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Why is it only chiral molecules that rotate plane polarized light?

  1. Dec 19, 2011 #1
    I just read that the reason molecules rotate plane polarized light is because the light interacts with the electron cloud of the molecule. That makes sense but why aren't achiral molecules optically active? Achiral molecules have electron clouds too so why don't their electron clouds cause optical rotation? As an example, lets say a 2 carbon alkene with 4 different substituents i.e. 1,1-hydroxychloro-2,2-iodoaminoethylene. This molecule is achiral but it has an irregular shaped electron cloud so why doesn't plane polarized light rotate when it interacts with that irregular shaped electron cloud?
  2. jcsd
  3. Dec 19, 2011 #2
    See, for example, this review from Bart Kahr (now at NYU):

    "Optical Rotation of Achiral Compounds."

    Some relevant quote mining....

    Anyway, there are a bunch more details - as well as how to determine whether an achiral molecule would be optically active - in the review, as well as a look back through the literature on this topic up until 2008.
  4. Dec 20, 2011 #3


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    You are right, every molecule scatters light and may also rotate the polarization, however what one usually calls optical activity is a property not of a single molecule but of a macroscopic sample of molecules. So even if a single molecule can do all kinds of sort of scattering to a photon, an isotropic sample of some substance or solution will only rotate the polarization of a macroscopic amount of photons if the molecules are chiral.
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