I know that meso compounds have chiral centers but don't rotate plane-polarized light, and I know that there can be non-traditionally chiral compounds (e.g. ones with large substituents that prohibit much rotation around single bonds) that can still rotate light, but I haven't been able to find any molecules that are chiral but don't rotate plane-polarized light (or rotate it impossibly little). I'm imagining something like an enantiomeric pair connected by one single bond with a slight modification to one that makes it have similar optical activity but not actually be identical (so the final molecule is indeed chiral), but I don't know exactly how to make it work.
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$\begingroup$ Achiral compounds don't rotate light. $\endgroup$– Ivan NeretinCommented Oct 16, 2018 at 20:40
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$\begingroup$ @IvanNeretin My source for that claim was the first answer to this post on Quora. The reasoning seems OK to me, but I'm not knowledgeable enough about stereochemistry to accurately confirm or dispute it. $\endgroup$– Carl SchildkrautCommented Oct 16, 2018 at 20:41
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2$\begingroup$ @IvanNeretin More precisely (as I suspect the questioner intended) compounds with no chiral centres can be chiral because of restricted rotation or other geometric features eg hexahelicene. $\endgroup$– matt_blackCommented Oct 16, 2018 at 20:56
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2$\begingroup$ Depends on what you mean by "do they rotate?" What if they don't rotate light by a measurable amount? I'm sure I can contrive a number of compounds where that is the case. $\endgroup$– ZheCommented Oct 16, 2018 at 21:13
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2$\begingroup$ Take octane and replace one of the hydrogen atoms on the 2-carbon with a deuterium. $\endgroup$– ZheCommented Oct 17, 2018 at 11:47
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1 Answer
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Good question.
There's a phenomenon named cryptochirality[1] (meaning “hidden chirality”), when a compound, though chiral, has practically unmeasurable optical rotation activity.
It can happen to molecules with chiral center(s) bearing very similar substituents. (So, no tricks with bonded slightly modified enantiomeric pairs are needed.)
An example is 5-ethyl-5-propylundecane $\ce{CH3[CH2]5-\overset{$*$}{C}(CH2CH3)(CH2CH2CH3)-[CH2]3CH3}$,[2] don't call it “butyl(ethyl)hexyl(propyl)methane”, found e.g. in beans. Its specific rotation is $[\alpha] < 0.001$.
Another, more common example are fats, i.e. triglycerides, $\ce{RCOOCH2\overset{$*$}{C}H(OCOR')CH2OCOR''}$,[1] if containing e.g. only palmitic, oleic and similar long acyls, optical rotation is not demonstrable.[3]
(Related topic is chirality in polymers, see e.g. Q: Chirality on Carbon of PVC molecule.)
References:
- Mislow K. & Bickart P.: An Epistemological Note on Chirality. Israel Journal of Chemistry 15, 1–6 (1976)
- Wynberg H., Hekkert G.L., Houbiers J.P.M. & Bosch H.W.: The Optical Activity of Butylethylhexylpropylmethane. Journal of the American Chemical Society 87, 2635–2639 (1965)
- Schlenk W.: Synthesis and analysis of optically active triglycerides. Journal of the American Oil Chemists’ Society 42, 945–957 (1965)
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4$\begingroup$ Wow this answers a question I've mulled over for years! I had no idea there was a term for this. Thanks! $\endgroup$ Commented Oct 16, 2018 at 23:28
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4$\begingroup$ @NicolauSakerNeto, slightly off-topic, but you might also be interested to know that these cryptochiral molecules are capable of asymmetric induction in the Soai reaction. That's one of the few reactions we know (I think it might be the only one) where a tiny enantiomeric excess in the system (which could theoretically be just stochastic) can be propagated, leading to significant levels of enantioenrichment in the product, so it's somewhat relevant to the origin of homochirality. $\endgroup$ Commented Oct 16, 2018 at 23:36