Question Video: Identifying the Name of a Branched Alkyne Chemistry

State the name of the molecule.


Video Transcript

State the name of the following molecule. (A) 4-Chloro-3-methylbut-1-yne, (B) 1-chloro-2-methylbut-3-yne, (C) 3-methyl-3-chloromethylprop-1-yne, (D) 1-chloro-2-ethynyl propane, or (E) 2-chloromethylbut-3-yne.

To answer this question, we need to identify the name of this organic molecule. Organic molecules can be named using the International Union of Pure and Applied Chemistry, or IUPAC, naming rules.

First, we need to identify the primary functional group in this molecule. We can see that the molecule contains a carbon–carbon triple bond. This group is known as an alkyne and is the primary functional group in this molecule.

Next, we need to identify and number the longest continuous chain of carbon atoms in the molecule. The longest continuous chain must contain the alkyne. And the alkyne must have the lowest possible position number in the carbon chain. Following these rules, we can identify two continuous chains of four carbon atoms that each contain the alkyne at a position number of one. In the case of a tie like this, we choose the chain that has the most branches or substituents. The green-numbered chain has one large branch, but the pink-numbered chain has two branches. So we should choose the pink-numbered chain to be the longest continuous chain of carbon atoms.

Next, we name the parent alkyne. This name consists of a prefix that indicates the number of carbon atoms in the carbon chain, a position number that indicates the location of the alkyne in the carbon chain, and the suffix Y-N-E. The alkyne is located on carbon number one. The prefix for four carbon atoms is but-. So the name of the parent alkyne is but-1-yne.

Next, we name each substituent. These names consist of a position number, followed by the substituent name. Bonded to carbon number three, there is a CH3 group, or a methyl group. So the name of the substituent is 3-methyl. Bonded to carbon number four is a chlorine atom, giving us the name 4-chloro-. To complete the name of the molecule, we write the substituent names in alphabetical order in front of the parent name.

Therefore, the name of the molecule shown is 4-chloro-3-methylbut-1-yne, answer choice (A).

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