1.3.5- Triazine nomenclature/numbering

According to the triazine nomenclature, 5-azauracil is 2,4-dioxo-l,2,3,4-tetrahydro-l,3,5-triazine (2). The subject index of Chemical Abstracts prefers s-triazine-2,4(lH,3H)-dione. Furthermore, some authors use a name derived from the lactim structure, 2,4-dihydroxy-s-triazine (3). The numbering of the substituents is the same for all these types of nomenclature. 

In all types of nomenclature based on triazine the numbering of the substituents is shifted by one as compared with the nomenclature of 6-aza analogs of pyrimidines. 

The 1,3,5-triazines are amongst the oldest known organic molecules. Originally they were called the symmetric triazines, usually abbreviated to s- or sym- triazines. The numbering follows the usual convention of beginning at the heteroatom as shown for the parent compound 1,3,5-triazine (1). Rather non-systematic nomenclature is prevalent even in the current literature, because some of the compounds have been known for at least 150 years. The non-systematic names of some of the more important 1,3,5-triazines are listed in Table 1. The terms melamine, cyanuric acid and cyan uric chloride will be used throughout this chapter, and the term triazine will refer to 1,3,5-triazines only. In addition to the above names, 2,4,6-trialkoxy-l,3,5-triazines (2) are called cyanurates. Similarly, 1,3,5-trialkyl-1,3,5-triazines (3) are called isocyanurates. 

The systematic nomenclature used originally the term imidazo-1,2,3-triazine. The Chemical Abstracts indexes use the more accurate name imidazo[4,5-d]-Z -triazine (141). The numbering of the substituents is different in the two systems of nomenclature as may be seen in the formulas. 

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