Preparation: Diborane can be prepared in almost quantitative yields by the
reduction of boron trifluoride etherate (BF3,0Et2)with lithium aluminium
hydride (LiAIH,) or sodium borohydride (NaBH,):

diglyme
4BF,.0EtZ + ' ~ N ~ B H-, 2B2H6 + 3NaBF, + 4Et20
where. diglyme is diethyleneglyco! dimethyl ether, (MeOCH,CH2)20. Diborane
can also be prepared by treating NaBH, with conc. H2SO4or H3PO4:
2NaBH, + H2S04 ---------+ B2H6 + 2H2 + Na,SO,
2NaBH, + 2H3P0, + B2H6 + 2Hz + 2NaH2P0,
b) Properties of diborane: Diborane is a colourless gas (b.p., 183K). It is rapidly ,

decomposed by water with the formation of H3B03 and HZ:

B,H, + 6HZ0 2H,BO, + 6H2
Mixtures of diborane with air & oxygen inflame spontaneously producing large
amount of heat. Diborane has a higher heat of combustion per unit weight of fuel

B2H6 + 30, - --
than most other fuels. Therefore, it is used as a rdcket fuel.
B2O3 + 3H20, A H = -2165 kJ mol-I
Pyrolysis of B2H, in sealed vessels at temperatures above 375 K is an exceedingly
complex process producing a mixture of various boranes, e.g., B4Hlo,B5H9,
B5H,,, B6HIo,B4H12and BloHI+ By careful control of temperature, pressure
and reaction time, the yield of various intermediate boranes can be optimised.
For example, by storing BZH6under pressure for 10 days. B4HI,,is produced in
15% yield according to the following equation:
~B,H, B4H~o +
Diborane undergoes a facile addition reaction with alkenes and alkynes in ether
solvents at room temperature to form organoboranes: '
6RCH=CH2 + B2H, 2B(CH2C'H2R)i
 This reaction known as hydroboration reaction was discovered by Brown and Elements of Group 13
Subba Rao in 1956. It is regiospecific, boron atom showing preferential
attachment to the least substituted carbon atom. You may compare this addition
Hydrobration reaction has
with polar additions to the double bond, e.g., addition of HX, which obey proved to be of outstanding
Markownikoff's rule. Reaction of the resulting organoborane with an anhydrous synthetic utility. H.C. Brown was
carboxylic acid yi'elds the alkane corresponding to the initial alkene whereas awarded the 1979 Nobel Pr~zein
oxidative hydrolysis with alkaline H 2 0 2yields the corresponding primary alcohol: chemistry for developing
hydrobr'tion reaction.
EtC02H
B(CH2CH2R)3 3RCH,CH3
NaOHM202
(CH,CH2R)3 3RCH2CH20H

c) Structure of diborane: The structure of diborane is of great inierest since it cannot

be explained on the basis of simple theories of bonding. As mentioned earlier,
there are not enough valence electrons in B2H6 to form the expected number of
covalent bonds, in other words, B2H6 is an electron-deficient coppound. Since H H
boron atom has three unpaired electrons in the outermost orbit in the excited 1 I
state, it can form three covalent bonds. If each of the boron atoms in diborane H-B-B-H
links itself to three hydrogen atoms, there will be no electrons left to form a bond I I
H H
between the two boron atoms. Thus, diborane cannot have an ethane type
Not possible
structure as shown in the margin.
Diborane, B2H6, has been found to possess a bridge structure (Fig. 6.2) in which each
B atom iq bonded to two H atoms called terminal H atoms by regular electron-pair
bonds. The resulting two BH, fragments are bridged by two H atoms as indicated by
electron diffraction studies and by Raman and infrared spectroscopy. The two boron
atoms and the four terminal hydrogen atoms lie in the same plane while the two
bridging hydrogen atoms lie in a plane perpendicular to this plane as shown in Fig.
6.2. The bridging hydrogen atoms prevent rotation between the two boron atoms.

