Our chemistry teacher taught us that hexyl (five letters) is preferred to heptyl (six letters) as hexyl has lesser number of letters. After searching many sources I could not find any such rule.

Which one would be preferred while writing IUPAC names?

For example, will it be 8-heptyl-10-hexylheptadecane or 10-heptyl-8-hexylheptadecane?

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    $\begingroup$ Hexyl comes from the Greek name "hexa* which means "six". "Heptyl" comes from the Greek name "hepta" which means seven. It has nothing to do with the number of letters. The next radical, after "heptyl", is "octyl", coming from the Greek, since "octa" means "eight" in Greek. $\endgroup$ – Maurice Aug 13 at 14:27
  • $\begingroup$ @Maurice Nit picking, but I believe it is "octo." $\endgroup$ – Zhe Aug 13 at 17:05
  • $\begingroup$ @ Zhe. OK. You may be right. It does not change the reasoning. $\endgroup$ – Maurice Aug 13 at 17:08
  • $\begingroup$ Please read the example I gave,I wanted to ask-between hexyl and heptyl which is given more preference according to the Alphanumeric order of IUPAC. $\endgroup$ – James Aug 13 at 17:19
  • $\begingroup$ @James I think you would like to review your examples. $\endgroup$ – Habib Aug 13 at 17:39

According to the IUPAC nomenclature, preference for placing prefixes is given in accordance with alphabetical order (just like in a standard English dictionary). It has nothing to do with the number of letters in a prefix. Since "P" in heptyl comes before "X" in hexyl, heptyl will be placed before hexyl in IUPAC nomenclature.

Hence, the correct name will be the one where heptyl is placed before hexyl (8-heptyl-10-hexylheptadecane).

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    $\begingroup$ Every time you back up your answer referring to "IUPAC nomenclature", you also need to provide a citation. This is not only considered a good practice in general, but also makes your post more future-proof since IUPAC rules are often a subject to change. $\endgroup$ – andselisk Aug 13 at 18:52
  • $\begingroup$ Is there any such rule in IUPAC of counting the number of letters to decide priority between two substituents? $\endgroup$ – James Aug 13 at 19:00
  • $\begingroup$ As far as I know, there isn't, at least not for determining the order of prefixes in an organic compound. $\endgroup$ – Habib Aug 13 at 19:06

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