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I am struggling with these type of questions where in some cases we rotate a molecule's mirror image by 180 degrees to check superimposability.

Like when checking any tetrahedral chiral compound, we draw its mirror image and rotate it by 180 degrees and say that it's not superimposable. Suppose the mirror image of the left-handed version is the right-handed version, hence both are not superimposable. If we rotate the right-handed version by 180 degrees and then try to check, will it be superimposable now? Similarly, trans 1,2 dimethyl cyclopentane is chiral but if we rotate its mirror image by 180, is it superimposable?

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    $\begingroup$ There is probably some misinterpretation or error.. if you talk about deciding whether a compound with chiral center(s) is optically active, you take the two stereo isomers, an just try to superimpose them (to check if they in fact are the same), NOT mirroring one of them beforehand. They already are mirror images. These might OR might not be the same. Hands are not. E.g. eyes are (in the first simplified approach, when ignoring anatomical details). $\endgroup$ – mykhal Jun 21 '17 at 12:03
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You're being confused by the rotation. You take the mirror image. Any rotation or sequence of rotations that superimpose the mirror image over the original means that the molecule is achiral. It's not just a 180-degree rotation.

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  • $\begingroup$ Nice Explanation, that was my confusion too. Many thanks $\endgroup$ – VKatz Oct 5 '18 at 15:27
  • $\begingroup$ But I have one question: Does it also apply on enantiomers ? Please take a look here: youtu.be/PoeNd6YhMuA?t=93 $\endgroup$ – VKatz Oct 5 '18 at 16:56
  • $\begingroup$ @VKatz Does what apply? Does this statement apply to enantiomers? Sure, it does. Enantiomers are not super impossible on their mirror image, no matter how you rotate the mirror image. $\endgroup$ – Zhe Oct 5 '18 at 17:15
  • $\begingroup$ That is what my confusion is. In the link: youtu.be/PoeNd6YhMuA?t=103 If I rotate that the compound which is, on right has side will be superimposable. And the video that is saying is totally opposite. Can you please clarify . $\endgroup$ – VKatz Oct 5 '18 at 19:00
  • $\begingroup$ @VKatz That is not a rotation. Remember that a projection on a flat surface is a representation of a 3 dimensional structure. You can "flip" a drawing of a person's face, but does that look exactly like the back of their head? $\endgroup$ – Zhe Oct 5 '18 at 20:30

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