Fig, 6.2: Bridge structure of diborane

In the biidge structure, diboral~ehas eight bonds but there are 12 electrons available
for bonding, three per B and one per H. Hence, all the bonds in the molecule cannot
\be electron-pair bonds which would require 16electrons for the structure in Fig. 6.2.
The terminal B-H bond lengths are the same as the bond lengths in non-electron
deficient compounds. This means that the four terminal B-H bonds are normal
electron pair bonds accounting for a total of eight electrons. Thus, electron deficiency
must be associated with the bridging B-H-B bonds in which a pair of electrons binds
three atoms, viz., B, H and B. Therefore, these bridging B-H-B bonds are called
three centre electron pair bonds abbreviated as 3c-2e.
Alternatively, we can give a simple molecular orbital description of bonding in B2H6
as follows. Each boron atom is sp3 hybr~disedgiving four ~ p " ~ b r i d orbitals, one of
which is vacant and the other three are singly filled. Two of the sp3 hybrid orbitals
on each boron atom are used to form terminal B-H bonds with singly filled 1s orbital
of hydrogen. Two BH, units are then brought together so that all six atoms are
coplanar. Then one singly filled sp3 hybrid orbital on one B atom and one vacant sp3
hybrid orbital on the other B atom overlap with the singly filled 1s orbital on
hydrogen atom to f o r d a bonding orbital shaped like a banana and covering all three
atoms, viz, R , H and R . Similarly the other bonding orbital iz :!so formed (Fig. 6.3).
 This orbital binding three atoms contains only two electrons; the bonding between
the bridging H atom and the B atom is thus only about half as strong as in the
conventional two-centre two-electron terminal bonds. The B2H6 molecule contains
two such three-centre electron pair bonds. Due to repulsion between the two H nuclei.
the delocalised orbitals are bent away from each other giving it the banana'shape.
6.4.2 Borohydrides
. ,Borohydrides like NaBH,, Be(BH,), and Al(BH& are the salts of complex
tetrahydridoborate anion, BH,. Since in BH;;, boron has a complete octet, the
borohydrides are more stable than the boranes. sodium borohydride is obtained by
the reaction of NaH and methyl borate:
4NaH + B(OMe)3 NaBH, + 3CH30Na
Other borohydrides are prepared from sodium borohydride. The alkali metal
borohydrides are white, non-volatile, ionic solids which are stable in dry air. In
contrast to the alkali metal borohydrides. Be(BH,), and Al(BH,), are covalent and
volatile in nature. The alkali metal borohydrides react wZIh water with varying ease.
Thus, LiBH, reacts violently with water, NaBH, can be recrystallised from cold water
while KBY, is even more stable.
LiBO, + 4H,

. inalcoholic and aqueous solutions make .it a useful reagent in the reduction of
aldehydes and ketones to primary and secondary alcohols, respectively. Other
functional groups such as >C=C<, -COOH and -N02.are not attacked normally:

RCHO ---

6.,4.3 Hydrides of Aluminium

The extensive chemistry of the boron hydrides finds no parallel with the hydrides of
heavier elements of Group 13. Out of the four possible binary hydrides of Al, Ga,
In and TI, only aluminium hydride, (AlH,),, is known. It is prepared by the action .
of either 10O0/~pure H2S04 or AlCI, on lithium aluminium hydride in fin ethereal
solvent:
2LiAIH4 + H2S04 ' ,' 2/n(AIH3), + L~,SO, + 2H2
3LiAIH4 + AICl, 4/n(AIH3), + 3LiC1
replaceable proton; examples are
acids, water, alcohols, etc.

2(A1H3), + 6nH20 2nAI(OH)3 + 9nH2

6.4.4 Lithium '~luminiumHyddde

Lithium aluminium hydride is much more useful than aluminium hydride. It is
 L-ts d Group I3
4LiH + AICl,
ether + LiAIH, + 3LiCli -

Lithium aluminium hydride is a greyish-hite solid which decomposes into its

elements above 400 K. It' is stable in dry air but reacts violently with water:
LiAlH, + 4H20 LiOH +
AI(OH), + 4H2
LiAlH, is readily soluble in ethers. It is a very important reducing agent for both
inorganic and organic compounds. It reduces inorganic halides to hydrides, e.g.:
4BC1, + 3LiAlH4 2B2H, +
3LiCI + 3AlC13
MCI, +LiAlH, -----+ MH, + LiCl AICI, +
(M = Si, Ge, Sn)
LiAlH, is one of the most important reuucing agents in organicchemistry because it
reduces as many as 60 functional groups including ethylenic >C=C< double bonds.
But, it is less selective than sodium borohydride. Before we discuss the halides of
boron and aluminium, you may like to attempt the following SAQ based on the
hydrides of boron and aluminium.
SAQ 4
a) Explain briefly the difference between a two centre electron pair bond and a three
,centre electron pair bond.

....................................................................................................
b) Compare NaBH4 and LiAIH, as reducing agents.
...................................................................................................
v

....................................................................................................
c) Which of the above two would ypu u s e m h e following conversions:
i) R-CH=CH-CHO + R-CH=CH-CH20H
ii) R-CH2-COOEt R-CH2-CH20H

6.5 HALIDES OF BORON AND ALUMINIUM I

All the elements of Group 13 form binary halides. All the four trihalides of each
element are known, with one exception. The compound Tl13 is not thallium(II1)
iodide, but rather thallium(1) triiodide T1+(1,)-, which is similar to potassium
triiodide, K + I ~You
. will learn more about triiodides in Unit 10. Thallium(II1)
chloride and bromide are also very unstable and decompose into the thallium(1)
halides and the free halogen. Thus, the only stable trihalide of thallium in +3
oxidation state is the trifluoride, TIF3, which is an ionic solid. This is in keeping with
the general trend that Tl(1) compounds are more stable than TI(I1I) compounds. Let
us now briefly discuss the halides of B and Al.

