Chiral Alkene

Consider this compound, which is in E-configuration and that red asterisk denotes the chiral carbon. If I were to somehow cause the E-configuration of the alkene part to Z-configuration keeping the rest of the molecule same, would it have the same optical rotation?


Not necessarily. Although the change you suggest would not alter the absolute stereochemistry (S in both cases) of the asymmetric center starred, the two molecules are diastereomers and therefore will have different physical and optical properties. Even if they did both rotate light the same way, they probably won’t have the same degree of rotation for the same concentration (specific rotation).

The problem boils down to whether we can predict the direction (or degree) of rotation from a drawn absolute stereochemical configuration. The answer to this is still “no”. We don’t currently have a theory that will allow us to say that, for a certain molecule, R gives us d or l, etc.

Some studies have been able to come up with some rules of thumb for small molecules, but there is no comprehensive theory to make this prediction in general. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2704476/

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  • $\begingroup$ So there is no clear way of predicting the effects of such a change? $\endgroup$ – user79161 Apr 26 '19 at 10:49
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    $\begingroup$ Not consistently. The rotation of plane polarized light it thought to originate from the interaction of the four groups off the asymmetric center with the two directions of circularly polarized light that are coincident in the plane polarized light. If one direction is slowed more than the other (circular dichroism: ja01.chem.buffalo.edu/~jochena/research/opticalactivity.html) then the plane of the polarized light is seen to rotate. We can't yet accurately predict how changing one of the four groups (like your E to Z change) will affect this interaction. $\endgroup$ – Withnail Apr 26 '19 at 11:41
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    $\begingroup$ There is a great post on Physics Stack Exchange that deals with more of the details of how this effect could, in principle, be predicted from a molecule's structure. But I don't believe that studies in this field are comprehensive yet: physics.stackexchange.com/questions/15503/… $\endgroup$ – Withnail Apr 26 '19 at 11:47

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