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

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 metaborate3)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

1) HYDRIDES:

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 two- dimensional 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 DIBORANE1)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 DIBORANEFrom 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 DIBORANE-

There 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:

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 +12H₂2) It can also be prepared by the direct reaction of alkali metal borohydrides with ammonium chloride. 3NaBH4+ 3NH4Cl------------- B₃N₃H₆+ 3NaCl + 9H₂