6.5.1 Halides of Boron

Boron trihalides of the type BX3 exist for all the four halogens. Boron trifluoride can
be prepared on a large scale by the fluorination of boric oxide or borax with fluoaspar
and conc. H2S04:
B203 + 3CaF2 + 3H2S0,
Na2B4O7 + 6CaF2 + 8HzS04 -----4BF3
- 2BF3 + 3CaS0, + 3H20
, + 6CaS0, + 6NaHS0, + 7H20 4 ,,
In the laboratory, pure BF3 is best prepared by thermal decomposition of G e n e
diazonium tetrafluoroborate, PhN,BF4:
PhN2BF4 PhF + N2 + BF3
BCI, and BBr, are prepared on a large-scale by direct halogenation of B2O3 in the
presence of C, e:g.:
B203 + 3C + 3C12 775 K -+ 2BCI3 + 6 C 0
In the laboratory BC13 and BBr, are prepared by halogen exchange reaction between
BF3 and A12X6:
/
 BI3 is prepared in good yield by reacting-LiBH, or NaBH, with I2 at 4 0 0 # ; i n a 4 3
K, respectively:
LiBH, + 41, B13 + LiI + 4HI
The boron trihalides are highly volatile; BF3 (b.p., 173 K) and BCI, (b.p., 260.5 K)
are gases, BBr3 a volatile liquid (b.p., 319 K) and B13 a low melting solid
(m.p., 323 K). Boron halides are all hydrolysed by water giving H3B03, and
hydrohalic acids, HX or hydrofluoboric acid, HBF,:
H3B03 + 3HX, (X = CI, Br, I)
4BF3 + 3H20 > ,H3BO3 + 3HBF4
In addition to the trihalides, boron forms lower halidesof formula, B2X4.But only
B2F4 and B2C14 have been studied in some detail. B2F4 is a colourless gas whereas
'B2c14is a colourless liquid at room temperature. These halides are much less stable
than the corresponding trihalides. B2F4is the most stable of all the B2X4compounds.
These arespontaneously inflammable in air and react with Hz, H 2 0 , ROH, CI,, etc.
Unlike BH3 which is unstable, boron trihalides are monomeric molecular compounds
and have no tendency to dimerise. In this respect boron trihalides resemble
organoboranes, BR3, but differ from diborane, B,H, and the aluminium halides,
AI2X,. Thus, boron trihalides having three electron pair B-X bonds are electron
deficient. However, the interatomic distances, B-X, are substantially shorter than
those expected for single bonds. For example, the B-F bond length in BF3 is 130 pm
which is shorter than the sum of the covalent radii of B (80 pm) and F (72 pm). This
shortening of bonds has been explained in terms of appreciable pr-pn bonding
between an empty p orbital of the sp2 hybridised boron atom and filled p orbitals of
one of the fluorine atoms.
All the four trihalides are trigonal planar molecules. This can be explained on the
basis of sp2 hybridisation of the boron atom.

>.
Electronic configuration of
boron atom in state

Electronic configuration of
boron atom in excited state

The three hybrid orbitals of boron overlap with singly filled 2p orbitals of three
halogen atoms giving rise to three B-X bonds. The empty 2p orbital of boron which
is not involved in hybridisation in perpendicular to the plane of the triangle. Its energy
is comparable to that of the filled 2p orbitals of halogen atom. Thus it can accept a pair
of electrons from a filled 2p orbital of any one of the three halogen atoms, forming a
dative n bond. This makes an octet of electrons around the boron atom. The Bxj
molecule exists as a resonance hybrid of the following three structures as shown in

16 Fig. 6.4: Structure of boron trihalides

 Elementsof Croup 13
Because of lack of efficient overlapping, the extent of rr bonding decreases as the size
of atoms involved in bonding increases. Thus, the extent of rr bonding in boron halides
decreases from BF, to BI?.

6.5.2 Halides of Aluminium

All the iour trihalides of aluminium, i.e. AIF,, AICI,, AlBr, and AlI, are known. AIF,
is made by treating A1203with HF gas at lOOOK and the other trihalides are prepared
by the direct exothermic reaction of the elements, e.g.:

A1203 + 6HF loMK , ~ A I F ,+ ~ H ~ O

2A1 + 3X2 2A1X3, (X = C1, Br, 1)

AlCl, is also obtained by heating a mixture of alumina and coke in a current of Clz:
A1203 +3C + 3C12 2A1C13 + 3 C 0
Aluminium trifluoride differs from the other trihalides of A1 in being ionic and Fig. 6.5: Structure of A12C16
nonvolatile in nature. Other halides of Al, as also of Ga and In, are covalent in nature
when anhydrous and are relatively more volatile. AlCI,, AlBr, and AlI, exist as
dimeric species formed by pairing of two AIXRunits as shown in Fig, 6.5. The pairing
occurs by formation of a coordinate covalent bond from the halogen on one AlX3 unit
to the A1 atom of another. Thus, for AlCl,, the species Al,CI, is formed. This is
similar to the linking together of BeCI, units in solid BeCl, which you have already
studied in the preceding unit.

