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How does trans-cyclooctene exhibit chirality if there are no stereocenters?

Related follow-on questions:

  1. Are all higher cycloalkenes chiral?
  2. Do more double bonds cause a bigger number of stereoisomers in cycloalkenes?
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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).

enter image description here

These two molecules are mirror images of each other but are not superimposable. Therefore trans-cyclooctene is chiral despite not having a chiral center.

Source


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:

  1. point chirality: typically a carbon center with four different substituents;

  2. axial chirality: such as allenes with different substituents on each carbon (see this question);

  3. planar chirality: such as the case of trans-cyclooctene;

  4. 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. $\endgroup$ – hBy2Py Aug 25 '15 at 16:51
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    $\begingroup$ 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. $\endgroup$ – hBy2Py Aug 25 '15 at 17:10
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    $\begingroup$ 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/… $\endgroup$ – jerepierre Aug 25 '15 at 21:14
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    $\begingroup$ @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 '15 at 1:34
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    $\begingroup$ @Marko, your edit would possibly have been better posted as a new question, possibly even two, with crosslinks posted among them. $\endgroup$ – hBy2Py Aug 26 '15 at 2:19

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