IJRPC 2015, 5(1), 41-76 Jawad K Shneine et al. ISSN: 22312781 41 INTERNATIONAL JOURNAL OF RESEARCH IN PHARMACY AND CHEMISTRY Available online at www.ijrpc.com CHEMISTRY OF FORMAZAN Yusra H Al-Araji, Jawad K Shneine*, and Ahmed A Ahmed Department of Chemistry, College of science, Al-Nahrain University, Baghdad, Iraq. 1. INTRODUCTION The history of the tetrazolium salts and formazans goes back more than one hundred years, when Friese (1875) reacted benzene diazonium nitrate with nitro methane, to produce a cherry-red "Neue Verbindung". This was the first formazan(Altman, 1976). Nineteen years later, Von Pechmann and Runge oxidized a formazan to produce the first tetrazolium salt. Many hundreds of tetrazolium salts and formazans were prepared in the following years(Pechmann and Runge, 1894). Formazans are compounds which contain the characteristic chain of atoms (N=N–C=N–NH), formazans arecoulored compounds ranging from red to orange or blue depending upon their structures. Most reports on formazans originate in the early 1900s, preceding modern characterization techniques. As a result, there is relatively little known about their properties. However, the common feature of all formazans is their intense colour, which allowed chemists to identify them over 100 years ago(Gilroy, 2008). Redox behaviors of formazans were evaluated in detail. Formazans form tetrazolium salt when they are oxidized. Tetrazolium salts are reduced back to formazans by the enzymes in the cell and stain the tissue. Tetrazolium–formazan system is classified as a marker of vitality. This feature enabled thedetermination of activity on tumor cell, which caused an increasing interest in the chemistry, and especially electrochemistry of formazans(Tezcan et al., 2008). Formazans have been studied extensively because of their ready accessibility, diverse chemical reactivity, broad spectrum of biological activities and wide range of applications(Chavanet al., 2012, Anandet al., 2009, Lipounovaet al., 2004). They are widely used as dyes, as ligands in complex formation reactions, and as analytical reagents, where their deep colour makes them good indicators of redox reactions. Formazans form salts and complexes with several metal ions, and especially the transition metal ions. Their derivatives with electron donating and withdrawing group attached to 1,3,5-phenyl ring were synthesized and the effects of substituents on the absorption λmax values were examined. They are also biologically active and there is much interest in their biological applications. The biological activity of formazan makes the knowledge of its oxidation potentials and possible mechanisms very important(Tunҫa and Yıldırım, 2010). Research Article ABST RACT Formazans was first synthesized over a century ago, but still attract attention of chemists, biologists, technologists and other specialists. In recent years, antiviral, anti-inflammatory, anti-fertility, anti- tubercular activity, antimicrobial activities, anti-cancer and anti-corrosion properties of formazans have been published. This review aims to describe the structures, synthesis, reactions and spectral properties of formazans for highlighting the future applications in several bioactive phenomena and analytical uses. Keywords: Formazan, Tetrazolium salt, Coupling reaction, Phase transfer catalyst.