The dimerisation of AlX, occurs because these halides are electron def~cient.By
dimerisation, the halides attain' an octet of electrons. You have studied that the .
trihalides, BX3 are also electron deficient and attain an octet by prr-prr bonding. This
is not possible in case of A1 and other larger elements because-of lack of efficient.
rr-overlap, and hence, they dimerise. This dimerisation is retained when the halides
dissolve in non-polar solvents such as C6H6 and CCl,. In coordinating solvents, such
as, diethyl ether, trimethyl amine and phosphorus oxochloride, AlCl, forms
..
complexes like AlC13.0Et2, AlCI,.NMe, and A1Cl3.OPCl3, e.g.:

A12C16 + (C2H5)2? 2 (C2H5)20.AlC13

Alkyl and acyl chlorides, RCI and RCOCl, react with AlCl, to form complexes of
the type R+AICI, and RCO+AICI,, respectively; these are formed as intermediates
in Friedel-Crafts alkylation and acylation reactions.
As explained in Section 6.3, due to high heat of hydration of A],+, the covalent Aqueous solutions of salts of most
dimers are broken into [ A I ( H ~ o ) ~ ]and
~ + 3 X (aq) ions, when the halides dissolve in heavy metals are acidic in nature
water. ~ q u e o u solutions
s bf aluminium halides and othe~aluminiumsalts are acidic because of dissociation of their
aqua ions as shown in the case of
in nature. This is because the hexa-aquo complex or the aqua ion [AI(H,o),]~+ aluminium.
dissociates readily in solution giving hydroxonium ions by a series of changes as
shown below:

If a base like N H 4 0 H is added to an aqueous solution of salts of aluminium, the Gelatinous nature of A1(Ol$)3 is
H 3 0 + ions are neutralised and hydrated aluminium hydroxide is precipitated as a due to its hydrated nature.
Because of its gelatinous nature.
gelatinous precipitate: AI(OH), is used for purifying
water and as a mordant in dyeing

An exdess bf a strong alkali like NaOH, causes the above reaction to continue further
'with the formation of the soluble aluminate anion, [Al(H20)2(OH)4]-:

[A1(H2O),(OH),] + NaOH Na[A1(H20)2(OH)41

The above reactions can be reversed by the addition of an acid. Thus, when a basic
solution containing aluminate ion is slowly neutralised, the hydroxide
..--
[AI(H20)3(0H)3]precipitates and then redissolves as more acid is added:
[Al(H20)2(OH),]- + H3O+ [A1(H20)3(OH)314 + H2O
[AI(H20)3(OH)31 + 3H30f [ A I ( H ~ o ) ~ ]+
~ +3 H 2 0
I

This explains the amphoteric nature of AI(OH)3.

SAQ 5 /
Explain briefly why boron trichloride is a gas and aluminium trichloride is a dimeric
solid.

6.6 OXIDES OF BORON AND ALUMINIUM

All the elements of Group 13 form trioxides also known as sesquioxides of the
formula, M2O3. Thallium forms stable T120 also. The basic character of oxides
increases down the group with increase in the atomic number. Thus, B2O3is acidic,
A1203and Ga203are amphoteric whereas In203and Tl2O3are basic. T120 is soluble
in water and the resulting hydroxide TlOH is, in fact, as strong a base as KOH. Let
us now study the oxides of boron and aluminium in a little more detail, this will be
followed by a discussion of the acids of boron and the borates.
6.6.1 Boric Oxide
Boric oxide, B203,is the principal oxide of boron. It is also known as boron trioxide,
boron sesquioxide and boric anhydride. It is prepared by burning boron in oxygen or
by heating boric acid to red heat:
2B + 30, p B203

2H3BO3 B203 + 3 H 2 0
Boric oxide is a white hygroscopic solid. It is acidic in nature and dissolves in water
to form boric acid:

When fused with metal salts, it forms, metaborates known as borate glasses.
Metaborates of coloured cations have characteristic colours. This forms the basis of
the borax-bead test in qualitative inorganic analysis, e.g.:
COO + B2O3 C O ( B O ~ )deep
~ , blue
CuS04 + B203 C U ( B O ~ )blue
~,
Cr2(S04)3 + Bz03 ~ C T ( B O ~green
)~,
The bond energy of the B-0 single bond is very high (523 kJ mol-'). Therefore,
unlike carbon and nitrogen, boron does not form stable BO double bonds to oxygen.
Instead of forming small volatile covalent molecule, B203(g) and small anion, B0:;
it forms pol mers having -B-0-B-0-B-0- chains. Thus, boric oxide is a
7
polymeric solid.

