Intermolecular interaction may result in high melting point whereas intramolecular interaction may result in higher melting point.
Phthallic acid having two carboxyl group attached adjacent to each other which helps in formation of strong intramolecular interaction between them. Apart of intramolecular h bond interaction, phthallic acid also forms H bond with polar solvent as well due to which strong interaction present between the molecules. This strong interaction rises the melting point quite higher than naphthalene.
Though naphthalene having symmetric structure but it melting point is 176.5 F which is lower than phthallic acid with 405 F.
