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Saturday, 11 February 2017

The p-block elements IV


Some important characteristics of boranes:
i) Lower boranes are colourless gases while higher boranes are volatile liquids or solids.

ii) They undergo spontaneous combustion in air due to strong affinity of boron for oxygen.
            B2H6  + 3O2  → B2O3  + 3H2O + Heat


iii) Boranes react with alkali metal hydrides in diethyl ether to form borohydride complexes.
           B2H6 + 2MH →2M+[BH4]- (M= Li or Na)
               Metal borohydride

(iv) Diborane reacts with ammonia to give borazine at 450
           K.B2H6 + 6NH3 → 3B3N3H6 + 12H2
  • Borazine has a cyclic structure similar to benzene and thus is called inorganic benzene.
  • The other elements of this group form only a few stable hydrides. The thermal stability decreases as we move down the group.
  •  AlH3 is a colourless solid polymerized via Al - H - Al bridging units. These hydrides are weak Lewis acids and readily form adducts with strong Lewis base (B:) to give compounds of the type MH3 (M = Al or Ga). They also form complex-tetrahydrido anions, [MH4]-. The most important tetrahydrido compound is Li[AlH4] 
                        4LiH + AlCl3 ――(ether)―>  LiAlH4  + 3LiCl


OXIDES & HYDROXIDES

M2O3&  M(OH)3

HALIDES: Structure of boron trihalides

Dimeric structure of aluminium chloride
Boron halides do not form dimers because the size of boron is so small that it is unable to coordinate four large-sized halide ions.

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