[Joules23]

Can someone please explain how this is a cyclic 4-octyne isomer and not
1,2-dimethyl-hexene

[macman104]

I see no way for that to be possible.  Do you mean a cyclic 4-octene?

[Joules23]

no, octyne, at least that what is says on the worksheet

[Joules23]

and i meant 1,2-dimethyl-cyclohexene

[macman104]

Ah, I was reading it as an isomer of "cyclic 4-octyne", not a cyclic isomer of "4-octyne".  In that case, this compound and 4-octyne are isomers of eachother because they have the same molecular formula (both have 8 carbons and 3 elements of unsaturation).  Wikipedia calls these types of isomers "functional group isomers".