Exercise 1.16 Numbering Carbons in Parent Chain

[Shannon Han 2B]

On #16, I thought if different and equivalent numbering was possible (1,2 and 2,1 in this case), you make the first-named substituent (which is isopropyl) have the lower number? Why do the solutions say 2-isopropyl and 1,1-dimethyl?

[martha-1I]

In this case, we start counting at the dimethyl site because we want the overall lowest count thus we get 2-isopropyl-1,1-dimethylcyclopentane. If we start counting at the isopropyl site, we get 1-isopropyl-2,2-dimethylcyclopentane which gives us an overall count of 5 instead of 4 (from the first example). We chose the one that has 4 because it has the overall lowest count.

[Chem_Mod]

Hi, Martha's explanation is actually incorrect even though it ends up with the correct answer.

When numbering the carbon chain, we choose the number set with the smallest element at the point of first difference, NOT the smallest sum.
For the molecule 2-isopropyl-1,1-dimethylcyclopentane, we could have chosen either the number set (1,1,2) or (1,2,2). To apply the rule of first difference, we compare the lowest elements of each set. But they are both 1 so they tie. Then we look at the 2nd lowest elements, and here 1 beats 2. So we choose (1,1,2).

Here is an example where using the sum rule results in the INCORRECT name!!
Consider 2-ethyl-8,8-dimethyldecane. If we were to number the decane chain the other way, we would get 9-ethyl-3,3-dimethyldecane which is incorrect. The set (2,8,8) defeats (3,3,9) at the first difference because 2<3, even though 18>15 !! Be warned, and do not sum up the numbers.

[Justin Le 2I]

You can usually get the lowest numbering scheme if you start at the C with the most substituents to follow the number rule that Chem Mod said above. So here, yo u see that there are two methyl groups on a C so if you start there, you'll probably get the right answer.

[martha-1I]

I was just about to post that the way I previously described how to number the carbon chain is incorrect, so dismiss it because it does not always work. Thank you Chem_mod for clearing that up.

[704564167]

Would it be wrong to name this 1,1-dimethyl-2-propylcyclopentane? Does there need to be the iso- in front of the propyl or is it sufficient enough to just say propyl since there are 3 carbons on that substituent.

[dyyjenny]

If you were to use the IUPAC name, it would be 2-(1 methylethyl)-1,1-dimethylcyclopentane