Two forms of (naphthalen-1-yl)boronic acid.

Two polymorphs of the title compound, C10H9BO2, were prepared by recystallization from different solvents at room temperature. Both forms demonstrate nearly identical mol-ecular structures with all naphthalene group atoms located in one plane and all boronic acid atoms in another: the dihedral angles between these planes are 39.88 (5) and 40.15 (5)° for the two asymmetric mol-ecules of the ortho-rhom-bic form and 40.60 (3)° for the single asymmetric mol-ecule in the monoclinic form. In each extended structure, mol-ecules form dimers, connected via two O-H⋯O hydrogen bonds. The dimers are connected by further O-H⋯O hydrogen bonds, forming layered networks in the (001) plane and the (100) plane in the ortho-rhom-bic and monoclinic forms, respectively. The resulting layers are practically identical in both forms. However, these layers are shifted along the [010] axis in the two forms, resulting in a slightly more effective packing for monoclinic structure (packing index = 0.692) compared to the ortho-rhom-bic form (0.688).