Question

In the reaction $B{{F}_{3}}+N{{H}_{3}}\ to [B(N{{H}_{3}}){{F}_{3}}]$.
(A)- Bond angle F-B-F increase.
(B)- HNH bond angle decrease.
(C)- Regular geometry of $B{{F}_{3}}$ changed.
(D)- Regular geometry of $N{{H}_{3}}$ changed.

Answer 1

Hint: Boron trifluoride electron deficient molecule with B-X bond length as $1.30A{}^\circ $ which is shorter than usual and expected single bond length and this shortness may indicate B-X pi-bonding in the fluoride. The F-B-F bond angle is $120{}^\circ $. Ammonia molecule has a trigonal pyramidal geometry with the bond angle of $106.4{}^\circ $. The bond length of N-H is $1.008A{}^\circ $.

Complete Step by step answer:
-Boron trifluoride which is an inorganic compound with pungent, colourless and toxic gas is a useful Lewis acid. It is a versatile building block for other boron compounds. The geometry of boron trifluoride is a trigonal planar with no dipole moment.
-Ammonia is a stable binary hydride and is the simplest pnictogen hydride. Ammonia is a colourless gas with a pungent smell and is a common nitrogenous waste.
-When boron trifluoride is reacted with ammonia, ammonia loses an electron pair and the boron trifluoride gains an electron pair due to which a dipolar bonding takes place between them. This reaction takes place at a temperature lower than $0{}^\circ C$ and form trifluroamine boron (III) adduct.
-In $B{{F}_{3}}$, there are three bond pairs and 0 lone pairs, so the hybridization is $s{{p}^{2}}$. In $N{{H}_{3}}$, there are 3 bond pairs and 1 lone pair, so the hybridization is $s{{p}^{3}}$. As the transformation takes place, the hybridization of boron changes from $s{{p}^{2}}$ to $s{{p}^{3}}$, but the hybridization of nitrogen remains the same because its lone pairs are shared with electron-deficient boron atom. Due to change in the hybridization of boron, its geometry changes from trigonal planar to tetrahedral geometry.

So, the correct answer is option C.

Note: Boron trifluoride is a very important reagent used as a Lewis acid in many organic synthesis reactions. It initiates polymerization reactions of unsaturated compounds such as polyether. It acts as a catalyst in some isomerization, acylation, alkylation, esterification, dehydration, condensation and many other important reactions.