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?

1 Answer
P dilip_k
Jul 22, 2018

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'

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