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A molecule is chiral when it is different from its mirror image, and achiral if it is identical to its mirror image. To check if the mirror image is identical to the original object, we have to check if the two are super-imposable. Sometimes no rotation is necessary to show the molecule and mirror image are idential, and sometimes we need to rotate the mirror image, for example by 180°. This is so confusing. Can someone explain exactly what is to be done in simple words?

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closed as unclear what you're asking by Mithoron, Nuclear Chemist, Todd Minehardt, Soumik Das, A.K. Feb 24 at 15:54

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  • $\begingroup$ Spatial perception is confusing, unless you're good at it naturally or have lots of practice. You are trying to check if you objects in possibility different orientations are the same. Are they the same? It's easiest if you can get them into the same orientation first. $\endgroup$ – Zhe Feb 23 at 14:33
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    $\begingroup$ If you need to rotate it, then rotate it. After all, no matter how much you rotate something, it is still the same thing; your left hand (hopefully) does not turn into your right hand if you turn it around. $\endgroup$ – orthocresol Feb 23 at 14:34
  • $\begingroup$ @orthocresol From your comment I am implying that it does not matter if I do not rotate or rotate 1° or 2° or 180° if I can fit it, it is achiral. Tell me if I am correct and thank you for responding. $\endgroup$ – user8550821 Feb 23 at 14:39
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    $\begingroup$ You can do what you want. If something is not superimposable it remains so. Your confusion is due to the fact that we must use projection to paper planes. Other is what symmetry of an object is. $\endgroup$ – Alchimista Feb 23 at 15:37
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Whether you need to rotate a molecule to show that it superimposes with its mirror image depends on how you position your mirror.

Take human bodies, for example. They have approximate mirror symmetry (bilateral symmetry), with the left side looking similar to the right side. If you stand in front of a mirror with one of your shoulders touching it, an observer will be able to see the symmetry without rotating the mirror image in their mind. If you face the mirror, you have to imagine turning the mirror image by 180 degrees along the long axis (head to toe, like a pirouette). If you are standing on a mirror placed on the floor (careful not to break it...), you have to imagine turning the mirror image 180 degrees around a front-back axis (like a cart wheel) to see that you and mirrored you are super-imposable.

If the plane of the mirror is not aligned with two of your body axes (top-bottom, front-back, right-left), the necessary rotations will be different from 180 degrees.

Aliens have bilateral symmetry too

This reminds me of the question why a mirror switches left and right, and not top and bottom (see this article). Once you understand why this is a silly question, you can return to checking molecules for chirality by placing them in front of a mirror.

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The superimposability check (as is often done to determine if a molecule is chiral) can necessitate one or more rotations of the molecule. The rotational axis is irrelevant in this. Absence or presence of such rotations has no further meaning in the context of chirality.

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  • $\begingroup$ Can we also rotate along any axis or we are stuck to axis perpendicular to a fixed plane. $\endgroup$ – user8550821 Feb 23 at 14:56
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    $\begingroup$ Any axis is fine $\endgroup$ – Karsten Theis Feb 23 at 21:31

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