Chemical and Ionic Equilibrium 2 Question 9
9. Which of the following are Lewis acids?
(2018 Main)
(a) $\mathrm{PH}{3}$ and $\mathrm{BCl}{3}$
(b) $\mathrm{AlCl}{3}$ and $\mathrm{SiCl}{4}$
(c) $\mathrm{PH}{3}$ and $\mathrm{SiCl}{4}$
(d) $\mathrm{BCl}{3}$ and $\mathrm{AlCl}{3}$
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Solution:
- Key Idea Lewis acids are defined as,
“Electron deficient compounds which have the ability to accept atleast one lone pair.”
The compound given are
$\mathbf{P H}_{3}$-Octet complete although $\mathrm{P}$ has vacant $3 d$-orbital but does not have the tendency to accept lone pair in it. Hence, it cannot be considered as Lewis acid.
$\mathbf{B C l}_{3}$-Incomplete octet with following orbital picture.
Hence, vacant $p$-orbital of $\mathrm{B}$ can accept one lone pair thus it can be considered as Lewis acid.
$\mathrm{AlCl}{3}$-Similar condition is visible in $\mathrm{AlCl}{3}$ as well i.e.
$\mathrm{Al}($ Valence orbital only $)=$
Hence this compound can also be considered as Lewis acid. $\mathbf{S i C l}{4}$ - 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 $\mathrm{SiCl}{4}$ is visible in following reaction.
Lone pair acceptance in $d$-orbital
Thus, option (b) and (d) both appear as correct but most suitable answer is (d) as the condition of a proper Lewis acid is more well defined in $\mathrm{BCl}{3}$ and $\mathrm{AlCl}{3}$.
