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

In summary, the reason molecules rotate plane polarized light is because of the interaction between the light and the electron cloud of the molecule. This explanation also addresses the question of why achiral molecules are not optically active, as they do have electron clouds but it is the overall asymmetry of the molecule that determines optical activity. While some non-enantiomorphic point groups may exhibit optical activity for certain directions of incident light, this fact has not been extensively studied in molecular chirality. The review by Bart Kahr delves into this topic and provides more details on how to determine if an achiral molecule would be optically active. Overall, optical activity is a property of a macroscopic sample of molecules, not a single molecule, and is determined by
  • #1
CrimpJiggler
149
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, 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?
 
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  • #2
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.
 
  • #3
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.
 

1. Why do only chiral molecules rotate plane polarized light?

Chiral molecules have a unique three-dimensional structure that causes them to interact with polarized light in a specific way. This interaction is known as optical activity, and it is only present in molecules with chirality.

2. What is chirality and how does it relate to optical activity?

Chirality refers to the property of a molecule having a non-superimposable mirror image. In other words, it is a molecule that cannot be rotated or flipped to match its mirror image. This property is what allows chiral molecules to interact with polarized light in a way that causes it to rotate.

3. Can achiral molecules also rotate plane polarized light?

No, achiral molecules do not have the same three-dimensional structure as chiral molecules, so they do not interact with polarized light in the same way. Achiral molecules are symmetrical and their mirror images can be superimposed, making them unable to rotate plane polarized light.

4. How do we measure the amount of rotation caused by a chiral molecule?

The amount of rotation caused by a chiral molecule is measured using a polarimeter. This instrument shines polarized light through a sample of the molecule and measures the angle of rotation. The specific angle of rotation can be used to identify the molecule and its concentration in a solution.

5. Are all chiral molecules optically active?

No, not all chiral molecules are optically active. Some chiral molecules have equal amounts of both the left-handed and right-handed versions, which cancels out any rotation of polarized light. These molecules are known as racemic mixtures and are not optically active.

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