What is diborane
Borazine (Cyclotriborazane) is a cyclic compound of boron, nitrogen and hydrogen with the empirical formula B.3H6N3. With its lone pair of electrons, nitrogen contributes to the fact that this compound has a formal resemblance to benzene, although it does not meet the criteria of aromaticity. According to a suggestion by Nils Wiberg, the basic structure of these compounds is also known as “inorganic benzene”. The systematically not admissible designation is derived from this Borazole from.
The name borazine / borazine has also been given to the group of substances iminoboranes, based on the structurally related alkynes.
Borazin was first made by Alfred Stock in the early 20th century. He obtained it by heating diborane and ammonia.
Extraction and presentation
Borazine can be produced by heating a mixture of diborane and ammonia in a molar ratio of 1: 2 to 250-300 ° C. The yield of this chemical reaction is 50%:
- Diborane reacts with ammonia to form borazine and hydrogen.
Alternatively, lithium borohydride and ammonium chloride can also be used as starting materials, which leads to a higher yield:
- Lithium borohydride and ammonium chloride are converted to borazine, lithium chloride and hydrogen.
Another borazine synthesis is as follows:
- First, boron trichloride and ammonium chloride are converted to trichloroborazine and hydrogen chloride.
- In a second step, trichloroborazine is reduced to borazine with sodium borohydride. Diborane and sodium chloride are produced as by-products.
Borazine decomposes in water to form boric acid, ammonia and hydrogen. Borazine (enthalpy of formation ΔHf = −531 kJ / mol) is thermally very stable.
Borazine is isostatic to benzene. This means that the bonds and bond conditions are the same or very similar to those in benzene. The C-C distance in benzene is 0.1397 nm. The bond distance between boron and nitrogen in borazine is 0.1436 nm; thus, as expected, it lies between the value for a B-N single bond (0.151 nm), as found in boron nitride, and that for a B-N double bond (0.131 nm).
The electronegativity difference between boron (2.04 on the Pauling scale) and nitrogen (3.04) as well as the electron deficiency on the boron atom and the free electron pair in nitrogen favor the formation of mesomeric borazine structures. Boron plays the role of a Lewis acid, nitrogen that of a Lewis base.
Due to the difference in atoms and thus the polarity of the B-N bonds, borazine is chemically much more reactive than benzene. Borazine reacts easily with polar compounds such as hydrogen chloride, water and methanol. Hydrogen chloride reacts with borazine in an addition reaction. With benzene this reaction would not take place. The reaction with bromine does not require a catalyst.
Borazine and its derivatives are of interest as potential precursors on the way to boron nitride ceramics.
Categories: Boron Compound | Nitrogen compound
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