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Does there exist a chiral compound with zero optical activity? It seems possible that for some (chiral) arrangement of atoms, the optical activity of one section of the molecule could be exactly compensated by another section of the molecule, even if these two sections are not mirror images of each other. Although this is just a mathematical coincidence, does any chiral molecule have an optical activity of exactly zero?

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    $\begingroup$ Are you asking for compound with smallest specific rotation? $\endgroup$
    – Safdar Faisal
    Commented Aug 8, 2020 at 6:33
  • $\begingroup$ Edited it... I wanted to ask something else $\endgroup$
    – user97495
    Commented Aug 8, 2020 at 6:34
  • $\begingroup$ It seems you're asking if "a molecule with chiral centres can have zero optical activity?". But, I'm not sure. $\endgroup$
    – Rahul Verma
    Commented Aug 8, 2020 at 7:54
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    $\begingroup$ I very much doubt that there is a compound with exactly zero activity. Meso compounds are achiral (by definition) even though they contain stereo centers. $\endgroup$
    – Martin - マーチン ♦
    Commented Aug 8, 2020 at 8:09
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    $\begingroup$ Do you mean allenes? they do have a stereocenter. $\endgroup$
    – Safdar Faisal
    Commented Aug 8, 2020 at 14:55

2 Answers 2

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The intent of the question appears to be to ask whether any chiral molecules could have exactly no optical activity. But this is problematic and the best we can do is test whether any compounds don't have observable optical activity.

As the Link by Nicolau Sater Neto to the concept of crypto chirality shows, on this criterion the answer is yes. The molecule 5-ethyl-5-propylundecane shown here has no observable optical rotation even when a pure enantiomer is isolated.

wikipedia image, creative commons licence

One can intuit that the nature of the compound is such that its rotation probably exists but is very very small.

it seems improbable from first principles that non trivial examples (eg not meso compounds) exist where the optical rotation is exactly zero.

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  • $\begingroup$ If the reason for the chirality were far from the center of chirality I would think the effect would approach zero or become not measurable. $\endgroup$
    – jimchmst
    Commented Feb 20 at 20:48
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Yes, tertiary amines where all groups are different are optically inactive (due to amine inversion) even though it's a chiral molecule. Eg. N(CH3)(C2H5)(H) or NHDT

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    $\begingroup$ While technically true this feels like a cheat as the reason why optical activity isn't observed is the lability of the molecules under normal conditions. At lower temperatures they might well show optical activity. $\endgroup$
    – matt_black
    Commented Feb 20 at 13:30