(E)-3-Chloro-4-fluoro-7-methyloct-3-en-5-yne

[Andrew Afyouni]

Can someone justify their logic in naming (E)-3-Chloro-4-fluoro-7-methyloct-3-en-5-yne? (I have attached a picture to this post from p. 50 of the organic chemistry orange book) My question stems from why the double bond is given the lower carbon # rather than the triple bond...It was my impression that triple bonds get priority over single and double bonds? I would appreciate some clarification. Thank you in advance!

[Umair Khan 2G]

You are right in thinking that in a normal carbon chain a triple bond would get priority over a double and single bond. However, recall that when naming structures and assigning numbers we have to look at the numbering of the substituents first. Since we give chlorine and fluorine the smaller numbers, we have to start with the 1st carbon on the right-hand side, thus making the double bond come before the triple.
If we wanted the triple bond to come first, the structure would have to be called 6-Chloro-5-fluoro-2-methyloct-5-en-3-yne.
Now are substituents have the numbers 6,5, and 2 rather than 3,4 and 7. Obviously 3 and 4 are smaller, so the name 3-Chloro-4-fluoro-7-methyloct-3-en-5-yne is preferred.
Also note that triple and double bonds have the same priority.

[Hayley Hammons 1L]

Since when do substituents take priority? I thought double and triple bonds always took priority in numbering over substituents. Or is this only true when the substituents are alkyl groups?

[Umair Khan 2G]

Reading over my answer, I can I wasn't very clear.
Your are right in that double and triple bonds have priority over substituents in all cases.
Also double and triple bonds have equal priority.
As such, in this example, prioritize the the double bond is given the lower number, because it give the substituents Cl and F lower numbers when naming.
In order of priority it is:
Function Groups > Bonds > Substituents