I'm new one of chemistry. I want to know the IUPAC nomenclature rules. That is if we write alkane with halogen must be indicate the numerals? Are the following names right or wrong?

  • chloropentane
  • trichloropentane
  • 5
    $\begingroup$ Yes, you must use the numerals to avoid ambiguity. 1-chloropentane is a different molecule with different properties than 3-chloropentane. Similarly 2,2,3-trichloropentane is a different molecule with different properties than 1,1,1-trichloropentane. $\endgroup$
    – Curt F.
    Feb 13, 2016 at 19:45
  • $\begingroup$ Exceptions, where the locants are not included, would be halogenated methane, monohalogenated ethane, hexahalogenated ethane (and all other perhalogenated alkanes). Theoretically also e.g. pentahalogenated ethane would be unambiguous, but the preferred name should contain the locants. $\endgroup$
    – mykhal
    Nov 19, 2018 at 8:55

1 Answer 1


You must use the numbers unless your substance in a mixture of isomers; otherwise your molecules structure is ambiguous. For chloropentane this name alone does not specify weather the chlorine is on the 1,2,or 3 position which is important as the position of a chlorine can dictated physical and chemical properties as well as reactivity in various organic reactions. Same is true for trichloropentane. there are there chlorine but without numbers I dont know if the chlorines are all on one carbon, if they are in the 2,3,4; 1,3,5; 1,2,5; 1,2,3; 2,3,5; 1,1,2; 1,3,3 1,1,4 positions or any of the many more possibilities. all of these molecules give different products in various organic reactions.

Again though if is a mixture of isomers (as is typical of hexane solvents) it is not important to give number


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