Question

In IUPAC nomenclature, which bond is given more priority?

In Pent-3-ene-1-yne, triple bond is given priority but in Pent-1-en-4-yne, double bond is given priority. What is the reason?

Answer 1

drawn

Actually in placing locants along the longest `C`-chain containing both `C=CandCequivC` bonds for setting IUPAC name of a hydrocarbon the preference is given as unsaturation first irrespective of `C=CandCequivC`.
In case of compound-I `CequivC` should be given locant -1 as it comes first as unsaturated bond and its IUPAC name becomes

`pent"-3-en-1-yne"`

But if both `C=CandCequivC` are in the same positions from both ends as for compound II then `C=C` is given the first preference in placing the locant, So the name of compound -II will be

`pent"-1-en-4-yne"`

But in all cases the names end up with 'en' followed by 'yne'