# The IUPAC name of vinyl acetylene is:A) $Pent-1-en-4-yne$ B) $Pent-4-yne-1-en$ C) $but-1-en-3-yne$ D) $but-1-yn-3-ene$

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Hint: Vinyl acetylene contains one carbon-carbon double bond and one carbon-carbon triple bond.

Alkenes are named by replacing the suffix –ane from the name of the corresponding alkane with suffix –ene. For example, alkene containing 2 carbon atoms is named as ethene by replacing the suffix –ane from the name ethane with suffix –ene.
Alkynes are named by replacing the suffix –ane from the name of the corresponding alkane with suffix –yne. For example, alkyne containing 3 carbon atoms is named ethyne by replacing the suffix –ane from the name ethane with suffix –yne.
Vinyl acetylene contains four carbon atoms, one carbon-carbon double bond and one carbon-carbon triple bond. The structure of vinylacetylene is as follows:

Since 4 carbon atoms are present in vinyl acetylene, the parent compound is butane. Since one carbon-carbon double bond and one carbon-carbon triple bond is present, the parent compound is butenyne. Here, the suffix –en represents a carbon-carbon double bond and a suffix –yne represents a carbon-carbon triple bond.
When multiple bonds are present, number the carbon chain from the end, which gives lowest possible locants to the carbon atoms joined by multiple bonds. A multiple bond is a double bond or a triple bond.
When both carbon-carbon double carbon-carbon and triple bonds are present in the same molecule, the carbon chain is numbered from the side which gives lowest locants to the double bonded carbon atoms.

The IUPAC name of vinyl acetylene is $but-1-en-3-yne$.

Hence, option C is the correct answer.

Note:
Do not number the chain from the side containing carbon-carbon triple bond.When both carbon-carbon double carbon-carbon and triple bonds are present in the same molecule, the carbon chain is numbered from the side which gives lowest locants to the double bonded carbon atoms.