Transcript H, B 2 -H, B 2 - Smita Asthana

DIBORANE
Methods Of Preparetion

It is prepared by treating boron Trifluoride with
LiAlH4 in diethyl ether
4BF3 + 3LiAlH4
2B2H6 +3LiF +
3ALF3

In a laboratory the diborane is prepared by the
oxidation of sodium borobhydride with iodine
2NaBH4 + I2
B2H6 +2NaI +H2

In industry it is prepared by the reaction of BF3 with
sodium hydride
450 K
2 BF3 + 6 NaH
B2H6 + 6 NaF
Properties
1) Diborane is a colorless, highly toxic gas. It has boiling
point ok 180 K
2) Diborane catches fire spontaneously when it is exposed
to atmospheric air
it burns with oxygen. The reaction is exothermic
releasing a lager amount of energy
B2H6 + 3O2
B2O3 + 3H2O
ΔH= - 1976 K J mol-1
3)
it is readily hydrolyzed by water to form boric acid
B2H6 + 6H2O
2B(OH)3 + 6H2
4)
With Lewis bases, diborane first undergoes
cleavage (breaking) to form borane (BH3)
which then reacts to form adducts
B2H6 + 2NMe
2BH3 . NMe
B2H6 + 2CO
2BH3 . CO
5) with ammonia an addition product B2H6.NH3
which then decomposes on heating at 473K to
give a volatile compound called borazole
(Borazine)
B2H6 + NH3
B2H6.NH3
473K
3 B2H6 + 6NH3
2B3N3H6 + 12H2
( borazole)
B2H6 + 6NH3
(BN) n Boron nitride
Structure
H
H
H
B
B
H
H
B in GS
ES
2s
H
H
2p
1S
Sp3
B undergoes sp3 hybridization and 3 hybridized orbitals have one electron each
and one orbital is vacant. Each Boron form two sigma bonds by overlapping with s
orbital of H and a normal 2e2c covalent bond is formed. These are coplanar.
Two more bonds are formed by overlapping of one boron sp3 orbital with one
electron, one hydrogen s orbital with one electron and another boron sp3 with no
electron. Thus a 2e3c bond is formed , also called banana or tau bond.
Structure of Higher Boranes
Types of Bonds in Boranes
1.
Normal covalent bonds -2c-2e- B-H
2. Normal covalent bonds - 2c-2e- B-B
3. Bridge bonds -3c-2e- B-H-B
4. Bridge bonds - 3c-2e- B-B-B
5. Closed bridge bonds - 3c-2eB
B
B
Structure of Higher Boranes
1. Tetraborane – B 4H10
Bonds –
(i) Four bridging (3c-2e) B-H-B
bonds viz, B1 –H-B3 , B2-H –
B3, B1-H-B4 and B2- H-B4
(ii)One direct (2c-2e) B-B
bond (B1-B2 bond)
(iii)Six terminal (2c-2e) B-H
bonds namely B3-H, B3-H,
B2-H, B2-H, B4-H and B4-H
bonds.
2.Pentaborane – B5H9
Bonds –
1. Five terminal B-H bonds
(viz., B1-H, B4-H, B2-H, B3-H
and B5-H bonds)
2.Four bridging B-H-B bonds
(namely B1-H-B2, B2-H-B3, B3H-B4, and B4-H-B1 bonds),
3. Two B-B bonds (B1-B5 and
B5-B4 bonds)
4.One closed (3c-2e) B5-B3-B2
bonds.
3. Decaborane – B10H14
Bonds –
1.Four bridging (3c-2e) B-HB bonds namely B5-H-B6, B6H-B7, B8-H-B9 and B9-H-B10.
2. Each of B-atoms is linked
with one H-atom by terminal
B-H bonds. Thus the molecule
has ten terminal B-H bonds
viz., B1-H, B2-H, B3-H, B4-H,
B5-H, B6-H, B7-H, B8-H, B9-H
and B10-H bonds.
3.Four B-B bonds (B2-B5, B2B7, B4-B8, B4-B10)
4.Four closed (3c-2e) B-B-B
bonds (B1-B2-B3, B1-B3-B4, B1B5-B10 and B3-B7-B8 bonds) +
BORAZINE – BORAZOLE – INORGANIC BENZENE
(HBNH)3
PREPARATION
PROPERTIES