Chapter 6th:- Preparation Properties & Structures of Borazine, Diborane, Hydrazine, Interhalogens, Polyhalides, and Fluorides of Xenon

B.sc 1st year Book
(Page 2)

Preparation Properties & Structures of Diborane

Structure:

Preparation:

By hydrogenation of boron chloride or bromide in the presence of Cu-Al catalyst

• By dropping an etherial solution of BF₃ on sodium borohydride

• By the action of dilute H₂SO₄ on NaBH₄

• By the reaction of Boron trichloride with Lithium aluminum Hydride in ether solution

This method gives a quantitative yield.

Physical properties:

1. Diborane is inflammable colorless gas having a stickily sweet odor.
2. It is extremely toxic.
3. It is an extremely reactive gas hence it must be handled carefully
4. It decomposes on heating and yields a number of higher boranes for example:

Chemical Properties:

1. Hydrolysis:

(i) With water:

(ii) With a concentrated aqueous solution of KOH at 0°C:

(iii) With dil. KOH solution: When B₂H­₆ is treated with dil. KOH, potassium metaborate is formed with the liberation of H₂ gas

The evolution of H₂ by the action of aqueous alkalies makes B₂H₆ as a useful reducing agent

2. By the action with Alkali metal hydrides:

B₂H₆ reacts with alkali metal hydrides in ether to form metallic borohydride.

3. By the action of O₂ (Combustion) or air:

Pure B₂H₆ does not undergo any reaction when mixed with dry air or O₂ at room temperature but when it is impure, ignites to give B₂O₃ and a large amount of energy is evolved.

The production of the large amount of energy in the above reaction makes B₂H₆ a useful rocket fuel

4. By the action of Halogens (l₂ does not react):

B₂H₆ reacts with Cl₂ (at 25 °C) and Br₂ (at 100 °C) to form HCI and HBr respectively.

5. By the action of Halogen acids. HX (X= Cl, Br, I):

B₂H₆ reacts with HCl readily in the presence of a catalyst (e.g., AICI₃) to form chlorodiborane, B₂H₅CI. With HBr, it reacts at 90 °C in presence of AlBr₃ to give B₂H₅Br while with HI it reacts at 50 °C in the absence of a catalyst to form B₂H₅I. Hydrogen gas is evolved in all the reactions.

6. By the action of Sodium or Potassium amalgam:

7. By the action with NH₃:

• When B₂H₆ reacts with an excess of NH₃ at-120 °C an addition compound, B₂H₆.2NH₃ is formed.

• When B₂H₆ reacts with an excess of NH₃ at high temperatures, boron nitride (BN) is obtained and H₂ is evolved.

• When one mole of B₂H₆ reacts with two moles of NH₃ at high temperature, an addition compound, B₂H₆.2NH₃ is formed. This additional compound gets decomposed when heated in a closed tube at 200°C and forms borazine (also called borazole), (BH)₃ (BH)_3, or B₃N₃H₆.

8. Methylation of B₂H₆ with trimethyl borane:

B₂H₆ reacts with B(CH₃)₃ at ordinary temperature and produces four methyl derivatives which are called ‘methyl diboranes’. In the formation of these derivatives, only four terminal H-atoms B₂H₆ are replaced by CH₃ groups while the two bridging H atoms remain as such. Thus, in this reaction, both three center-two-electron (B-H-B) bonds remain undisturbed.

The replacement of only four H-atoms by the CH₃ group shows that four H-atoms in the B₂H₆ molecule are different from the remaining two H-atoms.

9. Formation of adduct:

Due to its electron-deficient nature, B₂H₆ reacts with a number of molecules having lone pairs of electrons.

Structure:

The following structures are proposed:

• Ionic Structures:

The above structure could not be supported by experimental data. These structures are based on the reaction of diborane with ammonia (base). It was assumed that diborane contains one or two protons in its structures

• Ethane-like structure:

It was believed earlier that the molecular formulas of diborane and ethane are identical so both should have similar structures

Those types of structures are possible only for the molecules possessing 14 bonding electrons which may form seven covalent bonds. But in diborane, the number of banding electrons is twelve (6 from two 8-atoms and 6 from six H-atoms). This deficiency of two electrons is explained in terms of resonance between structures containing one electron bond and no electron bond.

