Chemistry PREPARATION, PROPERTIES, USES & STRUCTURE OF DIBORANE, BORIC ACID, BORAX AND BORON TRIFLUORIDE ETC.
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PREPARATION, PROPERTIES, USES & STRUCTURE OF DIBORANE, BORIC ACID, BORAX AND BORON TRIFLUORIDE ETC.

`B_2H_6` (Diborane) :

Structure of Diborane : See fig.1.

`B = 1s^2, 2s^2, 2p^1`


`(1s^2 , 2s^1 ,2p_x^1 , 2p_y^1 ,2p_z)/(sp^3 text(Hybridisation))`

(a) `4` Terminal `H`-are bonded by `sigma` bond & remaining `2H` are bridging hydrogens and of these are broken then dimer become monomer.

(b) Boron undergoes `sp^3` hybridisation. `3` of its `sp^3` hybridised orbitals contain one `e^-` each & fourth `sp^3` hybrid orbital is vacant.

(c) `3` of these `sp^3` hybrid orbitals get overlapped by `s` orbitals of `3` hydrogen atoms.

(d) One of the `sp^3` hybrid orbitals which have been overlapped by `s` orbital of hydrogen gets overlapped by vacant `sp^3` hybrid orbital of `2^(nd)` Boron atom and its vice versa.

(e) By this two types of overlapping take place `4(sp^3- s)` overlap bonds & `2 (sp^3 - s - sp^3)` overlap bonds

(f) `H` is held in this bond by forces of attraction from `B` & this bond is called `3` centered two electron bonds. It is also called Banana bonds. Due to repulsion between the two hydrogen nuclei, the delocalised orbitals of bridges are bent away from each other on the middle giving the shape of banana. See fig.2 & fig.3.


Uses of Diborane :

(i) Diborane is used for preparing substances such as high energy fuel & propellents.

(ii) Boron hydrides have been tried as rocket fuels.

(iii) It is used as a catalyst in polymerisation reaction.

(iv) It is used as a reducing agent in inorganic reaction.

(v) It is used for welding torches.

Preparation of `B_2H_6` :

(i) `Mg_3B_2 +underset(10%)HCl rightarrow B_2H_6 + B_4H_(10) + B_5H_9`

(ii) `B_4H_(10) underset(100%) overset(Delta) rightarrow B_2H_6 + H_2 + text(higher borane)`

(iii) `BCl_3(or BBr_3) + 6H_2 undersettext(discharge at low pressure)oversettext(Electric) rightarrow B_2H_6 + 6HCl`

(iv) `3underset[text(or)LiBH_4] [LiAlH_4] + 4BF_3 rightarrow 3LiF + underset[text(or) 3(BF_3)] [3AlF_3]`

Physical Properties :

(i) Diboranes (Melting point `=-165.5^oC` Boiling point `= -92.5^0C`) is a colourless gas with a foul smell & is extremely toxic.

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

(iii) Diborane reacts with sodium amalgum to form an addition product `B_2H_6Na_2`

(iv) On heating to `700^0C` diborane dissociates.

`B_2H_6 rightarrow 2B + 3H_2uparrow`

(v) Diborane readily adds at `0^oC` to `25^oC` to the olefinic acetylene compound to form trialkylborane. The reaction is known as hydroboration

`undersettext(Ethylene) (6CH_2 = CH_2)+B_2H_6 oversettext(ether) rightarrow undersettext(Triethylborane)(2B(CH_2-CH_3)_3)`

(vi) Diborane is electron-deficient molecule & hence it reacts with several molecule having lone pair(s) of electron (e.g. `CO`, ether, amines etc.) to form complex compounds.

(vii) Boranes have great affinity for water & `O_2` hence they do not occur in nature. Moreover borane cannot be prepared directly from its elements as they have positive enthalpy & positive free energy of formation.

Chemical Properties :

(i) `B_2H_6+O_2undersettext(air spontaneously)oversettext(burns in) rightarrow B_2O_3+H_2O`

(ii) `B_2H_6+H_2Otext(cold) rightarrow H_3BO_3+6H_2`

(iii) `B_2H_6+HCl text(dry)underset(AlCl_3)oversettext(anh.) rightarrow B_2H_5Cl + H_2`

(iv) `B_2H_6 + 2N(CH_3)_3 rightarrow 2(CH_3)_3N + BH_3`

(v) `B_2H_6+2Na (text((amalgam)) rightarrow B_2H_6Na_2`

(vi) `6NH_3 + 3B_2H_6 rightarrow undersettext(borazole) (2B_3N_3H_6) +12H_2`

Orthoboric Acid (`H_3BO_3`) :

`H_3BO_3` is soluble in water and behaves as weak mono basic acid. It does not donate protons but rather it accepts `OH^-`. Therefore it acts as a Lewis acid `(B(OH)_3)`

`underset(text(or) H_3BO_3) [B(OH)_3] + 2H_2O ⇋ H_3O^(+) + [B(OH)_4]^-`

Since `B(OH)_3` only partially reacts with water to form `H_3O^+` and `[B(OH)_4]^-` it behaves as a weak acid. Thus it cannot be titrated satisfactorily with `NaOH` as a sharp end point is not obtained. If certain polyhydroxy compounds such as glycerol , mannitol or sugar are added to the titration mixture then `B(OH)_3` behaves as a strong mono basic acid and hence can now be titrated with `NaOH` and end point is diluted using phenolphthalein as indicator

`B(OH)_3+NaOH ⇋ underset(NaBO_2+2H_2O) (Na[B(OH)_4])`

The added compound must be a cis diol to enhance the acidic proprieties. In this way the cis-diol forms very stable complexes with `[B(OH)_4]^-` formed in forward direction above, thus effectively removing it from solution. Hence reaction proceeds in forward direction (Le-Chatelier principle.)


`ast` Heating of boric acid

`H_3BO_3overset(100^oC) rightarrow undersettext(Metaboric acid) (HBO_2) overset(140^oC) rightarrow undersettext(tetraboric acid) (H_2B_4O_7)undersettext(red hot)overset(Delta) rightarrow undersettext(Glassy mass) (B_2O_3)`

`ast` See fig.2.

Borax `Na_2B_4O_7 . 10H_2O` or Tincal :

(a) It is also named as sodium tetra borate decahydrate.

(b) Common Indian name is Suhaga.

(c) `Na_2B_4O_7*5H_2O` is known as Jeweller's.

(d) `Na_2B_4O_7` is known as Boron glass.

Preparation : Form colemanite :

Preparation of Borax :

`undersettext(Colemanite)[2CaO (3 B_2O_3)] + 2Na_2CO_3 rightarrow 2CaCO_3 downarrow +Na_2B_4O_7+2NaBO_2`

Uses of borax :

(i) In making glass, enamel and glaze of pottery.

(ii) As antiseptic in medicinal soaps preparation.

Chemical Properties :

Action of Heat on Borax

(i) Borax swells up on heating.

(ii) On heating, borax loses water and swells into a white mass which on further heating melts to form transparent glassy solid called Borax glass and Borax bead.

`Na_2B_4O_7 . 10H_2O oversettext(heat) rightarrow Na_2B_4O_7 overset(740^0C) rightarrow Na_2O + 2B_2O_3`

(iii) The borax bead is due to the formation of `B_2O_3` which when fused with metal salts form corresponding metaborate.

`B_2O_3 + CuO rightarrow undersettext[Copper meta borate (Blue)] [Cu(BO_2)_2]`

(iv) Colour of meta borates

`undersettext(blue) (Cu) , undersettext(Green)(Fe), undersettext(blue)(CO), undersettext(Green)(Cr)`
 
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