6.6.2 Aluminium Oxide

Aluminium oxide, A1203is a highly ionic compound of aluminium. It is also tnbwn
as alumina. As you have studied earlier, it occurs in nature not only as bauxite and
colourless corundum, but also as coloured gem stones like ruby, emerald, sapphire
and topaz, etc. Colours of these gem stonesare due to the presence of transition
inetal ions. Anhydrous aluminium oxide can exist in two forms both of which ate
white. These are a-A1203 and 7-AI2O3.The 7-AI2O3is formed by dehydrating
AI(OH)3 below 750 K:
~AI(OH)~ * 7-Al2O3 + 3 H 2 0
The 7-A1203is quite reactive. It dissolves readily in both acids and bases. If the
7-AI2O3is heated to 1500 K, it changes to the a-AI2O3, which is called corundum.
Corundum has a very high melting point (about 2328 K). It is very hard and inert,-
especially towards acids.
SAQ 6
Discuss briefly the nature of oxides of Group 13 elements.

6.7 OXOACIDS OF BORON AND BORATES

Orthoboric acid H3BO3 commonly known as boricacid and metaboric acid HB02,
are two well-known and important oxoacids of boron. On a large scale, H3BO3is
prepared by the action of HCl or H2S04 on a concentrated solution of borax:
Na2B,07 + 2HC1 +
5H20 4H3B03 2NaC1 +
Boric acid is a flaky, white crystalline solid. It is moderately soluble in water. Boric
acid is a very weak monobasic acid (pK = 9.25), because it acts as an electron pair
acceptor (Lewis acid) from O H rather than as a proton donor (Arrhenius acid).
+
H3BO3 2 H 2 0 7[B(OH),]- + H 3 0 +
Its acid strength is considerably enhanced on complex formation with polyhydric
alcohols such as glycerol and mannitol. With mannitol K drops to 5.15 indicating an
increase in acid strength by a factor of more than 1.0 . b:

Fig. 6.6: Complex formation between borle add and a l:Md

On heating bonc acid at 375 K, metaboric acid, HB02 is formed. On further heating
above 500 K, B2O3 is formed:

In solution metaboric acid changes into orthoboric acid.

Boric acid has a two dimensional layer structure in which planar B 0 3 units are linked
to each other by unsymmetrical hydrogen bonds as shown in Fig. 6.7. In contrast to
the short 0-H....O distance of 272 pm within the plane, the distance between
consecutive layers is 318 pm. This is the cause of slippery and waxy feel of boric acid
which is also a good lubricant.

Fig. 6.7: Layer structure of H a O ,

~ - H I U C ~~:~ernrntr-~
~ Borax
Sdlts of boric acids are known as borates. As said earlier, hydrated hor;~tcsoccur
naturally, e.g., borax, Na2B,0,~10H20, kernite, Na+0,.4H20. colemanite.
Ca2B,0,,.5H20, etc. Anhydrous borates can be made by fusion of horic acid and
metal oxides.
Sodium tetraborate decahydrate, Na2B+07+10H20, is commonly known as hori~x.I t
occurs in certain lakes in India, Tibet and U.S.A: It is obtained by ext_ractinqimpure
borax with water and concentrating the solution until cry'stals of borax sepnratC out.
Borax can also be prepared from the mineral colkdanite by boiling it with Na,COI
solution:
Ca2B6O1, + 2Na2C03 Na2B407 + 2NaB02 + 2CaC03 4
Borax is crystallised from the filtrate after removing insoluble CaC03, By passing
COz, NaB02 present in the mother liquor is converted into borax:
' 4NaB02 + C 0 2 Na2B407 + Na2C03
Borax is a white crystalline solid. It is hydrolysed by water to give an alkaline solution:

On heating, borax loses, water to become anhydrous. Anhydrous borax on strong

heating with NH4Cl gives boron nitride and boron trioxitde:
Na2B407 + 2NH4Cl ----i------, 2BN'+ B2O3 + 2NaCl + 4H20
On heating alone, it decomposes to form NaBOz and B203
> 2NaB02 + B203

. SAQ 7 .
a) Explain why bofic acid behaves as a weak monobasic acid.
..........................................................................................................

b) Boric acid can be estimated by titration with standard alkali solutions, in the
presence of glycerol using phenolphthalein as an indicator. What is the function of

-
6.8 BORAZINE

borazine or borazole:
Borazine is best prepared by reducing B-trichloroborazme with sodium borohydride, Elements&Croup 13
NaBH4. B-trichloroborazine is first prepared by heating BC13 with NH4CI.

Borazine has many physical properties closely similar to those of isoelectronlc

benzene as shown in Table 6.4. Therefore, sometimes it is also called as inorganic
benzene.

Table 6.b: Comparison of physical properties of borazlne and benzene

Mol. wt.
MP (K)
BP (K)
Density
(10' x kg m-')

Borazine, however, is more reactive than benzene. It readily reacts with H 2 0 , MeOH
and HX to yield 1:3 adducts which eliminate Hz on being heated to 373 K, e.g.:

~ o r a z i n ehas a regular plane hexagonal ring structure. The B-N bond distance of
144 pm in borazine molecule is less than the sum of single-bond covalent radii of B
(82 pm) and N (70 pm). This indicates the presence of n bonding involving the lone
pair of electrons on nitrogen and an empty p orbital on boron. Thus, injyalence bond
terminology, the structure of borazine can be written as shown in Fig. 6.9.

