PREPARATION PROPERTIES AND STRUCTURES OF
Borazine (Inorganic Benzene)
Molecular formula: B3N3H6
The systematic name of borazine is s-tri-azatriborine. It is isoelectronic with benzene and some of its properties show striking similarities with benzene. Borazine is also known as Inorganic benzene because of its resemblance to benzene.
Total Electrons= 6C + 6H = 6 x 4 + 6 x 1 = 30
Total electrons = 3B + 3N + 6H = 3×3 + 3×5 + 6×1 = 30
Methods of Preparation:
(1) By Stock and Pohland method:
Borazine was first prepared by Stock and Pohland in 1926 by heating diborane with ammonia in a 1:2 molar ratio at 125°C. The adduct B2H8.2NH3 is first formed as an intermediate which then gets decomposed by heating in a closed tube at 250-300 °C to give borazine.
This method gives a low yield (50%) of borazine because of the simultaneous formation of solid polymeric by-products. Improved routes that utilize reactions of ammonia-borane (H3NiBH3) at 140-160°C or alternatively (NH4)2SO4 and NaBH4 at 120-140 °C, to afford very pure borazine on a multigram scale have been reported recently
(2) By the action of NH4Cl on BCI3: When BCl3 is heated with NH4Cl in chlorobenzene (C6H5CI) at 140-150 °C in the presence of catalysts like Fe, Ne, or Co to form B, B+, B++ trichloro borazine Cl3B3N3H3 which is then treated with NaBH4 to yield borazine B3N3H6
In this reaction 3/2 moles of diborane are liberated for every mole of trichloro borazine used Borazine so formed in this reaction is distilled in a vacuum and purified by fractional distillation method.
(3) Laboratory Method:
Borazine can be prepared in the laboratory by heating lithium borohydride LiBH4 with NH4CI in a vacuum at about 230-250 °C.
In this method, lithium borohydride is dissolved in diglyme and is then used as a reducing agent. The yield of this method is 80%.
(1) Borazine is a colorless, volatile mobile liquid with an aromatic odor. It boils at 64.5 °C and freezes at-57.9 °C.
(2) It decomposes slowly in storage even at -80 °C and small quantities of solid material are deposited over a period of several days.
(3) It resembles closely benzene in various physical properties like boiling point, melting point, liquid density, and surface tension.
(4) The molecule is planar with one B N bond distance of 1.44A throughout the ring compared with a C-C distance of 1.342A in benzene. The N-Hand BH distances are 1.02 and 1.20A respectively. The B-N distance is less than that expected for a single bond (1.60A in (H3N BH3) and this is attributed to delocalized P-P-bonding. Delocalization of the electrons in p-orbitals is also there keeping with the fact that the molecule shows magnetic anisotropy.
(5) Due to the difference in electronegativity between boron and nitrogen, the cloud in borazine is lumpy because more electron density is localized on the nitrogen atoms.
(6) The polarity of the B-N Bond is B+-N– and is the resultant of an electron drift from B to N associated with the -bond and from N to B associated with the -bond.
(7) The net effect is that the charge density on nitrogen increases. In addition to this, nitrogen retains its basicity and boron its acidity. Polar species such as HCl can therefore attack the double bond between nitrogen and boron. Thus, in contrast to benzene, borazine readily undergoes addition reactions.
Borazine is much more reactive than benzene. lt readily adds 3 moles HX (e.g., water, methanol, HCI or HBr) to its p bonds even at 20 °C. As expected, the hydrogen of HX migrates to the negatively polarized N-atom and the X group to the positively polarized B-atom. Thus, borazine is not so strongly aromatic as benzene.
1- Addition Reactions:
Borazine readily undergoes an addition reaction with HCI, HBr, H2O, CH3OH, C2H5OH, CH3Cl, etc. in cold without a catalyst. In such reactions more electronegative groups of the attacking molecules are attached with boron because boron is less electronegative than nitrogen in the B-N bond. For examples:
(i) Addition with HCI:
The hydrogen chloride derivative of borazine on heating at a temperature of about 50-100 “Closes three molecules of hydrogen to give B-trichloroborazine or B,B+, B++ trichloroborazine or -trichloroborazine.
