Question

How can I classify each species as either Lewis acid or Lewis base for $BeC{l_2},{\text{ O}}{{\text{H}}^ - },{\text{ B(OH}}{{\text{)}}_3}{\text{ and C}}{{\text{N}}^ - }$?

Answer 1

Species having positive charge signifies that the species is electron-deficient and the substance which accepts an electron pair is Lewis acid. Similarly species having negative charge or having lone pairs are electron-rich and they have excess electrons and substances that donate the extra pair of electrons are Lewis bases.

Complete answer:
$C{N^ - }$ is cyanide and this species has a negative charge which signifies the presence of free electrons in the species. So, it will act as a Lewis base.
$O{H^ - }$ have a negative charge which depicts that they can donate the nonbonding electrons. So $O{H^ - }$ is a Lewis base.
In $BeC{l_2}$ the electronic configuration of beryllium is $1{s^2}2{s^2}$ so 2 electrons are present in its valence shell and chlorine has 7 electrons in its outermost shell.
The central element Beryllium has 4 electrons (2 valence shell electrons from its own and two formed from the valence shell of chlorine). $BeC{l_2}$ has an incomplete octet and hence has the ability to accept electron pairs so $BeC{l_2}$ is a Lewis acid.
Boric acid $B{(OH)_3}$ is weak monobasic acid (Boric acid is not capable of releasing ${H^ + }$ions on its own).
$B{(OH)_3}$ acts as a Lewis acid as it accepts a pair of electrons from hydroxyl ions of water and then releases a proton.

Note:
Acid which undergoes ionization to produce hydrogen ions are known as protic acid. Electrophilic nature is generally shown by Lewis acids as they attract electrons and the bases are generally nucleophilic because with their lone pair they attack the positive charge.