CHEMISTRY ASSIGNMENT

FOR ROLL NO. 4980-4991

 Diborane

Diborane is the chemical compound consisting of boron and hydrogen with the formula B₂H₆. It is a colorless and highly unstable gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air at room temperature. Synonyms include boroethane, boron hydride, and diboron hexahydride.
Diborane is a key boron compound with a variety of applications. The compound is classified as "endothermic", meaning that its heat of formation, ΔH°_f is positive (36 kJ/mol). Despite a high thermodynamic instability, diborane is surprisingly nonreactive for kinetic reasons, and it is known to take part in an extensive range of chemical transformations, many of them entailing loss of dihydrogen.

SYNTHESIS OF DIBORANE

Extensive studies of diborane have led to the development of multiple syntheses. Most preparations entail reactions of hydride donors with boron halides or alkoxides. The industrial synthesis of diborane involves the reduction of BF₃ by sodium hydride, lithium hydride or lithium aluminium hydride:^[8]

8 BF₃ + 6 LiH → B₂H₆ + 6 LiBF₄

Two laboratory methods start from boron trichloride with lithium aluminium hydride or from boron trifluoride ether solution with sodium borohydride. Both methods result in as much as 30% yield:

4 BCl₃ + 3 LiAlH₄ → 2 B₂H₆ + 3 LiAlCl₄

4 BF₃ + 3 NaBH₄ → 2 B₂H₆ + 3 NaBF₄

Older methods entail the direct reaction of borohydride salts with a non-oxidizing acid, such as phosphoric acid or dilute sulfuric acid.

2 BH₄⁻ + 2 H⁺ → 2 H₂ + B₂H₆

Similarly, oxidation of borohydride salts has been demonstrated and remains convenient for small scale preparations. For example, using iodine as an oxidizer:

2 NaBH
4 + I
2 → 2 NaI + B
2H
6 + H
2

Another small-scale synthesis uses potassium hydroborate and phosphoric acid as starting materials.

Structure and bonding

Diborane adopts a D_2h structure containing four terminal and two bridging hydrogen atoms. The model determined by molecular orbital theory indicates that the bonds between boron and the terminal hydrogen atoms are conventional 2-center, 2-electron covalent bonds. The bonding between the boron atoms and the bridging hydrogen atoms is, however, different from that in molecules such as hydrocarbons. Having used two electrons in bonding to the terminal hydrogen atoms, each boron has one valence electron remaining for additional bonding. The bridging hydrogen atoms provide one electron each. Thus the B₂H₂ ring is held together by four electrons, an example of 3-center 2-electron bonding. This type of bond is sometimes called a 'banana bond'. The lengths of the B-H_bridge bonds and the B-H_terminal bonds are 1.33 and 1.19 Å respectively, and this difference in the lengths of these bonds reflects the difference in their strengths, the B-H_bridge bonds being relatively weaker. The weakness of the B-H_bridge vs B-H_terminal bonds is indicated by their vibrational signatures in the infrared spectrum, being ~2100 and 2500 cm⁻¹, respectively. The structure is isoelectronic with C₂H₆²⁺, which would arise from the diprotonation of the planar molecule ethene. Diborane is one of many compounds with such unusual bonding.
Of the other elements in Group IIIA, gallium is known to form a similar compound, digallane, Ga₂H₆. Aluminium forms a polymeric hydride, (AlH₃)_n, although unstable Al₂H₆ has been isolated in solid hydrogen and is isostructural with diborane.


VIKAS BHATI


NOTES ON GENERAL PRINCIPLES OF METALLURGY