Over the last years, ionic liquids have been used as solvent or catalyst in organic synthesis. Br?nsted acidic ionic liquids also have been found abrupt improvement as catalyst or dual rule of catalyst and solvent in many reactions. The pyridinium-based ionic liquids have less attention versus other cations. So, the synthesis, characterization and application of 3-carboxypyridinium hydrogensulfate [Hcpy]HSO 4 as a green, efficient and inexpensive pyridinium-based Br?nsted acidic ionic liquid has been reported. The synthesis of benzoxanthenes have attracted considerable interest due to their biological and pharmaceutical properties. 3-Carboxypyridinium hydrogensulfate has been employed as efficient catalyst for one-pot condensation of 2-naphthol and aromatic aldehydes at 120 o C for synthesis of the corresponding 14H-dibenzo[a.j]xanthene. The reaction carried out under solvent-free conditions and the procedure gave the products in good to excellent yields under green conditions. Using less catalyst, straightforward isolation of products only with water and also reusability of catalyst were other advantages of this method. 3,4-Dihydropyrimidin-2(1 H )-ones are one class of multicomponent reactions. One of the most important methods for synthesis of this class of compound is the one-pot condensation of an aldehyde, ?-ketoester, and urea under acidic conditions, first reported by Biginelli. 3-Carboxypyridinium hydrogensulfate has been used as acidic catalyst for synthesis of these compounds at 120 o C under solvent-free conditions. The procedure gave the products in very short time with excellent yields. Aryl azides are versatile intermediates that due to azide group have been found wide application in organic synthesis since they can be used for preparation of a variety of heterocycles. The general procedure for preparation of aryl azides is using aromatic amines and nitrous acid in two steps: diazotization of the amine and subsequent treatment with sodium azide. The reaction must carry out at low temperature (0 o C) due to instability of diazonium salt in aques media. In continuation of our effort to study of application of [Hcpy] HSO 4 acidic ionic liquid, it was employed as acid source for synthesis of aryl azides from corresponding amines under grinding condition. The reaction carried out under green conditions at room temperature with short reaction times and the products obtained with excellent yields. The products were isolated and purified only with water without using any organic solvent. Furthermore, not only the producing diazonium salts are stable at producing time, but also can be restored for long time at room temperature. In continuation, this procedure was used for employing in Sandmeyer reaction. iodoaromatic compounds are the most useful synthetic building blocks in organic synthesis.