Question

Why is phenylamine a weak base?

Answer 1

Weak bases, with small equilibrium constants for base equilibria (`K_b`), give rise to little `"OH"^(-)` after being placed into solution (whether through dissociation, such as the case of `"LiOH"`, or through deprotonating water, such as the case of `"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 `pK_b` of about `9.13`, so its `color(blue)(K_b)` is about

`10^(-pK_b) = 10^(-9.13) = color(blue)(7.41 xx 10^(-10))`.

Its association reaction in water is:

`"PhNH"_2(aq) + "H"_2"O"(l) rightleftharpoons "PhNH"_3^(+)(aq) + "OH"^(-)(aq)`

and its `K_b` can be written as:

`K_b = (["PhNH"_3^(+)]["OH"^(-)])/(["PhNH"_2])`

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, `"PhNH"_2`) than the products it makes in water (`"PhNH"_3^(+)` and `"OH"^(-)`).

Therefore, with little `"OH"^(-)` produced in water, it follows that phenylamine is a weak base from that perspective as well.