Why is phenylamine a weak base?

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.