P- BLOCK ELEMENTS
1) The elements belonging to groups 13 to 18
constitute p block elements.
2)The general electronic configuration for the atoms
of this block is ns²np¹.
3) The group 13 of the periodic table consists of
elements boron , aluminium, gallium, indium and
thallium.
4) Except boron, which is classified as a non metat,
all other elements of this group are metals.
GENERAL CHARACTERSTICS OF GROUP 13
ELEMENTS
1) ATOMIC AND IONIC RADII : On moving down the group, the atomic and ionic radii increases as
no. of shells increases.
2) MELTING AND BOILING POINT : The melting and boiling point decreases on moving down the group.
3) IONISATION ENERGY : On moving down the group,ionisation energies decreases.
4) ELECTROPOSITIVE CHARACTER : The group 13 elements have less electropositive character.
5) TENDENCY TO FORM COVALENT COMPOUNDS :In group 13 elements boron shows covalent character, the other
elements show ionic character.
Comparative study of compounds of group 13 elements
1) HYDRIDES:
Boron forms a no. of hydrides having the general formula BnHn+4 and
BnHn+6. The compounds of boron and hydrogen are called boranes. These
contain special types of bonds known as multicentre bonds.
3NaBH4 + AlCl3 ------------ Al[(BH4)3] + 3NaCl
2) OXIDES AND HYDROXIDES:
All the elements of group 13 form oxides of general formula M2O3. For
e.g. B2O3 , AL2O3. The oxide of boron is B2O3 and known as boric oxide.
2H3BO3 ------------ B2O3 +3H2O
The oxides and hydroxides of boron are weakly acidic and therefore , react with
alkalies B2O3+ NaOH--------- 2NaBO2 + H2O
Sodium metaborate
3)OXO ACIDS :- Boric acids
Among the oxyacids of group 13 elements, orthoboric acids B(OH)3 or H3BO3
is important. Some boric acids are known for e.g.
1) Orthoboric acid
H3BO3
2)Metaboric acid
HBO2
3)pyroboric acid
H₆B₄O₉
4)Tetraboric acid
H₄B₄O₇
Structure of Boric acid:
1) The electronic configuration of boron atom
is 2s² 2p¹.
2) In orthoboric acid, BO₃³- units are bonded
together through hydrogen bonds into twodimensional sheet.
3) The hydrogen bonding in one H₃BO₃ unit is
shown in fig (1) and complete structure is shown
in fig (2).
4) It is evident that each boron atom remains
bonded to three oxygen atoms and each oxygen
atom is bonded to a hydrogen atom.
CHEMISTRY OF SOME IMPORTANT
COMPOUNDS
 DIBORANE:
It is the simplest of the boranes and it forms
the starting material for the preparation of other boranes.
PREPARATION OF DIBORANE
1)It can be prepared in small quantities by the reaction of
iodine on sodium borohydride in diglyme
[( CH2OCH2CH2)2O].
2NaBH4 + I2----------> B2H6 + 2NaI + H2
2) It can also be prepared by the reduction of boron halides
with reducing agents like LiH, NaH, CaH2etc.in ether
solution.
8BF3 + 6LiH ----------> B2H6 + 6LiBF4
2BCl3 + 6NaH---------> B2H6 + 6NaCl
3) It can be prepared by passing silent electric discharge
through a mixture of hydrogen and boron trichloride at
low pressure.
2BCl3 + 6H2O----------- > B2H6 + 6HCl
4) On an industrial scale, diborane is prepared by
reducing gaseous BF3 with sodium hydride at 180⁰ C.
2BF3 + 6NaH-------------- > B2H6 + 6NaF
PROPERTIES OF DIBORANE
PHYSICAL STATE :- It is a colourless gas with a foul smell.
2) STABILITY : - It is stable only at low temperature when heated at
temperatures b/w 100 ⁰ C and 250 ⁰ C, it changes to a no. of higher
boranes.
B2H6 ---------------- > B4H10, B5H9, B5H11, B6H10 etc.
3) ACTION OF OXYGEN :- It is spontaneously flammable and burns in
oxygen liberating a lot of energy.
B2H6 + 3O2---------- > B2O3 + 3H2O
4) HYDROLYSIS : - it gets readily hydrolysed yielding boric acid and
hydrogen.so, it acts as a reducing agent.
