B.sc 1st year Book
Preparation Properties & Structures of Diborane
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.
- Diborane is inflammable colorless gas having a stickily sweet odor.
- It is extremely toxic.
- It is an extremely reactive gas hence it must be handled carefully
- It decomposes on heating and yields a number of higher boranes for example:
(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
By the action with Alkali metal hydrides:
B2H6 reacts with alkali metal hydrides in ether to form metallic borohydride.
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.
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.
By the action of Sodium or Potassium amalgam:
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.
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.
The following structures are proposed:
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
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 (Ht). 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 HbBHb bond angle is 97 and HtBHt 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 Ht are of one type while the remaining two namely Hb 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.