There are 5 carbon atoms in the main chain. So, the main chain would have a name ‘pent’ in it. Now, as the functional group of aldehyde is given more priority than double bonds, the numbering of carbon would start from there only.
So we start numbering from left hand side. We see that the double bond is shared between the 2nd and 3rd carbon. So, the name has ‘-2-en’ in it. Thus the IUPAC name stands as
Pent-2-en-1-al or pent-2-enal (as aldehyde group is on the first carbon atom).
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