Question

Explain why the following compounds behave as Lewis acid?

A) $BC{l}_3$

B) $AlC{l}_3$

Answer 1

a) Lewis acids are the species which accept a lone pair of electrons from other species.

Generally, they are electron deficient species.

In $BC{l}_3$ B forms three covalent bonds with 3 chlorine atoms by using its 3 valence electrons in the outermost shell.

B(ground state) $\to 1s^{2}2s^{2}2p^1$

B($1^{st}$ excited state)$\to 1s^{2}2s^{1}2p^2$





B has $3$ unpaired electrons in its outermost shell and forms $3$ covalent bonds. Due to this it has only $6$ valence electrons in its outermost shell

Hence, to achieve stability, it accepts lone pair of electrons from electron rich species and act as a Lewis acid.


b) Lewis acids are the species which accept a lone pair of electrons from other species.

Generally, they are electron deficient species.

𝐼𝑛 $AlC{l}_3,Al$ forms three covalent bonds with three $Cl$ atoms by using its $3$ valence electrons.

$Al$ (ground state) $\to 1s^{2}2s^{2}2p^{6}3s^{2}3p^1$

$Al$ (excited state) $\to 1s^{2}2s^{2}2p^{6}3s^{1}3p^2$



$Al$ also has $3$ unpaired electrons in its outermost shell and forms $3$ covalent bonds. Due to this it has only $6$ valence electrons in its outermost shell.

Hence, to achieve stability, it can accept lone pair of electrons from electron rich species.

So, $AlC{l}_3$ is a lewis acid.