For many years ethane like the structure of diborane was believed to be correct and it was often written as BH₃-BH₃. But this type of structure of diborane was later discarded because of its inability to explain the following

• Diborane should be paramagnetic, if it has one bond structure due to the presence of unpaired electrons, however, it is diamagnetic.
• There is no free rotation about the B-B bond however C-C bond rotation occurs in C₂H₆ This indicates the absence of B-B bond in diborane.
• In an ethane-like structure, all the six hydrogen atoms should be identical but in diborane not more than four H-atoms can be substituted by methyl group to give tetramethyl diborane (CH₃)2B₂H₂. This compound gives (CH₃)₂BOH on hydrolysis showing thereby that two methyl groups are attached to each boron atom. This shows that in B₂H₆ four H-atoms are equivalent and the remaining two H-atoms are different in nature.

Therefore, the structure of diborane should be such that it may explain all the points discussed above.

• Hydrogen bridge structure:

Dilthey 1921 proposed the hydrogen bridge structure of diborane. But recent works based on electron diffraction and spectral properties are more reasonable to explain the structure of B₂H₆ Thus it may be assumed to be formed from two irregular BH₄ tetrahedrons having one bridge common. The two hydrogen atoms lying on the common edge are called bridged hydrogen atoms (Hp) while others are known as terminal hydrogen atoms (H_t). Along with both the Boron atoms, all four terminal hydrogens lie in one plane while the bridge hydrogens lie in a perpendicular plane passing through boron atoms. I.e., If the terminal hydrogen atoms of boron are assumed to lie in the plane of the paper one bridge hydrogen lies above and the other below the paper.

As clear from the above structure that:

• Four hydrogen atoms two of each B-atom, are free and held directly with B-atoms through two-electron two center bonds (2e-2c, bonds). These hydrogen atoms are known as terminal hydrogens. On methylation of B₂H₆ all of these four terminal hydrogens can be replaced by methyl groups. Which shows they are equivalent, thus there are four (2e-2c) bonds.
• Two H-atoms lie one above and the other below the two BH₂^– groups are held with B-atoms through two electrons three centers (2e-3c) bonds or banana bonds. These two H-atoms are known as bridge hydrogens. It is due to the presence of 2e-3c, bonds. B₂H₆ is considered the best example of a molecule containing multi-center bonds or three-center bonds.
• There is no B-B bond.
• Both B-atoms are pseudo sp³

In B₂H₆ B-H, the terminal bond length is 1.19 Ǻ, B-H, bridge bond distance is 1.33 Ǻ and B-B inter-nuclear distance is 1.77 Ǻ. The H_bBH_b bond angle is 97 and H_tBH_t bond angle is 121.5.

Fig.6.01: Structure of di-borane

Experimental Evidence Supporting the Bridge Structure:

• NMR study and Raman spectra of B₂H₆ have shown that four hydrogens namely H_t are of one type while the remaining two namely H_b atoms are of different types.
• Specific heat measurements have shown that the two ends of the molecule cannot be rotated against each other. This indicates that the two bridging hydrogen atoms lie at right angles to the two BH₂ groups present in a plane.
• The reaction of diborane with trimethyl borane (CH₃)₃B and the formation of a series of methyl derivatives of diborane indicates that none of the bridge H-atoms in B₂H₆ is replaced by CH₃ In other words, both three center B-H-B bonds remain undisturbed when reaction occurs between B₂H₆ and (CH₃)₃B.

• Hydrolysis of B₂H₂Me₄ gives Me₂B(OH) showing that a maximum of two methyl groups may be joined to a given B-atom in the dimer, Hydrolysis of B₂H₂Me₂ gives Me₂B(OH) and MeB(OH)₂ e., 1, 1, 1-trimethyl diborane does not exist. This suggests that all the six hydrogen atoms in B₂H₂ are not equivalent.

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