Question

Which one of the following is a Lewis acid?
A.${{\text{N}}_{\text{2}}}{{\text{H}}_{\text{4}}}$
B.${\text{R - OH}}$
C.${\text{R - O - R}}$
D.${\text{AlC}}{{\text{l}}_{\text{3}}}$

Answer 1

The basic thing we need to know here is what a Lewis acid is. Lewis acid is a substance that can accept a pair of an electron to form a covalent bond. Thus it needs to have an empty outer orbital so that the electron can be accepted. Lewis base is also taken into consideration here.

Complete step by step answer:
First, let us discuss what a Lewis acid is. It is a substance that accepts a pair of electrons to complete its octet. We know what is meant by octet.
It accepts electrons so we can say that Lewis acid is an electron-deficient species.
Let us look at Option (A), ${\text{R - O - H}}$ in which each N is attached to 1 N and 2 H. But as we know that the valence electrons of N are 5. So there is a lone pair of electrons on both the N atoms.
Therefore, we can say that it is a Lewis base.
Let us look at Option (B) ${\text{R - OH}}$.As we know O has 6 valence electrons.
Here, we can see that 2 of the electrons are shared with R and H. Therefore, there are 4 electrons on O as 2 lone pairs. So they are electron-rich. Hence, not a Lewis acid.
Now, Option (C) ${\text{R - O - R}}$. It is the same as option B. The 2 electrons from the 6 valence electrons are shared. Therefore, it is a base. It is an electron-rich molecule
Option (D) is ${\text{AlC}}{{\text{l}}_{\text{3}}}$. We know that the valence electrons on the 3 electrons are shared and there is a vacant orbit. Therefore, it can accept electrons. So, it is a Lewis acid.
Thus, the correct answer is ${\text{AlC}}{{\text{l}}_{\text{3}}}$
The correct option is (D).

Note: The Lewis acid is also called an electrophile, which means it is electron loving. It can accept electrons. Lewis base is a substance that can donate its lone pair of electrons which means that it is electron-rich. Thus in this question, all other options except D is a Lewis base.