P BLOCK ELEMENTS 1 The elements belonging to

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 Bn. Hn+4 and Bn. Hn+6. The compounds of boron and hydrogen are called boranes. These contain special types of bonds known as multicentre bonds. 3 Na. BH 4 + Al. Cl 3 ------ Al[(BH 4)3] + 3 Na. Cl 2) OXIDES AND HYDROXIDES: All the elements of group 13 form oxides of general formula M 2 O 3. For e. g. B 2 O 3 , AL 2 O 3. The oxide of boron is B 2 O 3 and known as boric oxide. 2 H 3 BO 3 ------ B 2 O 3 +3 H 2 O The oxides and hydroxides of boron are weakly acidic and therefore , react with alkalies B 2 O 3+ Na. OH----- 2 Na. BO 2 + H 2 O Sodium metaborate 3)OXO ACIDS : - Boric acids Among the oxyacids of group 13 elements, orthoboric acids B(OH)3 or H 3 BO 3 is important. Some boric acids are known for e. g. 1) Orthoboric acid H 3 BO 3 2)Metaboric acid HBO 2 3)pyroboric acid H₆B₄O₉ 4)Tetraboric acid H₄B₄O₇

Structure of Boric acid: 1) The electronic configuration of boron atom is 2 s² 2 p¹. 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 [( CH 2 OCH 2)2 O]. 2 Na. BH 4 + I 2 -----> B 2 H 6 + 2 Na. I + H 2 2) It can also be prepared by the reduction of boron halides with reducing agents like Li. H, Na. H, Ca. H 2 etc. in ether solution. 8 BF 3 + 6 Li. H -----> B 2 H 6 + 6 Li. BF 4 2 BCl 3 + 6 Na. H-----> B 2 H 6 + 6 Na. Cl

3) It can be prepared by passing silent electric discharge through a mixture of hydrogen and boron trichloride at low pressure. 2 BCl 3 + 6 H 2 O------ > B 2 H 6 + 6 HCl 4) On an industrial scale, diborane is prepared by reducing gaseous BF 3 with sodium hydride at 180⁰ C. 2 BF 3 + 6 Na. H------- > B 2 H 6 + 6 Na. F

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. B 2 H 6 -------- > B 4 H 10, B 5 H 9, B 5 H 11, B 6 H 10 etc. 3) ACTION OF OXYGEN : - It is spontaneously flammable and burns in oxygen liberating a lot of energy. B 2 H 6 + 3 O 2 ----- > B 2 O 3 + 3 H 2 O 4) HYDROLYSIS : - it gets readily hydrolysed yielding boric acid and hydrogen. so, it acts as a reducing agent. B 2 H 6 + 6 H 2 O ------> 2 H 3 BO 3 + 6 H 2 5) ACTION WITH HALOGEN: B 2 H 6 + Cl 2 -----> B 2 H 5 Cl + HCl B 2 H 6 + HI----- > B 2 H 5 I + H 2 6) ACTION WITH HYDRIDES : 2 Na. H + B 2 H 6 ----> 2 Na[BH 4]

6) REACTION WITH PYRIDINE: - Diborane combines with pyridine to form salt. B 2 H 6 + 2 C 5 H 5 N------ > 2 H 3 B⁻‹—N⁺C 5 H 5 7) ACTION WITH BORON HALIDES: - Diborane reacts with boron halides to form halodiboranes B 2 H 6 + BCl 3 -------- > B 2 H 5 Cl + BHCl 2 8) ADDITION REACTION : - TO ALKENES- HYDROBORATION Diborane is added to the alkenes or alkynes in ether solvents at room temperature to form alkyl borane. 6 RCH=CH 2 + B 2 H 6 ----- > 2(RCH-CH 2)3 B C 2 H 5 COOH (RCH-CH 2)3 B -------- > 3 RCH 2 CH 3 (alkane) H 2 O 2/OH⁻ (RCH-CH 2)3 B -------- > 3 RCH 2 OH + H 3 BO 3 (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-HB 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 sp 3 hybridisation involving one 2 s and all the three p orbitals including one empty orbital. The four sp 3 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 1 s 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. 3 B 2 H₆+ 6 NH₃----- 2 B₃N₃H₆ +12 H₂ 2) It can also be prepared by the direct reaction of alkali metal borohydrides with ammonium chloride. 3 Na. BH 4+ 3 NH 4 Cl------- B₃N₃H₆+ 3 Na. Cl + 9 H₂

HALIDES OF Al. Cl₃: 1) Al. F 3 IS PREPARED BY TREATING AI 2 O 3 WITH HF GAS AT 700⁰ C. Al₂O₃ + 6 HF------- 2 Al. F 3+ 3 H 2 O 2) 2 Al + 3 Cl 2 ---- 2 Al. Cl 3 3) Al 2 O 3+ 3 Cl 2+3 C----- 2 Al. Cl 3+ 3 CO 4) STRUCTURE OF Al. Cl 3: 5) In crystalline state, anhydrous AICI 3 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, 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. Fe. S + H 2 SO 4 ----- Fe. SO 4 + H 2 S 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¯ H⁺+ S 3) Thermal stability: - It decreases from H₂O to H₂Te as: H₂O > H₂Se> H₂Te 4) Reducing character: - All hydrides except water are reducing agent. The reducing character increases from H₂O < 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 2 p-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 H⁺ + HSO₄ 2 H⁺ +SO₄²¯ STRUCTURE: It is dibasic acid. it has two hydrogens linked to oxygen atoms forming hydroxy groups. 1) PCI₅ reacts with H