Question

Which of the following molecule can act as lewis acid as well as lewis base?

• A. ${CO}_2$
• B. ${SO}_2$
• C. ${SO}_3$
• D. ${NH}_3$

Answer 1

The correct option is B ${SO}_2$

The compounds which have a lone pair of electrons which they can donate and at the same time have vacant d orbitals which can accept electrons can act as both Lewis acid as well as Lewis base.

In $S{O}_2$, molecule sulphur has 16 electrons

The electronic configuration is- $2,8,6$

It means S has 6 electrons in its valence shell

In $S{O}_2$, S forms a double bond with each O, i.e., $O=S=O$ even though 2 electrons (one lone pair) present over that can be donated hence it acts as Lewis Base.

But, when $S{O}_2$ interacts with $H_{2}O$, it accepts a pair of electrons to form $H_{2}S{O}_3$ (sulphurous acid).

Hence, $S{O}_2$ behaves as both, i.e., Lewis Acid as well as Lewis Base.