Whereas benzene does not undergo additional reactions with HCI.
(ii) Reaction with alcohols:
Borazine reacts with methanol; CH3OH to form an addiction. This adduct breaks down readily to yield a product with the alkoxy group (RO-group) attached to boron.
On the other hand, the reaction with ethanol C2H5OH is rapid and yields chiefly ammonia and triethoxy borane. Whereas benzene does not undergo these reactions.
(a) The adduct with water is formed at 0 °C, and the bond fission of this adduct occurs at a higher temperature to yield H2, boric acid, and ammonia
(b) Under proper conditions borazine reacts with 3 moles of water and gives B-trihydroxy borazine.
On the other hand, benzene does not undergo hydrolysis.
(iv) Addition with Bromine:
Bromination reaction of borazine occurs at 0 °C and yields B-tribromo N-tribromo borazine as an additional product which on heating at 60 °C gives B-tribromo borazine
Whereas benzene forms Bromo benzene when treated with Br2 in the presence of a suitable catalyst.
When borazine is heated to 340-440°C a mixture of two polynuclear borazines are formed which are B, and N analogs of diphenyl and naphthalene.
(vi) Reaction with aniline (C6H5NH2):
Borazine reacts with aniline to give tri aminoborane. This reaction is strongly exothermic.
Borazine may be hydrogenated to produce the saturated compound analogous to cyclohexane called ‘cycloborazane’. Hydrogenation of borazine results in polymeric products of indefinite compositions. Substituted derivatives of the saturated cycloborazane, B3N3H12.forms readily by addition to borazine through the following reaction.
(viii) Reaction with Grignard reagent:
Borazine reacts with phenyl magnesium bromide: C6H5MgBr to give B-arylate compounds such as B-mono phenyl, B-diphenyl, and B-triphenyl borazines.
X-ray crystallographic studies of borazine reveal that boron and nitrogen atoms in it are arranged alternatively in a planar hexagonal ring in which the B-N bond length is 0.147±0.007 nm. The bonding in borazine has been described in terms of resonance between the single bonded tri amino tri-borine ring (a) and the Kekule structures (d) and (e) in which double bonds are present due to the participation of lone pairs on nitrogen in the B-N. bonds. This results in a formal negative charge on the boron and a formal positive charge on the nitrogen atom (structures and g). This resonance hybrid description is based on the similarities which exist between borazine and benzene.
Borazine and benzene both have aromatic π clouds of electron density with the potential for delocalization over all of the ring atoms. However, due to the difference in electronegativity between boron and nitrogen, the π cloud in borazine is ‘lumpy, because more electron density is localized on the N atoms. This partial delocalization weakens the π-bonding in the ring. Each nitrogen atom receives more σ- electron density from neighboring boron than it gives away as a π donor. The net effect is that the charge density on nitrogen increases. In addition, nitrogen retains its basicity and boron its acidity. Polar species such as HCI can therefore attack the double bond between nitrogen and boron. On the other hand, since, carbon-carbon bonds are non-polar in benzene, benzene does not undergo addition reactions.
(a) Preparation of N-trimethyl Borazine.
Preparation of N-trimethyl Borazine
(b) By the reduction of monomethyl ammonium chloride with lithium borohydride
N-trimethyl borazine is a colorless mobile liquid. Its m.p.is -90 °C and b.p. is 130 °C. On hydrolysis, it gives the following product
By the reaction of ammonia on trimethyl boron.
At room temperature, B-trimethyl borazine is a colorless volatile crystal. Its m.p. is 31.8°C and b.p. Is 127°C. It is sensitive to moisture but is insoluble in water. At 100°C water replaces the NH groups with oxygen and forms B-trimethyl boroxine.