# 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?

Jul 22, 2018

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

$p e n t \text{-3-en-1-yne}$

But if both $C = C \mathmr{and} C \equiv C$ 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

$p e n t \text{-1-en-4-yne}$

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