greg1917 Posted February 11, 2003 Posted February 11, 2003 Right, this has been annoying me lately. In every chemistry textbook or website that covers optical isomerism, you'll find the same thing. A little paragraph explaining chirality, usually acoompanied by an explanation using hands, mirror images but not superimposable etc. The other section you'll see is how you can tell one enantiomer from another; by shining plane polarized light through them. One enantiomer rotates the plane through a certain angle, the other enantiomer through the same angle but in the opposite direction. This is all fair enough, but what I cant find anywhere is WHY? What is it about a certain arrangment of atoms that can rotate the plane of polarized light? I can understand dichroic crystals because they at least have an arrangement of ions, so in that situation the electric field would have an effect on the electric oscillation of light waves hence the polarization. But why would say, d-butan-2-ol have any effect on the orientation of oscillation of light waves and l-butan-2-ol the same but in the opposite direction? What character of the molecule does this? Any help would be much appreciated.
aman Posted February 12, 2003 Posted February 12, 2003 Good question, I gotta do some research myself cus I'm old and I forget. Just aman
greg1917 Posted February 12, 2003 Author Posted February 12, 2003 I thought that maybe all molecules do this, like when you shine plane polarized light through say, methane, maybe each molecule rotates the plane as well. But due to methane not being chiral there would be a totally random spread of the molecules in every possible orientation in space hence no net light plane rotation. I should stress that this is only me guessing and still doesnt come any where nearer to actually explaining what property it is of a certain arrangement of atoms that can have any effect on the plane of polarized light.
Radical Edward Posted February 12, 2003 Posted February 12, 2003 essentially, the refractive index in a crystal may be dependent on the polarisation of the light entering the crystal. If the refractive idex is different for two diffferent polarisations (say different for the x and y directions, where z is the direction that the light travels along), then a phase shift is experienced by one of the polarisations relative to the other (becaise one polarisation can travel quicker than the other), this may then result in the polarisation of the light entering the crystal being changed, depending of course on the thickness of the crystal and also the wavelength of the light. This is essentially what is responsble for quarter wave plates and so on. as an interesting note, sellotape is actually a pretty good quarter wave plate. orient two polarisers so that they are orthogonal, and no light passes through them, and then put a piece of tape between them and you'll see plenty light come through because the tape has changed the polarisation. nifty.
greg1917 Posted February 22, 2003 Author Posted February 22, 2003 So no-one knows why optical isomers rotate plane polarized light? I still cant find an answer to this anywhere, a friend at uni said hed ask his lab super but he hasnt got back to me yet.
Radical Edward Posted February 22, 2003 Posted February 22, 2003 I guess you didn't read what I said then.
greg1917 Posted February 22, 2003 Author Posted February 22, 2003 I did read what you said and while it mentions crystals and refraction, I was specifically looking for some kind of explanation for light plane rotation in optical isomers like D and L carvone, or butan-2-ol. I cant find the answer to this anywhere and no one has come forward with any sort of explanation. Do non chiral compounds rotate plane polarized light in both directions? Ive never had a problem with organic chemistry before but textbooks dont seem to want to say why this happens, only that it does.
Radical Edward Posted February 22, 2003 Posted February 22, 2003 the crystalline structures will set up structures that have differing refractive indices along different axes. the opposite chiral material will have the refractive indices arranged differently, and hence the rotation would be different too. whether a material rotates the plane of polarisation will depend on the arrangement of molecules, as I mentioned, sellotape does.
greg1917 Posted February 24, 2003 Author Posted February 24, 2003 the crystalline structures will set up structures what? Im specifically asking about optical isomers like carvone and most amino acids, why should a hydrogen or a hydroxyl group plus maybe an ethyl group and an amino group all around a carbon rotate light, while a different arrangement of the same groups around the same carbon atom has the opposite rotation? When naming optical isomers you go round groups by mass to say which is R and L, so surely it will be due to different electron densities in different groups which would exlpain why the electrical vector was rotated in the orietation of whatever that enantiomers chiral orientation is - the (+) enantiomer of one compund could only occupy certain orientations in space so its spread of electron rich or electron defficient areas could occupy the same limited orientations. Again the same being true for the (-) enantiomer but obviously a different set of orientations. If its not electron dense areas of chrial compunds what is it because there is very little else that could affect the electrical vector of a light wave to my knowledge. I presume ionic fields in dichroic crystals do similar things to the electrical vector but these are two different types of substances, carvone is not remotely crystalline.
Radical Edward Posted February 24, 2003 Posted February 24, 2003 well I don't know specifically about the materials that you are talking about since my work is in the optical rather than the solid state properties of the media. I took the crystalline stacking as a reasonable assumption to make, but I guess it looks like this isn't (at least) always the case. I would still guess that the arrangement of the molecules because of their shape has something to do with it, and also the ways in which the molecule is allowed to actually vibrate when in an excited state. Just as a point of interest, is emitted light (rather than transmitted) from these material polarised at all? I'd look into it, but I don't have access to journals for a couple of weeks.
greg1917 Posted March 2, 2003 Author Posted March 2, 2003 i eventually got all the answers i needed and also what happens when there are two chiral carbons in the one molecule. http://hexane.chem.uiuc.edu/cyberprof/public/chemistry/102x/Lecture/lect21c.html
Radical Edward Posted March 2, 2003 Posted March 2, 2003 ace, thanks. There are still some things I wonder about it though....
YT2095 Posted November 24, 2004 Posted November 24, 2004 Massive thread bump, but I have a question... Optical isomers can and do exist in Crystaline form, I think some sugars do this. as with polarised film or glass, a 90 degree rotation will block light and a 180 will allow it was through again (think numbers on a digital watch). why then with a random scattering of such crystals in a bag, can`t we have Black Sugar?
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