# For IUPAC nomenclature, does the number of multiple bonds take precedence over the chain length?

I've received a few mixed answers to this question. Some sources say when determining the parent chain for unsaturated hydrocarbons, the longest chain should be considered before the chain with the most multiple bonds, for example:

https://www.acdlabs.com/iupac/nomenclature/79/r79_933.htm

While others say what appears to be the opposite, such as this excerpt from Wikipedia:

The steps for naming an organic compound are:
Identification of the parent hydrocarbon chain. This chain must obey the following rules, in order of precedence:

1. It should have the maximum number of substituents or branches cited as prefixes.
2. It should have the maximum number of substituents of the suffix functional group. By suffix, it is meant that the parent functional group should have a suffix, unlike halogen substituents. If more than one functional group is present, the one with highest precedence should be used.
3. It should have the maximum length.
4. It should have the maximum number of multiple bonds.
5. It should have the maximum number of single bonds.

Would be great if I were to receive clarification regarding this.

• – user7951 Aug 29 '20 at 22:49
• In the 21st century, it is no longer important to create names that strictly follow IUPAC nomenclature. That made sense in the old days, when databases were printed on paper and came with thick volumes containing indexes. In the age of computer networks, unique human-readable names are largely irrelevant. Also chemists have always disregarded IUPAC in publications when an alternative order of precedence was more useful within a publication. – Karl Aug 30 '20 at 9:36

In all first three structures, the longest chain has chosen over the multiple bonds (the old names are in orange). In the typical case, the principle group is $$\ce{-CHO}$$ and hence hence to hexanal is the principal structure which has $$\ce{-OH}$$ substituent and longest chain. Thus, the name is 2-ethynyl-5-hydroxyhexanal.