Question

Select the option showing how to prepare (BN)_x from B₂H₆.
1. \(\rm B_2 H_6 \ + \ HNO_2 \ \xrightarrow{ \ \ h\nu \ \ }\)
2. \(\rm B_2 H_6 \ + \ Excess \ NH_3 \ \xrightarrow{ \ \ \Delta \ \ }\)
3. \(\rm B_2 H_6 \ + NH_3 \ \xrightarrow{ \ \ \Delta \ \ }\)
4. \(\rm B_2H_6 \ + \ HNO_2 \ \xrightarrow{ \ \ \Delta \ \ }\)

Answer 1

Correct Answer - Option 2 : \(\rm B_2 H_6 \ + \ Excess \ NH_3 \ \xrightarrow{ \ \ \Delta \ \ }\)

Concept:

• Boranes are the analogous series of hydrides of boron.
• About 25 neutral boranes and a large number of borane anions are known at present. The neutral boranes mostly belong to either of the two following general classes:
  • Nido boranes: B2H6, B5H9
  • Arachno boranes: B4H10, B5H11
  • Nido boranes: B8H12, B10H14B6H10 
• Boranes are prepared via the following reaction:

B2H6 (180 - 220)0C → B5H9 + B10H14

• Electrophilic substitution reactions of borane are also known. B5H9 undergoes Friedel-Crafts alkylation to give 1-alkyl derivatives.
• Electrophilic substitution by halogens is also known.

Explanation:

Aminoboranes:

• Aminoboranes contain B-N bonds in which the boron is trigonally hybridized.
• The lone pair on nitrogen may be donated to a vacant p orbital on boron, giving rise to B-N bond order greater than one.
• Aminoboranes are prototypes of alkenes.
• Aminoboranes can be prepared from amine and boranes.
• The reaction of diborane with ammonia gives borazines. the reaction is:

3B2H6 + 2NH₃ →​ 2B₃N₃H₆+ 12H₂

• When reacted with excess NH₃, the reaction is:

B2H6 + Excess NH3 →​ (BN)x + H₂

• The reaction takes place at high temperatures with the evolution of hydrogen.

Hence, we can prepare (BN)x from B2H6 by the reaction B2H6 + Excess NH3 →​ (BN)x