Solid-state reactions are commonly observed in organic crystals, including pharmaceutical and agricultural materials, fine chemicals, dyes, explosives, optics, and many other substances. The fact that these reactions are in general highly anisotropic with regard to the initiation and propagation in a crystal has led to this study for investigating the effect of crystal packing on the reaction mechanism and kinetics of organic crystals. We have used electron density-based concepts, including nuclear Fukui function, developed from density functional theory, for elucidating the effect of electronic structures of different polymorphs on the difference in their chemical reactivity. Two polymorphs of flufenamic acid were studied. The calculation results on major reacting faces of the two forms support their reactivity difference with ammonia gas. In addition, we calculated surface energies of reacting faces to discuss how the mechanical difference may affect the propagation of solid-state reaction.