Final answer:
Phenylamine is a weaker base compared to ammonia due to the electron-withdrawing effect of the phenyl group, which decreases the electron density on the nitrogen atom making it less likely to accept a proton.
Step-by-step explanation:
Phenylamine, also known as aniline, is a base of different strength compared to ammonia because of the influence of the phenyl group (C6H5-) attached to the nitrogen atom in phenylamine. This aromatic ring has a delocalized electron system that exerts an electron-withdrawing effect through resonance, which decreases the availability of the nitrogen's lone pair of electrons to accept a proton. Hence, phenylamine is less basic than ammonia. In contrast, ammonia, which is a weak base, has its lone pair readily available to bond with a proton forming ammonium ions and hydroxide ions in water.
Substitution effects are key to understanding base strength. While ammonia is a simple molecule with no substituents to affect its basicity, phenylamine's structure leads to a decreased tendency to accept protons. Amines like phenylamine and ammonia form substituted ammonium ions upon reaction with water, but the differing electronic environments make phenylamine a weaker base.