Question

Why is $BC{l_3}$ a Lewis acid?

Answer 1

Classification of acid and base according to the Lewis theory is based on electron transfer instead of proton transfer which states that a species which accepts an electron pair i.e., an electrophile is considered as a Lewis acid while a species which donates an electron pair i.e., a nucleophile is known as a Lewis base.

Complete answer:
We know that the Lewis acids have a tendency to accept electron pairs and are considered as electrophilic which means that these species are electron deficient and attract electrons to form bonds. Various species which can act as a Lewis acid are as follows:
1. All cations are considered as the Lewis acids because they are electron deficient and have a tendency to accept electrons. Example: $C{u^{2 + }}$, $F{e^{2 + }}$, etc.
2. An atom, ion or molecule with an incomplete octet of electrons acts as a Lewis acid. Example: $B{F_3}$.
3. Molecules in which the central atom has the tendency to have more than eight electrons are also electron acceptors and thus, are considered as the Lewis acids. Example: $SiB{r_4}$, $Si{F_4}$.
4. Molecules which have multiple bonds between the two atoms of different electronegativities are also considered as the Lewis acids. Example: $C{O_2}$, $S{O_2}$.
Now in the case of $BC{l_3}$, the electronic configuration of boron is $\left[ {He} \right]2{s^2}2{p^1}$ and after forming bond with three chlorine atoms, it exists in its trivalent state i.e., has only six electrons around it. Thus due to an incomplete octet of electrons, it is electron deficient and has a tendency to accept electrons in order to complete its octet. Hence, $BC{l_3}$ molecule acts as a Lewis acid.

Note:
It is important to note that $BC{l_3}$ is stronger Lewis acid than $B{F_3}$ because the boron atom in $B{F_3}$ molecule participates in $2p\pi - 2p\pi$ back bonding with fluorine atoms with a greater overlap and thus, reduces the electron deficiency on boron atom.