melamine is a tri-azine with 3 amine groups how would you name it,
as 1,3,5-triamino-2,4,6-triazine
or
2,4,6-triamino-1,3,5-triazine ?
please explain the reason too if possible
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1 Answer
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According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), cyclic parent structures containing one or more heteroatoms with no more than ten ring members are named by using the extended Hantzsch-Widman system. The locant ‘1’ is always is given to a heteroatom. The corresponding rule for cycles with more than one of the same heteroatom reads as follows.
P-22.2.2.1.2 A multiplicity of the same heteroatom is indicated by a multiplying prefix ‘di’, ‘tri’, ‘tetra’, etc., placed before the appropriate ‘a’ term. The final letter ‘a’ of a multiplying prefix is elided before a vowel, e.g., tetrazole, not tetraazole. Lowest possible locants are assigned to heteroatoms, locant ‘1’ being assigned to one of the heteroatoms. Locants are cited at the front of the name, i.e., before the skeletal replacement (‘a’) term and any preceding numerical prefixes.
Therefore, the systematic name of the parent structure in melamine is 1,3,5-triazine.
This numbering is also used in substituted heteromonocyclic compounds. A heteroatom keeps the locant ‘1’; then low locants are given to any substituents.
P-14.4 NUMBERING
When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:
(a) fixed numbering in chains, rings, or ring systems, i.e., when the numbering of a system is fixed, for example in purine, anthracene, and phenanthrene, this numbering must be used, both in PINs and in general nomenclature;
(…)
(c) principal characteristic groups and free valences (suffixes);
(…)
Therefore, the nitrogen atoms of the heterocycle in melamine have the lowest locant set ‘1,3,5’ according to (a), and then the lowest possible locant set ‘2,4,6’ is given to the amine groups according to (c). Thus, the preferred IUPAC name is 1,3,5-triazine-2,4,6-triamine.