Fig. 6.9: Valence bond structures for boraaine

Molecular orbital calculations, which are beyond the scope of discussion in this
course, indicate that in borazine the n electrons are only partially delocalised. This is
because of the difference in the energy of the n orbitals of B and N.
The experimental results of the reactions of water and HC1 with borazine indicate that
the reactions proceed by nucleophili~attack on boron atom. This suggests that the Nucleophiles are the electron
actual sign of net charges on B and W atoms in borazine should be opposite to that donating molecules or ions that
indicated in Fig. 6.9. This apparent paradox is explained by the existence of bse'or share electrons with
another atom or ion.
considerable polarity in the B-N a-bond (electronegativity B, 2.0, N, 3.05) in a
direction opposite to that in the B-N n-bond. In fact the drift in electron density in
B-N u-bond outweighs the drift in electron density in the B-N n-bond so that the
nitrogen atoms are relatively negative.

SAQ 8
Borazine is isoelectronic with benzene but it is much more reactive than benzene.
Explain briefly why this is so and what will be the nature of reagent attacking the
borazine molecule.
I

......................................................................................................... Y""
 6.9 COMPLEXATION BEHAVIOUR
As compared to the elements of Groups 1 and 2, elements of Group 13 show a greater
tendency of complex formation. Bec?use of lack of d-orbitals, boron is invariably
tetrahedrallv coordinated in these compounds. For example, in compounds like
NaBH,, NaBF,, NaB(C,H,),, BH3.NMe3, BF3-NH3as well as in chelates such as
[B(O-C,H,O~)~]-and [B(o-OC~H,COO)~]- the coordination number of boron atom
is four. Due to the presence of d-orbitals, the higher members of the group can
expand their coordination number even up to six. Thus, Al, Ga, In and T1 form
complexes such as (i) A1Cl3.NMe3,RCO+AI&(X= CI, Br), Et4N+M& (M = Al, Ga,
X=Cl, Br) in which the coordination number is four, (ii) AlC13.2NMe3,in which the
coordination number is five, and (iii) Na3[AlF6]in which the coordination number is
six. With chelating ligands like p-diketones, pyrocatechol, dicarboxylic acids and
8-quinolinol, Al, Ga, In and T1 form anionic or neutral complexes in which the
coordination number of the metal is six. Structures of some of these complexes are
shown in Fig. 6.10.
You know that the formation of AlCl, is important in Friedel-Crafts reaction
whereas the 8-quinolinol complex of aluminium is used in gravimetric estimation of
aluminium.

acetyl acetone oxalate 8-quinolinol

complex complex complex

Fig. 6.10: Some chelates of Al, Ga, In and TI

In addition to coordination complexes, aluminium forms a number of double

sulphates of general formula MAI(S04)2.12H20,where M is usually K , Rb, Cs or
NH,. These double sulphates are known as alums. For example, potash alum,
KA1(S04)2.12H20 and ammonium alum, NH4AI(S04)2.12H20.In alums, A1 can-
.also be replaced by a number of cations of the same charge and not too different in
size, e.g., Ga, In, Ti, V, Cr, Mn, Fe and Co. For example, chrome alum,
KCr(S04)2.12H20and ferric alum, NH4Fe(S04)2.12H20. The alums are
isomorphous with each other. It is important to realise that the alums are double salts
and not complex salts. In solution they behave simply as a mixture of component
sulphates and give reactions of their individual cations.

SAQ 9
a) Explain why boron cannot expand its coordination number beyond four

b) Work out the coordination number of the metal in the chelates given in Fig. 6.10.

6.10 ANOMALOUS BEHAVIOUR OF BORON

Just like lithium and beryllium in Groups Land 2, boron also shows anomalous
behaviour. In general, the boron chemistry resembles that of silicon (occupying a
diagonal position in the periodic table) more closely than that of Al, Ga, In and TI.
This is because of the small size and high electronegativity of boron as compared to
those of Al, Ga, In and TI. B ~ r o nresembles silicon and differs from Al, Ga, In and
TI in the following manner:
'\
 Both B and Si are nonmetals whereas Al, Ga, In and TI are distinctly metallic in Ekmene of Group 13
nature.
B(OH), and Si(OH)4 are acidic in nature, AI(OH), and Ga(OH), are amphoteric
and In(OH), and Tl(OH), basic in nature.
The hydrides of B and Si are volatile, spontaneously inflammable liquids. These
are readily hydrolysed by water and acids whereas aluminium hydride is a
nonvolatile polymeric solid. Hydrides of Ga, In and T1 are not stable.
BC13 and SiCI, are monomeric covalent compounds which are readily hydrolysed
by water to B(OH), and Si(OH)4. Anhydrous AlCl, is also a covalent compound
and exists in the form of a dimer, i.e. A12Cl,. On dissolving in water, it readily
gives ~ l , + ( a ~and
) C1-(aq) whereas ~ , + ( a does
~ ) not exist.
Both B z 0 3 and Si02 are acidic in nature and react with metallic oxides on fusion
to form borate and silicate glasses. On the other hand, A1203 is amphoteric.
Boron reacts with more electropositive elements, i.e. metals to form borides which
are very hard substances; Al, Ga, In and TI form alloys with metals.
SAQ 10
List four Crdperties in which boron differs from rest of the elements of the group.

