How would you name this molecule?

[756034]

Hi everyone,

I am reviewing orgo basics and have been pretty confident about naming cycloalkanes. Until I came across the following problem.

Name the following cycloalkane:

My response was 1,3-dichloro-4,5-diethylcyclopentane. But the answer key says the correct response is 1,4-dichloro-2,3-diethylcyclopentane. Maybe I am misunderstanding the first point of difference rule but...wouldn't you want that carbon location of the second chlorine to be as low as possible?

Thanks!

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[betterfuture]

The way you named it, 1,3-dichloro-4,5-diethylcyclopentane, was wrong because you did not give each substituent the lowest possible number. When you work you way with starting with the chlorines, either the left one or right one, you would then number the ethyl group, and then the next ethyl group, and lastly the final chlorine.

I don't think having the carbon where the chlorine is attached with the lowest numbering works well here because you generally want the lowest numbering possible for each substituent.

 

[756034]

The way you named it, 1,3-dichloro-4,5-diethylcyclopentane, was wrong because you did not give each substituent the lowest possible number. When you work you way with starting with the chlorines, either the left one or right one, you would then number the ethyl group, and then the next ethyl group, and lastly the final chlorine.

I don't think having the carbon where the chlorine is attached with the lowest numbering works well here because you generally want the lowest numbering possible for each substituent.

Thanks - I appreciate the explanation! I assumed that you would want to ensure that the numbers are as low as possible as you move through the name of the cycloalkane. Maybe I am misunderstanding the first point of difference rule? I thought the first point of difference between 1,3-dichloro-4,5-diethylcyclopentane and 1,4-dichloro-2,3-diethylcyclopentane was that second chlorine. And that because 3 is lower than 4, that the 1,3 would be correct.

 

[betterfuture]

Yeah, it would SEEM like you would next number the second chlorine, but like I said, that wouldn't give the other substituents the lowest possible number when naming.