Video Transcript
What is the name of the following alkyne? (A) 2-Chlorohex-4-yne, (B) 3-chlorohept-4- yne, (C) 4-chloropent-2-yne, (D) 4-chlorohex-2-yne, or (E) 4-chloropent-3-yne.
An alkyne is an unsaturated hydrocarbon that contains at least one carbon-carbon triple bond. The three lines in the skeletal formula representing the carbon-carbon triple bond are a clear indicator that this molecule is an alkyne. We need to determine the name of this alkyne. To name an alkyne, we’ll start by naming the longest continuous chain of carbon atoms. This name will consist of a prefix, which indicates the length of the carbon chain, and the ending Y-N-E indicating that the molecule is an alkyne.
We were given the skeletal formula of the alkyne, which does not explicitly show the carbon atoms. To find the longest continuous chain of carbon atoms in this molecule, let’s take a look at the displayed formula and skeletal formula of a different molecule. We can see the carbon-carbon triple bond in both structures. So in the skeletal formula, each side of the triple bond must represent a carbon atom. The ends of the skeletal formula must represent the carbon atoms on the ends of the displayed formula. And the point at which the skeletal formula bends or two lines meet with one another must represent the fifth remaining carbon atom.
Looking at the skeletal formula provided, we can identify one, two, three, four, five, and six carbon atoms in a continuous chain. The prefix for six carbon atoms is hex-, so an alkyne containing six carbon atoms in the chain is a hexyne. Next, we want to number the carbon chain so that the carbon-carbon triple bond has the lowest possible position number. If we number the carbon atoms in the chain from left to right, then the carbon-carbon triple bond starts with carbon atom number two. If we number the carbon chain from right to left, then the carbon-carbon triple bond starts with carbon atom number four.
Since we want the carbon-carbon triple bond to have the lowest possible position number, we should number the chain from left to right. We then add the position number to the name in between the prefix length and the ending Y-N-E. This gives us hex-2-yne. This is the name of the parent structure. Now, we can name any substituents or branches that stick off of the parent structure. We’ll need to write the position number of this substituent followed by the substituent name, which is then followed by the parent structure name.
Looking at the structure given in the question, we can see that this molecule has a chlorine substituent. Chlorine substituents are given the name chloro-. The chlorine atom is bonded to the fourth carbon atom of the chain. This gives us the name 4-chlorohex-2-yne, which matches answer choice (D). So the name of the alkyne given in the question is 4-chlorohex-2-yne or answer choice (D).
