Here's why:

* Structure: 1-Naphthylamine has a large aromatic ring system (naphthalene) and a primary amine group. The aromatic ring is nonpolar, while the amine group is polar.

* Polarity: Water is a highly polar solvent. While the amine group can form hydrogen bonds with water, the large nonpolar aromatic ring dominates the molecule's overall character.

* Solubility Rule: "Like dissolves like" - nonpolar substances are generally soluble in nonpolar solvents, and polar substances are generally soluble in polar solvents.

Therefore, due to the large nonpolar portion of the molecule, 1-naphthylamine has limited solubility in water.