p-Block Elements-I - Result Question 27

####27. Compound X on reduction with LiAlH4 gives a hydride Y containing 21.72 hydrogen alongwith other products. The compound Y reacts with air explosively resulting in boron trioxide. Identify X and Y. Give balanced reactions involved in the formation of Y and its reaction with air.

Draw the structure of Y.

(2001, 5M)

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Solution:

  1. Compound XLiAlH4Y a hydride + other compound. Hydride Y contains 21.72 hydrogen.

Y+O2ΔB2O3+H2O

Therefore, Y is a hydride of boron and it is obtained by reduction of X with LiAlH4. So, X is either BCl3 or BF3.

4BCl3X+LiAlH4B2H6+3AlCl3+3LiClOther products 

Molar mass of B2H6=2×11+6=28

B2H6+3O2B2O3+3H2O+ Heat  Y

Structure of Y(B2H6)

(a) There are 4 terminal BH bonds.

(b) There are two 3-centre-2-electron BHB bridged bonds.

(c) Terminal HBH planes are perpendicular to bridged BHB bonds.



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