Question

Explain the nature of boric acid as a Lewis acid in water.

Answer 1

Boric acid is a weak monobasic acid.

It is not a protonic acid or Arrhenius acid that produces $H^{+}$ ion in water.

Instead, it acts as a Lewis acid by accepting electrons from hydroxyl ion $\left({OH}^{-}\right)$ ion of water as boron has one empty $2p$ orbital to accept electrons and complete its octet.

$B(OH{)}_3+2H_{2}O\to [B(OH{)}_4{]}^{-}+H_3{O}^{+}$

Boric acid has $3OH$ groups, but none of them ionises in water to produce $H^{+}$ ion.

Hence, it is not a protonic acid.