Numbering of Substituents [ENDORSED]

[Christian_Makar_2D]

I just wanted to clarify something from class on Friday. When you have two substituents with the same numbering, you always have to give the first named group the lower number. But if they are not the same, do you give the lower number to the last group? I just got confused because of the example on page 96 of the course reader for the cycloalkane, 4-ETHYL-2-Methyl-1-Propylcyclohexane where the last group got the smallest number as opposed to the first group.

[Nick Foucrier 3J]

For naming/numbering substituents, you start by first listing all the substituents by name. Then you write them in alphabetical order. After listed in alphabetical order, you then proceed to number them. So, the numbers do not have to be in a particular order per se, unless they have the same name, in that case alphabetically it doesn't matter which one goes first so you must put the lower number first.

[Christian_Makar_2D]

If numbering order doesn't matter, then how is 1-ETHYL-3-Methyl-4-PROPYLCYCLOHEXANE wrong as opposed to 4-ETHYL-2-METHYL-1-PROPYLCYCLOHEXANE? Do you have to add all the numbers up and see which one gives the lowest?

[abram_wassily_1G]

yes, Christian. You have to add up all the numbers, and the name with the lower sum is the appropriate iupac name.

[csebastiani_1B]

If numbering order doesn't matter, then how is 1-ETHYL-3-Methyl-4-PROPYLCYCLOHEXANE wrong as opposed to 4-ETHYL-2-METHYL-1-PROPYLCYCLOHEXANE? Do you have to add all the numbers up and see which one gives the lowest?

No you don't add them up. They both have 1 as being the lowest value so then you look at the next lowest number possible and its the 2 on 4-ETHYL-2-METHYL-1-PROPYLCYCLOHEXANE so that is the correct name

[Vivian Nguyen 2A]

In the textbook, it states that there is no lowest sum IUPAC rule. An example of 2,7,8-trimethyldecane is given to show that even though the sum of the numbers (2+7+8=18) is larger than 3,4,9-trimethyldecane (3+4+9=16), the former is correct.

[ChristinaRoble3J]

I have been very confused on the numbering of substituients after reading these posts I hoped to be more sure on how numbering works, but alas I am even more confused. I noticed that someone endorsed Christians post about the rules for ordering numbers, but his post doesn't take into consideration what the textbook said about there being no sum rule for iupac naming. Pls help :/

[Amy_Bugwadia_3I]

As stated above, there is no lowest sum rule for IUPAC naming. Thus, we must try to number the carbons in order to give us the lowest numbers possible. In that respect, we simply look at the numbers individually. 1-ETHYL-3-Methyl-4-PROPYLCYCLOHEXANE and 4-ETHYL-2-METHYL-1-PROPYLCYCLOHEXANE both have 1 as their lowest possible number, but 4-ETHYL-2-METHYL-1-PROPYLCYCLOHEXANE has 2 as the next lowest number, versus 1-ETHYL-3-Methyl-4-PROPYLCYCLOHEXANE, which has 3 as its next lowest number. Since 2 < 3, 4-ETHYL-2-METHYL-1-PROPYLCYCLOHEXANE is correct.

[ChristinaRoble3J]

Ohhh that makes so much sense! Thank you Amy!