Let u: now summarise the main points of the chemistry of Group 13 elements which
I you have studied in this unit.
Boron, aluminium, gallium, indium and thallium are members of Group 13of the
periodic table. You have studied occurrence, extraction, uses and the general
characteristics of these elements and gradations in their properties.
Boron the first member of the group exhibits anomalous behaviour showing
1 resemblance to silicon and differing from other members of the group.
1 Hydrides of boron and aluminium are electron deficient compounds and exhibit
three centre electron pair bonding in addition to normal electron pair bonding.
Complex hydrides of boron and aluminium are important reducing agents.
Halides gf boron are monomeric covalent compounds which are hydrolysed by
water. Boron halides exhibit pn-pn bonding.
Aluminium trffluoride is an ionic solid whereas its other halides when anh drous
K
are dimeric covalent compounds. In aqueous solution, the halides furnish Al (aq)
ions.
Boron forms two stable acids, viz., orthoboric acid and metaboric acid.'In solution
metaboric acid changes into orthoboric acid which behaves as a w'eak monbbasic
acid. Salts of these acids are known as borates.
Boron forms borides, boron nitride and borazine. Borides are extremely hard
compounds. Boron nitride which b isoelectronic with C2 can exist in diamond and
graphite forms. Borazine is isoelectronic with benzene and is also known as
inorganic benzene.
Aluminium oxide is an extremely stable ionic compound which can exist in two
different forms: a-A1203 and 7-Al2O3.
Boron and aluminium form a large number of addition complexes and chelate
complexes with various nitrogen and oxygen donor ligands.

6.12 TERMINAL OUESTIONS

1 Describe various steps involved in the extraction of aluminium from bauxite.
Explain why it cannot be prepared by reduction of A1203 with C. 23
2 While Al, Ga, In and T I form both covalent and electrovalent compounds. boron
forms mostly only covalent compounds. How would you explain this behaviour?
3 Describe the structure of diborane molecule. What is the nature of bonding in this
molecule?
4 Write chemical equations amphoteric nature of AI2O3and AI(OH),.
5 How would you explain nature of boron?
6 How would you prepare borazine? Compare its.pmperties with those of benzene.
7 Diborane is added to C H ~ - C H = C Hand
~ the product treated with
(a) CH3COOH (b) H202/0H-. What will be the product formed in each case?

6.13 ANSWERS
\
Self Assessment Questions
1 a) Aluminium is the most abundant metallic element in the earth's crust. Its two
important ores 'are bauxite, AI20,.xH20 and cryolite, Na3AlF,.
b) i) - b) , ii) - c), iii) - a), iv) - e), v) - d) .
2 Due to insertion of ten 3d elements between Ca and Ga, the effective nuclear
charge of Ga is higher. Consequently its atomic radius is smaller as well as its
ionisation energies and electronegativity are higher than expected.
3 a) By the action of conc. HN03, a thin impervious, coherent, unreactive layer of
A1203is deposited over the surface of Al metal. This coating protects Al from
further attack and thus A1 becomes passive.
b) With halogens, oxygen, sulphur, nitrogen and carbon, boron and aluminium
form compounds having the formulae BX3 and AIX,, B203 and
A1203, B2S3 and A12S3, Bfd-and AIN, BI2C3and A14C3, respectively.
4 a) In a two centre electron pair bond, a pair of electrbns binds two atoms,
whereas in a three centre electron pair bond a pair of electrons binds three
atoms together.
b) LiAIH, is a more versatile reducing agent than NaBH,; it reduces as many as
sixty functional groups in organic chemistry. On the other hand, NaBHj is
more selective. For example, NaBH, reduces >C=O group in aldehyde and
ketones to alcohols, but functional groups such as >C=C<, -COOH aRd
-NO2 are not normally attacked.
c) i) NaBH, ,,~ii) LiAIH4
/

