Amines with nitrogen directly attached to benzene are less basic due to resonance. Benzylamine is more basic because \(-NH_2\) is not directly attached to the ring.
Step 1: Basis of basic strength in amines. Basicity depends on the availability of the lone pair of nitrogen for donation to a proton. Lone pair involvement in resonance with an aromatic ring greatly reduces basicity. Step 2: Why aniline-type (aromatic) amines are weak bases. In aniline, the lone pair of $-NH_2$ is conjugated with the benzene ring through resonance. This delocalisation reduces electron density on N, making aniline a much weaker base than aliphatic amines ($pK_b$ of aniline $\approx 9.4$ versus $\approx 3$ for aliphatic amines). Step 3: Rank the three aniline-type compounds. $p$-Nitroaniline: $-NO_2$ is a powerful electron-withdrawing group that further withdraws the lone pair from N through resonance, making it the weakest base among the three. Aniline: standard reference basicity. $p$-Toluidine: $-CH_3$ is an electron-donating group (by induction), slightly increasing electron density on N, so slightly more basic than aniline. Order: $p$-nitroaniline < aniline < $p$-toluidine (all very weak bases). Step 4: Consider benzylamine separately. Benzylamine ($C_6H_5CH_2NH_2$) is an aliphatic amine. The $-NH_2$ group is on a $CH_2$ group, NOT directly attached to the benzene ring. Therefore, the lone pair on N is NOT involved in resonance with the ring and is fully available for protonation. Step 5: Compare benzylamine with aniline-type compounds. Benzylamine behaves like a typical aliphatic amine. Its $pK_b \approx 4.7$, while all aniline-type compounds have $pK_b$ values from 9 to 13. A lower $pK_b$ means stronger base. So benzylamine is by far the strongest base among the four. Step 6: State the final answer. Benzylamine is the strongest base because its nitrogen lone pair is not involved in aromatic ring resonance, making it fully available for protonation. \[ \boxed{\text{Benzylamine}} \]
