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

[CrimpJiggler]

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, let's 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?

 

[Mike H]

See, for example, this review from Bart Kahr (now at NYU):

http://dx.doi.org/10.1002/anie.200704559

Some relevant quote mining...

Unfortunately, the link between optical activity and enantiomorphism is not only introduced early and reinforced relentlessly in a chemist's education, it is wrong.

It is well-known to some that oriented systems belonging to some non-enantiomorphous point groups will indeed be optically active for some directions of incident light, but this fact, albeit qualified, has failed to make substantive inroads in the science of molecular chirality...

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.

 

[DrDu]

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.