Order of naming

[Anjali_U_3L]

On page 102 of the course reader, why isn't the structure 1,6-dichlorocyclohexene called 2,3-dichlorocyclohexene? (starting at the double bond and going the other way)

[Megan Yabumoto 3K]

When determining the numbering for structures of compounds that have a double bond, you want the first number to be the smallest it can possibly be. In this case, you would start at one and go all the way around to six because you must go across the double bond.

[Amy_Shao_2D]

You want the set of numbers that has the lowest number. 1 is the lowest number, so use the set that has 1, which is 1,6.
Side note: you'd think it was 2,3 because that has lower numbers overall but IUPAC doesn't have a "lowest sum" rule or anything (ie 2+3 is less than 1+6), just lowest number.

[Greta Tobin 2F]

You want the set of numbers that has the lowest number. 1 is the lowest number, so use the set that has 1, which is 1,6.
Side note: you'd think it was 2,3 because that has lower numbers overall but IUPAC doesn't have a "lowest sum" rule or anything (ie 2+3 is less than 1+6), just lowest number.

Does this only refer to the first number listed? I.e. in that example, 1 is less than 2 so we would choose that order even though 3 is less than 6?

[Chem_Mod]

Remember that this molecule has an alkene which is a functional group. You want to give functional groups the lowest number possible and in the case of double or triple bonds, you must follow the direction of the bond for numbering. So another way of writing the correct name for this molecule is 1,6-dimethyl-1-cyclohexene which has lower numbers than 2,3-dimethyl-2-hexene (the alternative numbering you were proposing).

[Diana_Visco_1l]

According to IUPAC, double bonds and triple bonds take precedence in the order of which the structure is named (this is an exception where alphabetical order doesn't appear).

[Amy_Shao_2D]

You want the set of numbers that has the lowest number. 1 is the lowest number, so use the set that has 1, which is 1,6.
Side note: you'd think it was 2,3 because that has lower numbers overall but IUPAC doesn't have a "lowest sum" rule or anything (ie 2+3 is less than 1+6), just lowest number.

Does this only refer to the first number listed? I.e. in that example, 1 is less than 2 so we would choose that order even though 3 is less than 6?

Yes, first number listed in numerical order. So if you had like 1,2,5 and 1,3,4, you'd compare 1 and 1 but they're the same, so then you'd move to 2 and 3. 2<3 so you'd go with the 1,2,5.