[ve_90]

Is it correct to assign the following name: 4-ethyl-5-isopropyl-3-heptene? I fear not: the longest chain is the one with more substituents, if I'm not mistaken. Therefore, I would say: 4,5-diethyl-6-methyl-3-heptene.
Thanks for a confirmation.

[Borek]

hept-3-ene, other than that ChemSketch agrees with 4,5-diethyl version.

That being said: isopropyl version, even if not entirely correct, is unambiguous and there is no doubt what the molecule is.

[ve_90]

Sorry, can you explain it to me better?

[Borek]

It should be 4,5-diethyl-6-methylhept-3ene.

"Isopropyl" is one of these names that are still acceptable, but not preferred. 4-ethyl-5-isopropyl-3-heptene clearly and unambiguously describes the molecule, so using it won't produce serious problems in communication with other chemists (but can be disallowed in formal context of publication).

[ve_90]

Okay, now it's clear to me. However, my doubt is this, beyond the name: is it correct to write 4-ethyl-5-isopropyl-3-heptene, since according to IUPAC rules, the longest chain containing the double bond must not only be the longest but also the one with the most possible substituents?
If I consider the isopropyl group as a substituent, the substituents on the main chain are 2, not 3 (unlike 4,5-diethyl-6-methylhept-3-ene).

[Borek]

Yes, that's exactly the problem - but in the end it depends on what you consider a valid substituent. Once you assume isopropyl is an acceptable substituent you don't have to worry about its internal structure.