Among the following nitrogenous compounds, which one is the most difficult to protonate?

Step-by-step solution

The ease of protonation of a compound depends on the availability of the lone pair of electrons on the basic atom (nitrogen, in this case). In methylamine and dimethylamine, the alkyl (methyl) groups exert an electron-donating inductive effect (+I effect), which increases the electron density on the nitrogen atom. This makes their lone pairs more available for protonation compared to ammonia. In aniline, the nitrogen atom is directly attached to a benzene ring. The lone pair of electrons on the nitrogen atom is del