2. Borazine or Borazole (B3N3H6) which is isoelectronic with
benzene ,shows similarity with benzene as well as some
physical properties,
hence it is called INORGANIC BENZENE.
3. PREPERATION
• Borazine can conveniently be prepared by heating lithium borohydride (dissolved
in diglyme) with ammonium chloride at 230 degree celcius in vacuum
• 3LiBH4+3NH4Cl ——— B3N3H6 + 3LiCl + 9H2
• Borazine can be prepared by heating a 1.2 molar mixture of diborane and
ammonia at 250˚C - 300˚C
• 3B2H6 + 6NH3 2B3N3H6 + 12H2
4. STOCK AND POHLANDS METHOD
• By the action of NH3 on Diborane(B2H6) •
The adduct B2H6.2NH3 is first formed which
then gets decomposed by heating in a
closed tube at 200 degree Celsius.
3B2H6+ 6NH-3[B2H6.NH3 ]-2BN Ho+12H2
(Ratio=1:2) Adduct
This method gives low yield because of
simultaneous formation of solid polymeric by-
products.
5. STRUCTURE
• .
Electron diffraction and spectroscopic studies reveal that borazine has a
hexagonal planar structure very similar to that of benzene.
Xray crystallography ha shown that the B&N atoms in borazine are alternately
placed in a planar hexagonal ring in which the N-B-N bond angles are 120
and all the B-N bond length are 144pm which is very close to C-C bond
length in benzene (142pm)
In the structure of borazine both B&N atoms are sp2 hybridized .
Two of the sp2 orbitals of each B&N are involved in the formation of
hexagonal planar ring.
Boron has a vacant unhybridized p orbital,however in the case of N it is the
unhybridized p orbital that contains lone pair on N. These unhybridized p
orbital overlap in a sideways maner to form 3 B-N dative pi bond.
That is, it is the electron pair from each N that is utilized for the formation of pi
bond without any electron contribution from B
6. In fact there is partial delocalization of the pi electrons although not as
completely as that in benzene. As a consequence of the greater
electronegativity of N than that of B the B-N bonds are polar and hence
borazine exihibits less aromatic character than benzene.
By resonance concept borazine is a resonance hybrid
of resonance structure 1&2
7. PROPERTIES
• Borazine is a colourless liquid
• Boiling point = 63˚C
• Although isoelectronic with benzene several of its properties are different from
those of benzene
It is far more reactive than benzene.
It is thermally less stable than benzene.
It undergoes addition reactions for more readily than benzene.
8. THERMAL STABILITY
• Borazine is not thermally very stable.
• It decomposes slowly on storage even at room temperature to yield
diborane,H2,and other volatile products.
9. ACTION WITH HCL
• With HCL borazine adds 3HCL molecules to give the addition products B3N3H9Cl3.
• Here we may consider that the 3 molecules of HCL add at 3 B=N double bond of
borazine.
• B3N3H6 + 3HCL B3N3H9Cl3
In this addition reaction
the positive part of adding molecule namely H gets attached to the more electronegative atom N of borazine
the negative part of adding molecule namely Cl get attached to the less electronegative atom B of borazine
Thus 3 hydrogen atom get bonded to the 3N and 3Cl get bonded to the 3B ,thereby leaving all the π bonds
present in borazine to yield addition product B3N3H9Cl3
10. Benzene does not undergo this addition reaction.
when the above HCl adduct B3N3H9Cl3 is heated to 100 degree celcius it decomposes giving
off 3 molecules of H2 to yield B-trichloroborazine (B3N3H3Cl3)
B3N3H9Cl3 B3N3H3Cl3 + 3H2
11. ACTION WITH WATER
• Borazine react with water to yield an adduct ,
Where the OH group are attached to the B atoms,
However at high temperature borazine undergo hydrolysis to yield boric acid,ammonia and H2
B3N3H6 + 3HOH [BH(OH)NH2]3
B3N3H6 + 9HOH 3B(OH)3 +3NH3 +3H2
Benzene neither adds water nor does it undergoes hydrolysis with water
12. ACTION WITH BROMINE
• Borazine when treated with excess bromine at low temperature adds 3 bromine
molecule to yield an orange solid adduct B3N3H6Br6
• When the above bromine adduct B3N3H6Br6 is heated to 60 degree celcius it
decomposes giving 3 molecules of HBr to yield B-tribromoborazine (B3N3H3BR3)
• B3N3H6Br6 B3N3H3Br3 + 3HBr
13. CHEMICAL PROPERTIES
Addition reactions
One molecule of BORAZINE adds 3 molecules of HCI or HBr in the cold without a
catalyst. These molecules gets attached with all the three B atoms of borazine,since B
atom is more negative than than N atom in B-N or B=N bond and hydrogen chloride
derivative is obtained. This addition reaction is not shown by benzene.
14. . Hydrolysis
• Borazine gets slowly hydrolysed by water to produce boric acid,ammonia
and Hydrogen. Hydrolysis is favoured by the increase in temperature
15. 3.Pyrolysis
• When borazine is pyrolysed above 340 °C,B6N6H10 and B5N5H8 are produced.These products
are boron- nitrogen analogues of diphenyl and naphthalene respectively
16.
17. 5.Reaction with Aniline
• Borazine undergoes a strongly exothermic reaction with Aniline to produce tri-aminoborine
18. Physical properties
• Colourless
• Volatile liquid
• Boiling point: 64.5°C
•Melting point:-58 degree celcius
•Decomposes at -80° C
•Molecular weight: 78 g
20. USES
Borazine is used,
As a precurser to grow boron nitride filims on surface eg. Rhodium
In the preparation of organic borazine derivatives
As gasoline additives
As catalyst for polymerization of alkenes
As fire retardants in cotton and nylon textiles
As polymerization inhibitors of unsaturated alcohols and esters
As a starting material for potential ceramics such as boron carbonitrides
