I was practicing writing nomenclatures for several organic compounds with the help of Blue Book Essentials and I have some queries regarding the following structure: enter image description here

According to the blue book, the numbering of parent chain is as follows:

  1. Lowest locants for heteroatoms
  2. Lowest locant(s) for indicated hydrogen
  3. Lowest locant(s) for principal characteristic group(s)
  4. Lowest locants for ‘ene’, ‘yne’, and hydro prefixes
  5. Lowest locants as a set for all substituents cited by prefixes
  6. Lowest locants for substituents in the order of citation.

According to this order, the carbon bearing the methyl group should get the lesser number, hence '3'. So the name should be 3-methylhepta-1,4,6-triene?

Please clarify this for me. Thanks!

  • $\begingroup$ Since CH3 isn't a principal characteristic group, I would start with the 4th rule, but to infer the side i should start from, I would first number both ways, and add up position numbers of carbon bearing the functional group/bond, and the side which gives me the highest will be the order of numbering right? $\endgroup$
    – Akilan SS
    Commented Mar 13 at 5:25

1 Answer 1


Numbering is chosen so as to give the unsaturated sites (the highest priority C atoms) the lowest numbering. It can be a 1,3,6 triene, or a 1,4,6 triene. The first one is the preferred numbering, making the methyl substituent on the 5 atom.

  • $\begingroup$ Thanks for clearing it up! I have another query, should I consider every other group not mentioned principal characteristic group(as per Blue Book), in terms of alphabetical order? For example, does isopropyl group get higher priority than the methyl group? $\endgroup$
    – Akilan SS
    Commented Mar 13 at 5:31
  • $\begingroup$ I think they're equivalent (they give the same molecules) so I forget what the IUPAC convention is there. $\endgroup$
    – Ryan
    Commented Mar 13 at 5:37
  • 1
    $\begingroup$ See this answer? chemistry.stackexchange.com/questions/90644/… $\endgroup$
    – Ryan
    Commented Mar 14 at 1:53

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