Question

How do you count carbons on cycloalkanes?

Answer 1

Around the ring, starting at whichever carbon gives you the smallest numbers in the final name.

Explanation:

Imagine we have a cycloalkane ring of five carbon atoms, with two methyl groups attached to the ring. These two methyl groups might be adjacent on the ring: we'd choose to number the carbons so that the name would be 1,2-dimethylcyclopentane. Maybe there is one space between the two methyl groups: 1,3-dimethylcyclopentane. These are the only possibilities - if we had 1,4-dimethylcyclopentane we have just numbered the wrong way round the ring - numbering in the opposite direction would show that this was really 1,3-dimethylcyclopentane.