B2H6 + 6 H2O ------> 2H3BO3 + 6H2
5) ACTION WITH HALOGEN:B2H6 + Cl2----------> B2H5Cl + HCl
B2H6 + HI---------- > B2H5I + H2
6) ACTION WITH HYDRIDES : 2NaH + B2H6--------> 2Na[BH4]
6) REACTION WITH PYRIDINE:- Diborane combines with
pyridine to form salt.
B2H6 + 2 C5H5N----------- > 2H3B⁻‹—N⁺C5H5
7) ACTION WITH BORON HALIDES:- Diborane reacts with
boron halides to form halodiboranes
B2H6 + BCl3---------------- > B2H5Cl + BHCl2
8) ADDITION TO ALKENES- HYDROBORATION
REACTION :Diborane is added to the alkenes or alkynes in ether solvents at room
temperature to form alkyl borane.
6RCH=CH2 + B2H6 --------- > 2(RCH-CH2)3B
C2H5COOH
(RCH-CH2)3B --------------- > 3RCH2 CH3
(alkane)
H2O2/OH⁻
(RCH-CH2)3B ---------------- > 3RCH2CH2OH + H3BO3
(1⁰ alcohol) (boric acid)
STRUCTURE AND BONDING OF DIBORANE
From the electron diffraction and infra red spectroscopic
studies, it has been concluded that diborane has the following
structure—
In diborane structure,
1) There are two types of hydrogen atoms.
2) Four hydrogen atoms, two on the left and two on the right
known as terminal hydrogens, are different than the other two
hydrogen atoms, known as bridging atoms.
3) The two boron atoms and the four terminal hydrogen atoms
lie in the same plane.
4) While the two bridging atoms, one above and other below
are in a plane at right angle to the rest of the molecule.
NATURE OF BONDS IN DIBORANEThere are three centre electron pair bonds , B-H-B
involving one electron pair bonds which binds
three atoms : B, H and B.such a bond may be
indicated as H…..B…..H.
It was postulated that boron atoms undergoes sp3
hybridisation involving one 2s and all the three p
orbitals including one empty orbital.
The four sp3 hybrid orbitals adopt tetrahedral
arrangements.
The hybrid orbital containing an unpaired
electron of one boron and the vacant hybrid
orbital of the second boron atoms overlap
simuntaneously with 1s orbital of a hydrogen
atom to form a B…..H…..B bridge bond.
It is called a three centre electron pair bond.
This type of bond is also known as BANANA
BOND.
BORAZINE:It is isoelectronic with benzene because of its similarity in
physical properties and structure, it has been known as inorganic
benzene.
1)PREPARATION:it was first prepared by alfred stock in 1926 by the reaction of
diborane and ammonia in the molar ratio of 1:2 at 250-300⁰C.
3B2H₆+ 6NH₃---------- 2B₃N₃H₆ +12H₂
2)
It can also be prepared by the direct reaction of alkali metal
borohydrides with ammonium chloride.
3NaBH4+ 3NH4Cl------------- B₃N₃H₆+ 3NaCl + 9H₂
HALIDES OF AlCl₃:1) AlF3 IS PREPARED BY TREATING AI2O3 WITH HF GAS AT 700⁰ C.
Al₂O₃ + 6HF------- 2AlF3+ 3H2O
2) 2Al + 3Cl2----2AlCl3
3) Al2O3+ 3Cl2+3C----- 2AlCl3+ 3CO
4) STRUCTURE OF AlCl3:5) In crystalline state, anhydrous AICI3 has closely packed str.
6) It exists as dimer in which there is four co- ordination around each Al
atom.
7) Al is tetrahedrally surounded by four CI atoms.
8) In addition to three covalent bonds, two co-ordinate bonds are formed
9) By the donation of electron pair of CI to the vacant orbital of AI.
10) The dimer str. Exists in vapour state at low temp. but at higher
temperature,it dissociates to trigonal planar in which co- ordination no.
of AI is three.
COMPARATIVE STUDY OF GROUP 16 ELEMENTS:1)HYDRIDES:- All elements of group 16 form halides such as H₂O, H₂S,
H₂Se, H₂Te etc. The hydride of oxygen i.e. H₂O is available in abundance
in nature.
