Destroyer 2013 OC - Question 48

[Mufasha]

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So maybe I'm missing something…but I'm working through the Destroyer now and I came across this question about isomers. It's number 48 from in the GC section from Destroyer 2013 and it asks about the number of isomers (including both constitutional isomers and stereoisomers) that exist for 1,2-dimethylcyclopentane. I understand why the isomers they've presented are correct, but is it limited to that? The molecular formula is C7H14, so wouldn't the total constitutional isomer count include things with 7 carbons and 1 degree of unsaturation, like heptene?

 

[matty675]

Unsaturated compounds would not be included because they would have a different molecular formula with fewer Hydrogens.

 

[Mufasha]

But isn't the ring compound also unsaturated in that it has 1 degree of unsaturation (i.e., because of the ring, 2 hydrogens are lost as compared to the saturated formula)? If I'm counting correctly, its molecular formula should be the same as heptene's (C7H14 for both).

 

[matty675]

Saturated compounds have 2n+2 Hydrogens. Heptane would have 16 Hydrogens and Cycloheptene would have 14.

 

[matty675]

Oh I see what you're talking about now. I'm not sure about that, maybe someone else will chime in.

 

[Mufasha]

Yes, but heptene (7 carbons with one double bond) would have 14 hydrogens, no? If that's the case, I thought 1,2-dimethylcyclopentane would have (many) more isomers than what the solution is presenting. You'd have 7 carbons and 1 degree of unsaturation in both cases.

 

[Mufasha]

Oh wow, we posted at the same time! Oops haha

 

[Dotoday]

The question asked for only cyclo isomers.

 

[Mufasha]

Hmm, okay maybe they changed it in the 2014 version. It just says isomers, not specifically cyclo isomers, in the 2013 version, but that's good to know. I just wanted to make sure I wasn't missing something.

 

[discensdentibus]

It does not say "cycloisomer" in the 2014 edition. You are not missing anything. I've picked out two problems so far, that one and problem 85(2014 ed), that I would say are debatable for answer choices. If it has the same # of C's and H's, it is a constitutional isomer.

Also, if anyone is curious about a sweet degree of unsaturation formula: #C - (1/2x - N) + 1
where x = H + halo; N and C are Nitrogen and carbon respectively