CHEMICAL PROPERTIES OF DIBORANE:

The Boranes unddergo different type of chemical reactions like oxidation, pyrolysis, Nucleophilic and electrophic  and reactions with bases such as OH^- and NH₃.

(1) Reaction with Na: Diborane reacts with sodium amalgum to form an addition product B₂H₆Na₂.

(2) Reaction with air: the reaction of diborane with oxygen is highly exothermic and due to evolution of enormous amount of heat energy it can use as industrial fuel and propellants and not use as domestic due to its poisonous nature..

(3) Thermal Stability: B₂H₆ stable only at low temperature when heated 100 to 250 degree it changes into higher hydrides and On heating to 700ºC diborane dissociates.

Note: - Formation of B₂H₂Cl₄ shows that the 2H left in B₂H₂Cl₄ are responsible for dimmer formation (Bridge bond). Diborane has only four replaceable hydrogen and with their replacement, the dimeric structure continuous to be as such. Remaining  two hydrogen when they get displaced, the dimeric structure break indicating that these two hydrogen are act as bridging hydrogen.  

(9) Reaction with Ammonia (NH₃):

(1) Diborane react with excess NH₃ at temperature to form (inorganic graphite) Boron nitride (BN) _x.while when diborane and NH₃ react in 1:2 ratios at low temperature give   Inorganic Benzene (Borazole).

(2) Diborane is electron-deficient molecule and hence it reacts with several molecules having lone pair(s) of electron like CO, ether, amines, to form complex compounds.

(3) B₂H₆ give symmetrical cleavage with respect to only large size and weak amines CO, H¯, N(CH₃)₃ , THF, PH₃, PF₃ , OEt₃ OMe₂ ,Pyridine, Thiophene ,SMe₂, Set₃ etc.

(4) In the presence of small and strong base B₂H₆ undergo unsymmetrical cleavage like NH₃, H₂N (CH₃), HN (CH₃)₂ etc

ILLUSTRATIVE EXAMPLE:

(10) Methylation of Doborane:

(1) In diborane two boron atoms and four terminal hydrogen atoms lie in one plane. While two bridge hydogen atoms (encircled ) lie smmetrically above and below the plane.

(2) Total valence electrons in B₂H₆ is 12(6 from boron 3x2) and 6 from six hydrogen atoms) and there are two B-H-B (3C-2e) bridge bonds and four B-H (2C-2e) terminal bonds.

(3) Bond energy of B-H-B bond is 441 kjpermole which greater than bond energy B-H bond (341Kjpermole) hence methylation of diborane no more than four hydrogen.

(4) In above reaction it is clearly reveal that none of the bridge hydrogen in B₂H₆  has been replaced by –CH₃ .ie in this reaction both the bridge bond remaine undissociate.

Structure of "Inorganic Benzene" /Borazole/Borazine:

Structure of Boron Nitride:

Structure of Fulelrences:

Structure of Graphite:

PHYSICAL PROPERTIES DIBORANE:

(1) Diborane is a colourless gas with a foul smell and is extremely toxic.

(2) Melting point = -164.85^oc and Boiling point = -92.59^oc

(3) It is an extremely reactive inflammable gas which burns in air with a green flame

Structure of Boron nitride (Inorganic Graphite):

Diborane react with excess NH₃ at temperature to form boron nitride (BN) _x.while when diborane and NH₃ react in 1:2 ratios at low temperature give Borazole.

Boron Nitrides exist two forms just like allotropic forms of carbon (Graphite and Diamond) and both have formula (BN)_x.

(i) Boron Nitride (Inorganic Graphite):

(1) Boron nitride is a hexagonal 2D planar giant covalent network , slippery  and a white solid with a layered structure like graphite. Doe to similar structure with graphite it know as “Inorganic graphite” and due to white colour  it is also called “white graphite”

(2) The thermodynamically stable phase of boron nitride, BN, consists of planar sheets of atoms like those in graphite The planar sheets of alternating B and N atoms consist of edge shared hexagons and, as in graphite, the B-N distance within the sheet (145 pm) is much shorter than the distance between the sheets (333 pm,). The difference between the structures of graphite and boron nitride, however, lies in the register of the atoms of neighboring sheets:

(3) The B-N-B or N-B-N bond angle is 120^oc . It may be expected for perfect hexagonal ring bond network just like graphite. And boron and nitrogen atoms are sp2 hybridized.

