2 Substituents on a Ring

[Mackenzie Sober 2E]

The course reader says for 2 Substituents on a ring, if the substituent numbers are the same, the 1st named group gets the lower number. If there are only 2 substituents, won't they always have the same number? If this is not correct, can someone please give an example of when they would not have the same number.

[Alex Dib 4H]

An example of this would be with, lets say, a cyclohexane with a methyl on the left lower carbon and an ethyl on the right upper carbon. You can either number this as 4-ethyl-1-methylcyclohexane, giving a 1 to the methyl and giving a 4 to the ethyl , or you can alternatively name it 1-ethyl-4-methylcyclohexane giving a 1 to the ethyl and a 4 to the methyl. In both cases, the substituent numbers are the same (1 and 4), so what you have to do is assign the lower number to the first named substituent, in this case ethyl because of alphabetical order. So the correct answer would be 1-ethyl-4-methylcyclohexane. Hope this helps.

[Katie McCombs 3G]

Since it is in a ring structure, you are correct that the number will always be the same. Therefore it is the common practice of a ring with 2 substituents to give the first named substituent the lower number.