Borazine

It contains full explanation about borazine, which includes physical and chemical nature of borazine and it's applications. Which also includes CSIR and GATE questions.

1. TOPIC B ORAZINE ( B 3 N 3 H 6 ) PRESENTED BY K.MUTHU KANNAN

2. BORAZINE Introduction: Borazine is also known as borazole , Cyclotriborazaneborazol . It is a inorganic compound with the chemical formula B 3 H 6 N 3 . The compound is isoelectronic and isostructural with benzene. For this reason borazine is sometimes referred to as inorganic benzene . The compound was reported in 1926 by the chemists Alfred Stock and Erich Pohland by a reaction of diborane with ammonia. The IUPAC name of borazine is 1,3,5,2,4,6-Triazatriborinane.

3. Preparation: Stock and Pohlands method: By the action of NH 3 on diborane in 1:2 ratio. The adduct B 2 H 6 .NH 3 is first formed, which then gets decomposed by heating in a closed tube at 200°C. 3 B 2 H 6 + 6 NH 3 → 3[B 2 H 6 .NH 3 ] → 2 B 3 H 6 N 3 + 12 H 2 Heating BCl 3 with NH 4 Cl: Heating boron trichloride with ammonium chloride in chlorobenzene in the presence of Fe, Ni, or Co catalyst at 140 °C trichloroborazine is formed.

4. This derivative being reduced with sodium borohydrate in polyether gives borazine. By heating a mixture of LiBH 4 and NH 4 Cl: Borazine can be prepared in the laboratory by heating a mixture of lithium borohydrate and ammonium chloride in vaccum at 230 °C. 3NH 4 Cl + 3LiBH 4 → B 3 N 3 H 6 + 3LiCl + 9H 2

5. Physical Properties : It is a colourless volatile liquid. Melting point is -58 °C Boiling point is 53°C Density is 0.81g/cm 3 . Molecular weight is 77.5 g/mol. It decomposes at -80 ° C.

6. Structure: In borazine ,B-N bond (= 1.54 Å) and double bond, B=N (=1.36 Å) distances. The angles are equal to 120°. In benzene C-C bond length is equal to 1.42 Å. Boron behaves as a Lewis acid and nitrogen behaves as a Lewis base. Like benzene and borazine has a planar hexagonal structure, containing 6 membered ring, in which B and N atoms are arranged alternatively. In borazine both B and N atoms are sp2 hybridised.

7. Chemical reactions : Addition reaction: One molecule of borazine adds 3 molecules of HCl 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 N atom in B-N or B=N bond and hydrogen chloride derivative is obtained.

8. One molecule of borazine also adds three molecules of Br 2 at 0°C and gives B-tribromo –N- tribromoborazine which on being heated to 60° C, loses three molecules of HBr and forms B- tribromoborazine .

9. Hydrolysis: Borazine hydrolyzes readily, yielding boric acid, ammonia, and hydrogen.

10. Pyrolysis: When borazine is pyrolysed above 340 ° C , B 6 N 6 H 10 and B 5 N 5 H 8 are produced. These products are boron nitrogen analogues of diphenyl and naphthalene respectively.

11. Formation of adduct: Borazine forms an adduct with methanol. This adduct undergoes pyrolysis with the elimination of hydrogen and gives B-trimethoxy-borazine. Polymerization: Borazine heated at 70 °C expels hydrogen gas with formation of polyborazylene . n B 3 N 3 H 6 → [B 3 N 3 H 4 ] n

12. Applications: Borazines are also starting materials for other ceramics such as boron carbonitrides. Borazine can also be used as a precursor to grow hexagonal boron nitride (h-BN) thin films and single layers on catalytic surfaces such as Cu, Pt, Ni, Fe and many more, with chemical vapor deposition (CVD). Among other B-N type compounds mixed amino-nitro substituted borazines have been predicted to outperform carbon based explosives such as CL-20.

13. NET and GATE exam Questions: 1) BCl 3 with NH 4 Cl gives product A which upon reduction by NaBH 4 gives product B. Product B upon reacting with HCl affords compound C, which is Cl 3 B 3 N 3 H 9 ( ClBNH ) 3 (HBNH) 3 ( ClH ) 3 B 3 N 3 CClH 3 2) Heating mixture of NH 4 Cl gives one liquid product (X), along with other products. Compound (X) is NH 4 [BH 4 ] [(NH 3 ) 2 BH 2 ][BH 4 ] N 3 B 3 H 6 N 3 B 3 H 12

14. 3) BCl 3 and NH 4 Cl were reacted at 140 o C to give compound X, which then treated NaBH 4 gave compound Y, X and Y ? X= B 3 N 3 H 6 Cl 3 , Y= B 3 N 3 H­ 6 X= B 3 N 3 H 9 Cl 3 , Y= B 3 N 3 H­ 6 X= B 3 N 3 H 9 Cl 3 , Y= B 3 N 3 H­ 12 X= B 3 N 3 Cl 6 , Y= B 6 N 3 H­ 6 4) 3NH 4 Cl + BCl 3 X Y ; X,Y ? [HB(NH)] 3 , [H(OH)B(NH 2 )] 3 [HB(NH)] 3 , [HB(NH 2 OH)] 3 (NH 4 )(H) 2 (BH 2 ) 3 ,[H(OH)(NH 2 OH)] 3 (NH 4 )(H) 2 (BH 2 ) 3 , [HB(NH 2 OH)] 3

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