[THC]

In McMurry's Fundamentals of Organic Chemistry, pp. 75, a cyclopalkene is named 1,5-dimethylcyclopentene.

The correct name must be: 2,3-dimethylcyclopentene?

[Yggdrasil]

Why do you think the methyl groups should be labeled 2 and 3?  The 1,5 name gives the lower number to the first methyl group.  Also, ChemDraw says that the compound is 1,5-dimethylcyclopentene.

[THC]

Why do you think the methyl groups should be labeled 2 and 3?  The 1,5 name gives the lower number to the first methyl group.  Also, ChemDraw says that the compound is 1,5-dimethylcyclopentene.

Shouldn't I count the C atoms the other way around, so the overall numbering is the lowest number possible? 1+5=6 vs. 2+3=5?

[agrobert]

The alkene (olefin) has overall priority in the molecule starting from C-1.  One methyl is on C-1 and then counting in the direction across the double bond is C-2, C-3, C-4 and then C-5 (with the other methyl).  So as Yggdrasil said 1,5-dimethylcyclopentene.  Or 1,5-dimethylcyclopent-1-ene for clarification.

[THC]

The alkene (olefin) has overall priority in the molecule starting from C-1.  One methyl is on C-1 and then counting in the direction across the double bond is C-2, C-3, C-4 and then C-5 (with the other methyl).  So as Yggdrasil said 1,5-dimethylcyclopentene.  Or 1,5-dimethylcyclopent-1-ene for clarification.

I think I've misunderstood something fundamental, then.
Do I always name cycloalkenes so a substituent attached to one of the Cs in the double bond is numbered 1, and the remaining substituents are counted clockwise?

PS I've uploaded a structure of the molecule here: http://peecee.dk/upload/view/93946/full

[agrobert]

No you name a molecule with the numbering starting with the highest priority according to IUPAC rules.  In this molecule the alkene is the highest priority.  When naming alkenes you assign C-1 as the methyl carbon because it has the most functionality.  Then counting across the double bond to C-2 and the rest follows as before.  Clockwise or counterclockwise has nothing to do with it because you could easily flip this ring over and assign the same numbering in a clockwise fashion.

[Yggdrasil]

Shouldn't I count the C atoms the other way around, so the overall numbering is the lowest number possible? 1+5=6 vs. 2+3=5?

I don't think the overall numbering is important.  See rule A2.2 on the following page, especially the example with 2,7,8-trimethyldecane v. 3,5,9-trimethyldecane.

http://www.acdlabs.com/iupac/nomenclature/79/r79_36.htm

[THC]

No you name a molecule with the numbering starting with the highest priority according to IUPAC rules.  In this molecule the alkene is the highest priority.  When naming alkenes you assign C-1 as the methyl carbon because it has the most functionality.  Then counting across the double bond to C-2 and the rest follows as before.  Clockwise or counterclockwise has nothing to do with it because you could easily flip this ring over and assign the same numbering in a clockwise fashion.

Shouldn't I count the C atoms the other way around, so the overall numbering is the lowest number possible? 1+5=6 vs. 2+3=5?

I don't think the overall numbering is important.  See rule A2.2 on the following page, especially the example with 2,7,8-trimethyldecane v. 3,5,9-trimethyldecane.

http://www.acdlabs.com/iupac/nomenclature/79/r79_36.htm

Okay, so the overall conclusion is that I should always number the C atoms so the first substituent gets the lowest number - thanks for corretcing me, I've always thought otherwise!

[agrobert]

The highest priority functional group gets the lowest number.