Question

What happened when:
$B{{F}_{3}}$ is reacted with ammonia?

Answer 1

The $B{{F}_{3}}$ is a Lewis acid i.e. an electron deficient species and $N{{H}_{3}}$ is a Lewis base, a species which consists of lone pairs that can be donated for the formation of bonds.
- $N{{H}_{3}}$ molecule consists of a lone pair of electrons .

Complete Solution :
So here in the question we are asked to predict what happens when $N{{H}_{3}}$ and $B{{F}_{3}}$ reacts together.
Before predicting the product let’s see some characteristics of ammonia and boron trifluoride.
So in $B{{F}_{3}}$ molecule B is the central atom and we know that the$B{{F}_{3}}$ is a Lewis acid i.e. an electron deficient species.
- The atomic number of B is 5 and has an electronic configuration:
$\text{E}\text{.C}\,\text{of}\,\text{B=1}{{\text{s}}^{\text{2}}}\text{2}{{\text{s}}^{\text{2}}}\text{2}{{\text{p}}^{\text{1}}}$
- The valency possessed by the B atom is +3.
- The hybridization of $B{{F}_{3}}$ molecule is $s{{p}^{2}}$ and hence having a planar structure.
- In $N{{H}_{3}}$ molecule, N is the central atom having the atomic number 7 and the electronic configuration is,
$\text{E}\text{.C}\,\text{of}\,\text{N=1}{{\text{s}}^{\text{2}}}\text{2}{{\text{s}}^{\text{2}}}\text{2}{{\text{p}}^{\text{3}}}$
Ammonia is a $s{{p}^{3}}$ hybridized molecule in which one lone pair is present and hence having a trigonal pyramidal structure.
So as we discussed earlier $B{{F}_{3}}$ is a Lewis acid and requires two electrons for B to obtain the octet configuration and $N{{H}_{3}}$ consists of one lone pair of electron and it acts as the Lewis base according to Lewis theory.
Hence the $N{{H}_{3}}$ molecule donates its lone pair of electrons to the $B{{F}_{3}}$ molecule and forms a coordinate bond.
So the ammonia and boron trifluoride on reactions forms an adduct.

$N{{H}_{3}}$ The hybridization of this adduct formed is $s{{p}^{3}}$ hybridized adduct.

Note: The hybridization of a molecule is calculate using the formulae,
$\text{steric number = No}\text{. of bps + No}\text{.of lps}$
Bps refers to bond pairs and lps refers to lone pairs and the steric number obtained will give an idea about the hybridization of the molecule.