A triazole is a heterocyclic compound featuring a five-membered ring of two carbon atoms and three nitrogen atoms with molecular formula C2H3N3. Triazoles exhibit substantial isomerism, depending on the positioning of the nitrogen atoms within the ring. Many triazoles are versatile, biologically active compounds commonly used as fungicides and plant retardants. However, triazoles are also useful in bioorthogonal chemistry, because the large number of nitrogen atoms causes triazoles to react similar to azides. Lastly, the many free lone pairs in triazoles make them useful as coordination compounds, although not typically as haptic ligands. Isomerism There are four triazole isomers, which are conventionally divided into two pairs of tautomers. In the 1,2,3-triazoles, the three nitrogen atoms are adjacent; in the 1,2,4-triazoles, an interstitial carbon separates out one nitrogen atom. Each category has two tautomers that differ by which nitrogen has a hydrogen bonded to it. Preparation There are several methods to prepare triazoles. 1,2,3-Triazoles 1,2,3-Triazoles are usually prepared following (3+2) cycloaddition protocols. A common technique for unsubstituted triazoles is the Huisgen azide-alkyne 1,3-dipolar cycloaddition: a azide and an alkyne react at high temperature to form a ring. However, the Huisgen strategy produces a mixture of isomers (typically 1,4- and 1,5-disubstituted) when used to produce substituted triazoles. In order to selectively prepare a desired isomer, metal catalysts are employed. In the copper-catalysed azide-alkyne cycloaddition (CuAAC), copper(I) salts select for the formation of 1,4-disubstituted 1,2,3-triazoles. One such catalyst is CuBr(PPh3)3, which is relatively stable towards oxidation even at elevated temperatures and can produce triazoles with a broad range of substituents either in solvent or under neat reaction conditions. Conversely, ruthenium catalysts (RuAAC) select for 1,5-disubstituted 1,2,3-triazoles. 1,2,4-Triazoles Most techniques for producing 1,2,4-triazoles use the free energy of water, either by dehydrating a mixture of amides and hydrazides (the Pellizzari reaction) or imides and alkyl hydrazines (the Einhorn-Brunner reaction). Of those two, only the Einhorn-Brunner reaction is regioselective. Recent research has focused on grinding and microwave irradiation as greener substitutes. Applications Triazoles are compounds with a vast spectrum of applications, varying from materials (polymers), agricultural chemicals, pharmaceuticals, photoactive chemicals and dyes. Benzotriazole is used in chemical photography as a restrainer and fog suppressant. Cyclohexylethyltriazol was briefly used as an alternative to Cardiazol (Metrazol) in convulsive shock therapy treatment of mental illnesses during the 1940s. Importance in agriculture Many triazoles have antifungal effects: the triazole antifungal drugs include fluconazole, isavuconazole, itraconazole, voriconazole, pramiconazole, ravuconazole, and