Up to now, a few synthetic routs have been demonstrated for the construction of aryl azides. The older rout is based on the diazotization of aromatic amines and subsequent reaction with azide ions. Controlling reaction condition is very difficult when two or more amino group exist as substituents in aromatic rings. An efficient approach to azidation of aryl halides is described here. Good yields of aryl azides were obtained with CuI/EDTA as a catalytic system. Cost-affectivity of the EDTA compared to expensive DMEDA (1:500) along with the use of 7EtOH:3H 2 O as mixed solvent and green medium makes it to be a suitable method for selective synthesis of aryl azides. In the second part of this project, a one pot Cu(II)-promoted coupling reaction of electron-deficient and electron-rich halobenzenes with sodium azide were studied in details. The reduction of the intermediate aryl azides under the same Cu(II)-ascorbate redox conditions leading to anilines has been documented. Different experiments showed that both sodium ascorbate and EDTA play important roles in the reaction pathway. The result demonstrated that when electron-deficient haloarenes were used, the obtained products were usually aryl amines and in the case of electron-rich halobenzenes only aryl azide were produced with lower yields when it was compared with the aryl amines. In the third part of this project, The electronic structures and intramolecular proton transfers (PTs) in the biurea isomers have been investigated at B3LYP /6-311++G(d,p) level of theory. The results showed that biurea is not planar and the diketo is the most stable isomer. Furthermore, in the next step, conversion of most stable isomers to each other was also investigated. The results indicated that intramolecular proton transfer step is the rate-determining step in all paths. Study of the relative stabilities and mechanism of conversion of isomers have been performed with polarizable continuum model (PCM). The heights of barrier for PT mechanism assisted by one water molecule was also studied and calculated results showed that the water molecule assisting PT has much lower energy barrier than analogous barrier in the gas phase. A detailed study on tautomerism of allophanic acid was performed using MP2/6-311++G(d,p) level of theory. Kinetic, thermodynamic, structural and also spectral data of the five most stable tautomers of allophanic acid were calculated. The results showed that the tautomer 1 involving hydrogen in the central nitroge and in a trans-conformation of amino group and carbonyl group was more stable than the other possible tautomers at this level of theory. Then, interconversion of most stable tautomers to each other was surveyed using internal rotation and proton transfer (PT) routes through pathway A - E . Energetic data were used to calculate the energy barriers of tautomeric interconversions and by this way, higher value of energy barriers obtained for PT steps. Moreover, solvent effects were explored for all of the paths A - E . Results showed that tautomerism could be facilitates in the presence of solvents especially in water and also in acidic media.