Optical Rotation Demonstration (Karo & Carvone)

From WikidChem

Optical_rotation_demonstration_(karo_&_carvone)

see attachments: Polarized Films.JPG Yellow.JPG Blue.JPG Mirror image.JPG Kaliapparat and McBride.JPG (Sorry these links dont work anymore, I dont know how to fix them, -GJB

Sorry for that. the file upload function is disabled on this website. My pictures...  7o7)

We were shown a demonstration in class that illustrates the idea that certain materials can rotate polarized light. These materials are called "optically active." The demonstration in class consisted of an overhead projector with a sheet of polarizing material on top of the light source, causing the light that was projected on the screen to be polarized to a certain plane, meaning that the electric field all the light waves coming through was oscillating in a certain direction perpendicular to the direction of motion. Above that, another sheet of polarizing material was held. When it was oriented in the same way as the bottom sheet, light went through. When it was oriented 90 degrees in either direction, no light travelled through. The first demonstration consisted of a beaker of water placed on the projector. When the second sheet of polarizing material was held above it and rotated, the result was exactly as before, whereas light would not travel through when the sheet was rotated 90 degrees. Therefore water is not optically active.

Next, the water was replaced by a beaker of Karo syrup. Without a sheet of polarized material on top, the light travelling through the Karo syrup appears to be normal light. But when the top sheet is held above and rotated, a full spectrum of colors is seen depending on the angle of rotation, and when it is rotated a full 90 degrees still no light gets through. (Oops. This is not what happened. See if you and your critics can explain why there are colors.) This indicates that Karo syrup is optically active, and is changing the orientation of the oscillations of different wavelengths of light.

When the polarized paper was rotated above the karo syrup, a full spectrum of colors was seen. Because the syrup rotates each wave length of light a different amount, the polarized paper let through each of the different wave lengths (different colored light) at a different angle of rotation. For instance, say karo syrup rotated red light by 10 degrees, orange light by 15, yellow light by 20, and so on. When the paper was turned 10 degrees, red light would shine through, and the same with the rest. This is why there was a different colored light at every angle of rotation of the polarized paper. See the attached pictures

The final two demonstrations were of two types of carvone. The only noticeable difference between the two samples was odor, where one smelled of rye bread and the other of pepperment(spearmint?). One of the samples was placed on the projector and the polarization sheet was held above it and rotated. At a full 90 degree rotation, no light made it through the surroundings of the carvone, but light travelling through the carvone was visible. When the sheet was rotated about 10 degrees to one side then no light was visible through the carvone but instead some light was visible around it. That carvone sample was then replaced with the other sample. The same phenomenon as before occured when the two sheets of polarization material were oriented 90 degrees from each other, but only when the sheet was rotated 10 degrees in the opposite way as before did light not pass through the carvone. This demonstration indicates that carvone is optically active, and rotates light slightly in one direction. The direction of rotation depends on which optical isomer of carvone is used, one will rotate it to the left as it approaches the viewer ("levorotatory"), the other to the right ("dextrorotatory").

The powerpoint slide associated with the demonstration shows the way to write the "specific rotation" of a certain material. Specific rotation is a quality that all optically active materials have, and is a way to characterize them, because it is a quality that holds true for all solutions of that material (at least in a particular solvent). The way to calculate specific rotation is given as the expression "degrees/ g/mL / dm", where degrees is the observed rotation in degrees for a certain sample, g/mL is the concentration of that sample, and dm is the length of solution through which light is travelling, most likely the height of solution in the beaker, given in decimeters. To alleviate confusion that might arise from having 3 division operators in that expression, let us call concentration "c" and let us group the denominator into one line surrounded by parentheses, so that the expression is now "degrees/(c x dm)" where "x" denotes multiplication. This expression will give the same specific rotation for all tests on a certain material, as long as two conditions are held constant. Those two conditions are given as a superscript and subscript next to the a? (where a is alpha) notation. The top condition is color; the "D" shown on the powerpoint slide indicates sodium D light, which is yellow. The bottom variable is temperature of the solution in Celsius. To relate this concept to the demonstration, water would have a specific rotation of 0, and the two samples of carvone will have equal absolute values for specific rotation, except one will have a negative value and the other a positive one. AS 10:30pm 11/30/06