(4) Boron nitride (Inorganic graphite ) is a very good insulator (thermal and electrical) and chemically very inert , chemically posses great stability due to the  very strong B-N bonding  in the 2D layers structure. It melts under pressure at  3000^oc  so it is great thermal stability.

(5) In (BN)_x the hexagonal rings are stacked directly over each other, with B and N atoms alternating in successive layers; in graphite, the hexagons are staggered. Molecular orbital calculations suggest that the stacking

(6) In (BN)_x stems from a partial positive charge on B and a partial negative charge on N. This charge distribution is consistent with the electronegativity difference of the two elements.

 (7)In  Boron nitride(BN)x the Vander Waals forces holding the sheet in line with each other are stronger, so boron nitride is not as good a good lubricant as graphite. However , the use of boron nitride as a lubricant is noted as high temperature due to its chemical stability.

(8) As with impure graphite, layered boron nitride is a slippery material that is used as a lubricant. Unlike graphite, however, it is a colorless electrical insulator, as there is a large energy gap between the filled and vacant π bands

Uses:

(1) Boron nitride ceramics us in high temperature (range 2700-3000^oc)  equipment due to excellent thermal stability, thermal shock stability and chemical stability.

(2) Boron nitride based ceramics are stable in air at 1000^oc while carbon-graphite based materials ignited at that temperature.

(3) Hexagonal boron nitride can be made in single layers and can also be formed into nanotubes. And that nanotubes are used for wire sieving and a catalyst support.

(4) Hexagonal boron nitride can be incorporated in ceramics, alloys, resins, plastics, rubbers to give them self-lubricating properties. And plastics based hexagonal boron nitride  decrease thermal expansion, increased thermal conductivity, increased electrical insulation. 

Structure of "Borazon" (Cubic boron nitride):

Structure of "Inorganic Benzene":

DIBORANE-PREPARATION:

Boranes are hydride of Boron and diborane is famous borane. It is gas and is highly inflammable in air and poisonous Diborane is used for preparing substances such as high energy fuel and propellants.

Other Methods:

Structure of “Borazine/Borazole”/inorganic Benzene:

(1) Borazine is an inorganic compound with the chemical formula  (B₃N₃H₆).and also called Borazole. It is a heterocyclic compound, containing the 3-(BH) units and 3-(NH) units alternate.

(2) Borazine formed by reaction of B₂H₆ and NH₃ in the ratio of 1:2 at room temperature.

(3) Borazine is isoelectronic and isostructural with benzene. For this reason borazine is known as “inorganic benzene”. Like benzene borazine has delocalized electrons and aromatic character. And it is a colourless liquid.

(4) In Borazine molecule nitrogen is more electro negative than the boron. Nitrogen acquires partial negative charge and boron acquires partial positive charge and back bonding take place between boron and nitrogen.

(5) As compared with Benzene, Borazole/borazine is less stable and more reactive toward Electrophilic Aromatic Substitution reactions due to presence of polar B-N bond in it while in benzene all the C-C bonds are non polar.

(6) Borazine is a highly polar molecule due to high Electronegativity difference between Boron and Nitrogen.

Even though Borazine/Borazole  and Benzene have same stricture their chemical properties are different.

(1) Organic benzene is C₆H₆ while Inorganic benzene is Borazine having chemical formula B₃N₃H₆

(2) The pi bonds in borazine are highly polarized than pi bonds in benzene due to high polarity (B-N polar bond) of Borazine molecules. Thus borazine is more nucleophillic (Negative) hence more reactive than benzene with respect to “Electrophic addition reactions”.

Here protonation(H⁺) take place at nitrogen atoms due to more electron density (more negative)  and  chlorine attack at boron atoms.

(3) Aromaticity of borazine is less than benzene because some delocalization in Borazine/Borazole is not complete as benzene. One reason is that nitrogen atom has more electronegativity than boron ,the electron density is higher at nitrogen atoms then boron. The electron density is determined by both sigma and pi bonds, both of which have polarity, but opposite directions,   hence it is less reactive  toward “Electrophilic  substitution reactions” than Benzene. 

(4) Borazine undergo polymerization when strongly heated under vacuum and yield biborazonyl and naphthazine whose structure are similar to biphenyl and naphthalene.

Structure of "Borazon" (Cubic boron nitride):

Structure of "Inorganic Gaphite";

Related Questions: 

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