Discussion Forum



1)

Which of the following are Lewis acids?


A) $PH_{3}$ and $BCl_{3}$

B) $AlCl_{3}$ and $SiCl_{4}$

C) $BCl_{3}$ and $AlCl_{3}$

D) $PH_{3}$ and $SiCl_{4}$

Answer:

Option C

Explanation:

Key Idea Lewis acids are defined as, "Electron deficient compounds which have the ability to accept at least one lone pair,"

The compound given are

PH3- Octet complete although P has vacant 3d-orbital but does not have the tendency to accept lone pair in it. Hence, it cannot be considered as Lewis acid.

BCl3- In complete octet with following orbital picture.

482019700_orbit12.jpg

Hence, vacant p-orbital of B can accept one lone pair thus it can be considered as Lewis acid.

AlCl3 - Similar condition is visible in AlCl3 as well i.e.

Al( Valance orbital only)=

482019559_3d.JPG

Hence this compound can also be considered as Lewis acid.

SiCl4  - Although this compound does not have incomplete octet but it shows the tendency to accept lone pair of electrons in its vacant d- orbital. This tendency of SiCl4 is visible in the following reaction.

 482019849_sicl.JPG

Thus option (b) and (c) both appear as correct but most suitable answers is (c) as a condition of a proper Lewis acid is more well defined in BCl3  and  AlCl3.