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Are there any limits to rotating a compound in order to prove that its chirality or achirality? (assuming that you're rotating about a single bond)

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    $\begingroup$ Usually no, then you would enter the realm of conformations. Some conformation are notoriously unstable, but still you cannot really talk about stereoisomerism. $\endgroup$
    – Jori
    Apr 22, 2015 at 10:55
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    $\begingroup$ Limit is rotation energy - hence existence of atropoisomery. $\endgroup$
    – Mithoron
    Apr 22, 2015 at 13:09
  • $\begingroup$ Are you talking about bond rotation or rotation of the entire molecule? $\endgroup$
    – Zhe
    Nov 20, 2016 at 0:12
  • $\begingroup$ I'm talking about bond rotation $\endgroup$ Dec 13, 2016 at 9:16

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I think what you may be asking is: could there could be a nominally "meso" compound that has restricted rotation about its central single bond, causing it to exist in the form of two non-interconvertible stereoisomers. Example- start with meso-tartaric acid, and make the di-tert butyl ether from the OH groups and di-tert-butyl ester from the COOH groups. Maybe these groups would be bulky enough to restrict rotation. You could mentally rotate around the central C-C bond until the molecule has a plane of symmetry, so it is still a nominally "meso" compound, but physically you could separate the two stereoisomers.

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Rotating about a single bond would be a conformational change and not a stereochemical one. Meso compounds are not defined as having rotational symmetry, either way. For example, 1,2-diphenylethane-1,2-diol does not have rotational symmetry since one carbon is R and the other S. However, it's still a meso compound because its mirror image can be rotated to show that it is identical to the original compound.

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  • $\begingroup$ There is no need to be sarcastic. Your molecule would be indeed unstable, but main problem is are you talking about saturated or unsaturated molecule - trans suggests unsaturation - in this case it wouldn't have freedom of rotation around C=C, but rest of molecule's name says saturated $\endgroup$
    – Mithoron
    Apr 23, 2015 at 15:11
  • $\begingroup$ You're right, I'm sorry for being rude. I just used that molecule because it's a common example they give in the textbooks. I know it dissociates, though, but I think that's a discussion for a different topic. I'm talking about an unsaturated molecule, because trans-1,2-dichloro-1,2-ethane diol is (1R, 2S) or (1S, 2R) which are the same meso compound. It's just a common naming convention. $\endgroup$
    – Stagg C.
    Apr 23, 2015 at 20:16
  • $\begingroup$ Hmm, here it's with trans - chemwiki.ucdavis.edu/Wikitexts/Purdue/Purdue_CHM_26100/… but I'm pretty sure it shouldn't be, as it's not unsaturated or cyclic $\endgroup$
    – Mithoron
    Apr 23, 2015 at 20:30
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The properties of a molecule don't change upon rotation about a single bond and such rotations are very fast and frequent at room temperature, so you can consider the rotated form and decied the chirality of the molecule.

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