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I recently study organic chemistry and I read a book by Sana Jamshaid. I came across this compound and I cannot understand why it named 6-ethenyl-4-(prop-2'-enyl) octa-6-en-3-ol:

6-ethenyl-4-(prop-2'-enyl) octa-6-en-3-ol

I understand that the longest chain is 8, hence octa (but if we go up on the 4th longest can be 9) but I don't understand the 4-(prop-2’-enyl). Are there any explanations to why the compound named in such way? The double bond also sits on the 7th and not the 6th.

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    $\begingroup$ The name looks wrong. The longest chain is 10 the second longest nine. $\endgroup$ – matt_black Dec 10 '20 at 13:50
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    $\begingroup$ I cannot rationalize that name either, and the drawing looks off (angles at tertiary carbon are nonsensical). Probably it would help to draw the same molecule differently and proceed from there. It also wouldn't hurt if there were a complete reference to the book. "The country should know its heroes", as they say in Russia. $\endgroup$ – andselisk Dec 11 '20 at 6:46
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The suggested name
“6-ethenyl-4-(prop-2'-enyl) octa-6-en-3-ol”
is not correct; the closest valid name is
6-ethenyl-4-(prop-2-en-1-yl)oct-6-en-3-ol.

However, this name does not describe the compound that is shown in the picture. Note that the parent chain actually is non-7-en-3-ol instead of oct-6-en-3-ol, and the longest side chain actually is but-3-en-1-yl instead of prop-2-en-1-yl.

Therefore, the correct name for the compound that is shown in the picture is
(7E)-4-(but-3-en-1-yl)-6-ethenylnon-7-en-3-ol.

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I also agree with @Loong, the suggested name "6-ethenyl-4-(prop-2'-enyl) octa-6-en-3-ol" seems incorrect. The correct name for this compound should be (7E)-4-(but-3-en-1-yl)-6-ethenylnon-7-en-3-ol, as @Loong mentioned.

In this compound, the longest carbon chain has 10 carbon atoms, but we select the carbon chain with 9 carbon atoms as it has a functional group (-OH) attached. So, parent chain will be non- as there are 9 carbons.

enter image description here

Now, we number the chain from left to right, as the main functional group is always assigned the least possible locant.

enter image description here

So, we see that we get -OH at locant 3, -CH2CH2CH=CH2 at locant 4, -CH=CH2 at locant 6 and double bond at locant 7. So these substituent/functional groups can be named accordingly:

  • 3-ol
  • (but-3-en-1-yl) (as carbon atom attached to parent chain (alkyl) is always numbered 1)
  • ethenyl (vinyl group)
  • 7-ene (NOTE: the ending -e will be dropped while writing the complete IUPAC name)

Also, since it is a trans- (w.r.t double bond at locant 7), we put (7E)- in the beginning of the name. (Z is put for cis- )

So, now, the IUPAC name should be (7E)-4-(but-3-en-1-yl)-6-ethenylnon-7-en-3-ol

Here are a few resources:

NCERT Class XI Organic Chemistry - some basic principles and techniques

Systematic IUPAC nomenclature - Aditya Vardhan V

cis- and trans- nomenclature in organic compounds

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