Why is phenylamine a weak base?

Jun 1, 2017

Weak bases, with small equilibrium constants for base equilibria (${K}_{b}$), give rise to little ${\text{OH}}^{-}$ after being placed into solution (whether through dissociation, such as the case of $\text{LiOH}$, or through deprotonating water, such as the case of ${\text{NH}}_{3}$).

They tend to have little tendency to accept protons (as Bronsted bases) or to donate an electron pair (as Lewis bases).

Basicity can often be described by ${K}_{b}$, the base dissociation constant. Phenylamine has a $p {K}_{b}$ of about $9.13$, so its $\textcolor{b l u e}{{K}_{b}}$ is about

${10}^{- p {K}_{b}} = {10}^{- 9.13} = \textcolor{b l u e}{7.41 \times {10}^{- 10}}$.

Its association reaction in water is:

${\text{PhNH"_2(aq) + "H"_2"O"(l) rightleftharpoons "PhNH"_3^(+)(aq) + "OH}}^{-} \left(a q\right)$

and its ${K}_{b}$ can be written as:

${K}_{b} = \left(\left[{\text{PhNH"_3^(+)]["OH"^(-)])/(["PhNH}}_{2}\right]\right)$

Even without really having an absolute measure for what ${K}_{b}$ is a good guideline for when a base can be called weak or strong, ${10}^{- 10}$ is very small, so it is a good indication for phenylamine being a weak base.

This is because, again, a small equilibrium constant indicates a significantly greater quantity of reactant (the base itself, ${\text{PhNH}}_{2}$) than the products it makes in water (${\text{PhNH}}_{3}^{+}$ and ${\text{OH}}^{-}$).

Therefore, with little ${\text{OH}}^{-}$ produced in water, it follows that phenylamine is a weak base from that perspective as well.