5 In BCI, boron attains an octet of electrons'by means of pn-pn binding between B

and C1 atoms. Thus it exists as a monomeric gas. Due to the larger size of
aluminium iq aluminium trichloride, efficient pn-pn donding cannot take place \

between A1 and CI atoms. Consequently, aluminium trichloride dimerises to attpin

an octet of electrons around A1 by forming a dative bond between C1 and Al atoms.
Thus, it exists as a solid. \
6 The elements of Group 13 form oxides of the type M203which become more basic
6n moving down the group. Thus. B2O3is acidic, A1203 and Ga203 are
amphoteric, whereas In203and T1203 are basic.
7 a) Boric acid behaves as a weak monobasic acid because it ionises in water by
accepting a pair 'of electrons from the hydroxyl ion as given'below:
H3B03 + 2H20 -. [B(OH),]- + H30-
b) As explained above, boric acid is a weak monobasic acid. On addition of
glycerol to boric acid, a chelate complex is formed due to which the strength
of boric acid increases by a factor of lo4. Thus, in the presence of glycerol,
boric acid can be used as a primary standard using phenolphthalein as indicator.
8 In benzene, the n electrons are completely delocalised over all the c&b,on atoms
whereas in borazine these are only partially delocalised. Due to the difference in
electronegativities of B and N atoms, there is a drift in the electron density in both
 q- and a- B-N bonds. The drift in B-N u- bond is from B to N and it outweighs Elements of Group
the drift in B-N a- bond which is from N to B. Thus, nitrogen acquires a net
negative charge whereas boron acquires a net positive charge. This results in a
nucleophilic attack on boron in borazine.
9 a) As boron can accommodate at the most eight electrons in its outermost shell
(n = 2), it cannot form more than four electron pair bonds and expand its
coordination number beyond four.
b) The coordination number of the metal in all the chelates lshownin Fig. 6.10 is
six.
*
10 i) Boron is a nonmetal, whereas Al, Ga, In and T1 are metals.
ii) ~ , + ( a q )does not exist, whereas Al, Ga, In and TI easily furnish M3+(aq).
iii) B(OH), is acidic whereas Al(OH), and Ga(OH), are amphoteric and
In(OH), and TI(OH)3 are basic in nature.
iv) The halides of boron exhibit pa -pa bonding whereas those of Al, Ga, In
and TI do not.
Also see other differences in Section 6.10.
Terminal Questions
1 Purification of bauxite and electrblysis of fused A1203in cryolite are the tw&steps
involved in extraction of Al. ~lumihiurncannot be prepared by reduction of A1203
with C, because on heating A1203 with C, AI4C3 is formed.
2 Because of their high ionisation energies, all the elements of Group 13 form
covalent compounds. But due to their high enthalpies of hydration, compounds of
Al, Ga, In and TI ionise in aqueous solution to furnish M3'(aq)'ions, also.
.
3 See sub-section 6.4.l(c).
4 + 3H2SO4 A12(S04)3 + 3 H 2 0
\
AI2O3 + 2NaOH + 3 H 2 0 2NaAl(OH),
Al(OH), + 3HC1 AICI, + ~ H , O
Al(OH), + NaOH NaAI(OH)4
5 Because of its extremely small size and high electronegativity boron exhibits
anomalous behaviour.
6 See sub-sec. 6.8.1. ,
 UNIT ELEMENTS OF GROUP 14
I 4

Structure
7.1 Introduction
Objectives
7.2 Occurrence, Extraction and Uses
Occurrence
Extraction
Uses
7.3 General Characteristics
Physical Properties
Multiple Bonding
Catenation
Chemical Properties
Complex Formation
7.4 Anomalous Behaviour of Carbon
7.5 Silica and Silicates
Silica
Silicates
Sillcones
7.6 Chemistry of Divalent Silicon, Germanium, Tin and Lead Compounds
7.7 Summary
7.8 Terminal Questions
9
7.9 Answers

7.1 INTRODUCTION
In Units 4 and 5 you have studied the general characteristics of the elements of
. s-block, as well as the periodicity in their properties. You have also studied in Unit
,

6 these aspects of the chemistry of elements of Group 13, which belongs top-block.
Yo? would have noticed that while s-block elements show a regular gradation in
properties down the group, the elements of Group 13 stlow some irregularities. It
- was also pointed out that the first element in each group shows some anomalous
behaviour .
Now we extend our study to another group of p-block elements, namely, Group 14,
which consists of carbon, silicon, germanium, tin and lead. This is the first group in
which the transition from non-metals, C and Si through typical metalloid, Ge, to
weakly electropositive metals, Sn and Pb, can be clearly seeri. However, this does
not imply that the properties of carbon are completely non-metallic; its crystalline
forms are lustrous, one allotrope (graphite) conducts electricity. Tin and lead, on the
other hand, form amphoteric oxides and volatile chlorides. We will discuss these
properties in this unit. In the next unit you will study the chemistry of Group 15
elements. -

I Objectives
After studying this unit you should be able to :
explain the occurrence, extraction and uses of the elements of carbon family,
explain allotropy and describe different forms in which these elements exist,
compare the general characteristics of the elements of carbon family,
explain catenation with special reference to carbon,
describe internal n-bonding and the concept of complex formation by elements of
this group,
explain the nature of bonding in carbides,
describe the chemistry of halides, hydrides and oxides of these elements,
describe the structure and uses of silicates and silicones, and
26 describe the chemistry of divalent compounds of these elements.
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