What is the IUPAC name of the compound shown below?
A) (1S,3
S)-1,3-dichloro-1-ethyl-3-methylcyclopentane
B) (1R,3
S)-1,3-dichloro-1-methyl-3-ethylcyclopentane
C) (1R,4
S)-1,4-dichloro-1-ethyl-4-methylcyclopentane
D) (1R,3
S)-1,3-dichloro-1-ethyl-3-methylcyclopentane
E) (1S,3
R)-1,3-dichloro-3-ethyl-1-methylcyclopentane

Question

What is the IUPAC name of the compound shown below? 
A) (1S,3
S)-1,3-dichloro-1-ethyl-3-methylcyclopentane 
B) (1R,3
S)-1,3-dichloro-1-methyl-3-ethylcyclopentane 
C) (1R,4
S)-1,4-dichloro-1-ethyl-4-methylcyclopentane 
D) (1R,3
S)-1,3-dichloro-1-ethyl-3-methylcyclopentane 
E) (1S,3
R)-1,3-dichloro-3-ethyl-1-methylcyclopentane

Quizplus · Accepted Answer

The compound is a cyclopentane with two chlorine atoms attached to the first and third carbons. There is also an ethyl group attached to the first carbon and a methyl group attached to the third carbon. To name the compound using IUPAC nomenclature, we first need to determine the stereochemistry at the chiral centers. The first chiral carbon is labeled as C-1 and has two substituents attached to it: chlorine and ethyl group. The second chiral carbon is labeled as C-3 and has two substituents attached to it: chlorine and methyl group.

To assign the stereochemistry at C-1, we look at the substituents attached to it and rank them based on atomic number, with the highest atomic number getting the highest priority. Here, chlorine has a higher atomic number than carbon, so it gets the highest priority. The ethyl group has a carbon attached to a carbon, so we need to compare the atoms attached to the carbons. We see that one carbon is attached to a hydrogen and a methyl group, while the other carbon is attached to two hydrogens. Therefore, the methyl group gets the second priority. This means that the chlorine has a higher priority than the ethyl group, and the lowest priority (4th) is assigned to the hydrogen atom. To determine the stereochemistry, we need to draw a circle from the 1st to the 2nd to the 3rd priority. If the circle goes clockwise, it is labeled as R, and if it goes counterclockwise, it is labeled as S.

Similarly, we can assign the stereochemistry at C-3. The methyl group has a higher priority than the chlorine because of the atomic numbers, and the ethyl group has a lower priority because it has a carbon attached to a carbon, which has a lower atomic number than chlorine. Drawing a circle from the highest to the second-highest to the third-highest to the lowest priority shows that the stereochemistry at C-3 is S.

Putting it all together, we get the name (1R,3S)-1,3-dichloro-1-ethyl-3-methylcyclopentane. Choice D matches this name and is, thus, the correct answer.

What Is the IUPAC Name of the Compound Shown Below