How does trans-cyclooctene exhibit chirality if there are no stereocenters?
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$\begingroup$ related chemistry.stackexchange.com/a/31296/9961 $\endgroup$– MithoronAug 25, 2015 at 18:38
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$\begingroup$ Related: chemistry.stackexchange.com/questions/24901/… $\endgroup$– jerepierreAug 25, 2015 at 21:09
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$\begingroup$ Especially related, since trans-cyclooctene undergoes ring flip very slowly, unlike cyclohexane: chemistry.stackexchange.com/questions/34475/… $\endgroup$– jerepierreAug 25, 2015 at 21:12
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$\begingroup$ chemistry.stackexchange.com/questions/24901/… $\endgroup$– MithoronAug 26, 2015 at 1:23
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$\begingroup$ I absolutely agree with Brian's comment on the answer, it would be better, if you roll back to the original question and ask a follow up question with the added ones. $\endgroup$– Martin - マーチン ♦Aug 26, 2015 at 3:06
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Very interesting question! The key word you are looking for is planar chirality. In trans-cyclooctene, the polymethylene bridge can either go "in front of" or go "behind" the plane of the double bond, assuming you fix the double bond and the two hydrogens in place.
As pointed out by @jerepierre, they are considered different molecules due to a high-energy barrier which prevents the interconversion. Cyclooctene is the first cycloalkene to have both stable cis- and trans- isomers. The chain in trans-cyclooctene is not long enough to swing over the double bond. As the chain gets longer, the energy barrier to rotation decreases.
Here are the two mirror images of trans-cyclooctene (image source: own work).
These two molecules are mirror images of each other but are not superimposable. Therefore trans-cyclooctene is chiral despite not having a chiral center.
Edit: this is the first time I learned that a chiral molecule does not have to have a chiral center. After making some searches online, I feel a need to expand the answer to clarify the concept of chirality.
A chiral molecule is one that has a non-superimposable mirror image. Mathematically a molecule is chiral if it is not symmetric under an improper rotation. Chirality arises due to:
point chirality: typically a carbon center with four different substituents;
axial chirality: such as allenes with different substituents on each carbon (see this question);
planar chirality: such as the case of trans-cyclooctene;
inherent chirality: due to the presence of a curvature in a structure that would be devoid of symmetry axes in any bidimensional representation, such as fullerenes.
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$\begingroup$ Hm. Technically, though, it does have a chiral center (at the center of the ring), right? It just doesn't coincide with a nucleus. $\endgroup$– hBy2PyAug 25, 2015 at 16:51
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1$\begingroup$ I'm probably splitting hairs, or up against a nomenclatural definition. Formally speaking, does a 'chiral center' only exist at a nucleus? If so, then the center of mass doesn't qualify as a chiral center, and my initial comment is incorrect. $\endgroup$– hBy2PyAug 25, 2015 at 17:10
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1$\begingroup$ I'd give +1/2 if I could. An additional key feature of trans-cyclooctene is that it doesn't undergo ring flip (or does so slowly). For a similar question, see: chemistry.stackexchange.com/questions/34475/… $\endgroup$ Aug 25, 2015 at 21:14
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1$\begingroup$ @Brian As I was unsure myself, I consulted the goldbook, where it is explicitly stated as "an atom". In the case of planar chirality, the definition also clearly states plane. One rational about this is, that there are multiple atoms that lie in the same plane when considering enantiomers, which is not the necessarily the case for point chirality. $\endgroup$– Martin - マーチン ♦Aug 26, 2015 at 1:34
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1$\begingroup$ @Marko, your edit would possibly have been better posted as a new question, possibly even two, with crosslinks posted among them. $\endgroup$– hBy2PyAug 26, 2015 at 2:19