The hydrides of other elements can be obtained by the action of acids on
metal sulphides, selenides and tellurides.
FeS + H2SO4---------- FeSO4 + H2S
k₂Se + H₂SO₄ ---------- K₂SO₄ + H₂Se
2) TRENDS IN CHARACTERSTICS OF HYDRIDES:1) Volatility :- All hydrides are volatile. The volatility increases from H₂O
to H₂Se and then decreases.
2) Acidic character:- The hydrides of this group are weakly acidic.for e.g.
H₂S
H⁺+HS¯
HS¯
H⁺+ S
3) Thermal stability:- It decreases from H₂O to H₂Te as:H₂O > H₂S> H₂Se> H₂Te
4) Reducing character:- All hydrides except water are reducing agent. The
reducing character increases from
H₂O < H₂S < H₂Se < H₂Te
OXYACIDS OF SULPHUR:1) Sulphurous acid, H₂SO₃
It contains sulphur in +4 oxidation state. It is a strong acid. It is diatropic and
ionise in two stages.
H₂SO₃ ------- H⁺ + HSO₃¯
HSO₃⁻ ----- H⁺+ SO₃²¯
STRUCTURE OF SULPHUROUS ACID :Sulphurous acid forms two types of isomeric diethyl derivative. They are
symmetrical and unsymmetrical derivative.
The sulphite ion has a pyramidal str. Involving sp³ hybridisation of S- atom in
which one of the sp³ hybrid orbital occupies alone pair of electrons.
Two sp³ hybrid orbital overlap with 2p-orbitals of oxygen forming two S-O bonds.
The half filled d- orbital sulphur forms pπ- dπ bond with third O- atom.
2) SULPHURIC ACID:It contains +6 oxidation state. Because of its wide
applications in industry, it is called as king of chemicals.
H₂SO₄
H₂O + SO₃
ACIDIC CHARACTER:H₂SO₄
H₂SO₄
H⁺ + HSO₄
2H⁺ +SO₄²¯
STRUCTURE:It is dibasic acid.it has two hydrogens linked to oxygen atoms forming
hydroxy groups.
1) PCI₅
THIOSULPHURIC ACID:It contains S in +2 oxidation state.
H₂S₂O₃ -------- H₂SO₃ + S
STRUCTURE:It is derived from sulphuric acid by the replacement of one of the oxygen atoms
by a sulphur atom.
DISULPHURIC ACID:It is a stronger oxidising agent and more powerful dehydrating agent. It
combines with water to give sulphuric acid.
H₂S₂O₇ + H₂O ------- 2H₂SO₄
PEROXODISULPHURIC ACID, H₂S₂O₈:It contains S in +6 oxidation state. It is also called MARSHALL’S ACID.
STRUCTURE:-
HYDROGEN PEROXIDE H₂O₂:It was discovered by french chemist J. L. THENARD in l8l8.
Physical properties:1) It is a thick syrupy liquid with pale blue colour.
2) It is more viscous, less volatile and dense than water.
3) Its density is 1.44 g cm⁻³
4) Its melting point is 272.4 K and boiling point is 358 k at 68 mm of Hg pressure.
CHEMICAL PROPERTIES:1) DECOMPOSITION:-
2)ACIDIC NATURE:-It is a weak acid and dissociates as:-
USES OF HYDROGEN PEROXIDE:1) It is used in industry as a bleaching agent for textiles, paper, pulp, straw
leather, oils fat etc.
2) It is used as ahair bleach.
3) It is used as a antiseptic for washing wounds, teeth and ears under the
name PERHYDROL.
4) It is used for restoring the colour of lead paintings.
5) It is used for preserving milk, wine and other liquors.
6) Recently it is used in environmental chemistry such as in pollution control
treatment of domestic and industrial effluents ; oxidation of cyanides and
restoration of aerobic conditions of sewage wastes.
STRUCTURE OF HYDROGEN PEROXIDE:Hydrogen peroxide has a skew structure in which 2
hydrogen atoms are arranged in two directions almost
perpendicular to each other and to the axis joining the two
oxygen atoms.
The fact that two O—H bonds lie in different planes is due
to repulsion b/w the various bonding and non bonding
orbitals.
There is free rotation about O—O bond because there is
no barrier to internal rotation.