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I wonder, what are the exact IUPAC organic chemistry nomenclature rules that give the preference to the chain length over number of unsaturated bonds, i.e. that justify, or prefer following names:

structure 1

4-ethenyl-4-ethynylheptane over
3,3-dipropylpent-1-en-4-yne

structure 2

3,5-dimethylideneheptane over
2,4-diethylpenta-1,4-diene

?

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1 Answer 1

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In Chapter P-4 "Rules for Name Construction", section P-44 "Seniority Order for Parent Structures", of Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013, it is stated that (p 490):

P-44.3.2 The principal chain has the greater number of skeletal atoms [criterion (b) in P-44.3].

The Blue Book lists several helpful examples, one of which is that the principal chain "octane" is senior to "hept-1-ene". Therefore, the following compound

Compound to be named

is named 3-methylideneoctane instead of 2-ethylhept-1-ene.

The authors mention that this preference is in contrast to previous recommendations (e.g. 1979):

In acyclic parent structures the order of seniority between unsaturation and length of chain given in earlier recommendations is reversed. Thus, the first criterion to be considered in choosing a preferred parent acyclic chain is the length of the chain; unsaturation is now the second.

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  • $\begingroup$ Thanks. I wonder if the reasons leading to this change are mentioned somewhere.. $\endgroup$
    – mykhal
    May 10, 2017 at 16:17

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