Optical Activity & Carvone

From WikidChem

[edit]

Slide 22

The amount by which a material rotates polarized light is another quantity used to distinguish between isomers (called specific rotation). It is measured in deg cm2/g. A good measurement must include the color of light used (usually the D line produced by a sodium lamp), the temperature, and the solvent used. Positive values indicate clockwise (right) rotation, negative values indicate counterclockwise (left).

[edit]

Slide 23

The two samples of carvone passed around in class had two different odors and two different specific rotations. They are isomers, though they are extremely similar in behavior (as all the other measurements are the same). The rye bread odor-isomer has a positive specific rotation, the spearmint odor-isomer has a negative specific rotation.

[edit]

Slide 24

This slide is the payoff for the last two. The only discernible difference in the two isomers is the position of one hydrogen atom; their chemical formulas, and the 2D structural drawings we can make of them, are the same. The 3D models are what show a difference.

The whole reason why we're interested in specific rotation is because it provides another tool to understand what's going on with isomers. It's something that often differs in isomers that are extremely similar, but with minute structural differences. So it also illustrates the importance of considering a molecule's configuration, as the 19th-century scientists refused to do.

The obvious question is whether our 3D models correspond to reality. But the simple match of such a complicated model and the molecules' behavior argues that they may be plausible.

Vocabulary note: In section, Josh used the term "enantiomer", which refers to this behavior. Enantiomers are just isomers like these, that are structural mirror-images of each other. Stereoisomers are isomers that differ only in configuration. We have not covered these terms in lecture, but they do appear on the practice tests.

BRS, 11:02 AM 16NOV06