Stereochemistry?

Question

What does polarize (as in plane-polarized light) mean?

What is the purpose of the polarizer?

and Why do we need to know specific rotation?

and please don't quote from a text book, i need to understand this!!!!!????

Answer 1

light is a particle/wave. Its wave characteristic is as an electromagnetic wave which has an electric field and a magnetic field which are at right angles to each other. These fields are at right angles to the path of travel of the wave [think X Y Z of three dimensional space - if X is the electric field and Y is the magnetic field then the wave is going in direction Z]

In unpolarized light, the fields can point in any direction perpendicular to the direction of travel. A polarizer only lets through the light waves that have a specific direction [which due to quantum effects is half the ones arriving]

An optically active substance will change the orientation of the fields, rotating them slightly. With unpolarized light, this effect could not be seen, light goes in with random orientation, light comes out with random orientation.

But with polarized light, which only has one direction, any change can be seen and measured. The amount of optically active substance determines the amount of rotation.

So if the rotation of a known sample can be determined, the concentration of an unknown sample can be worked out - eg if the light rotated by twice the angle, the unknown sample was twice the concentration of the known sample.

Specific rotation is simply the rotation of a known sample under known conditions. If you reproduce the known conditions (same light, same temperature, same path length etc) you can analyze an unknown sample. And even if the conditions are not the same, you can usually apply corrections.

Answer 2

Suppose that you had a special kind of eyesight that could see actual light waves. Suppose that I shined a flashlight into your eyes. You would see some light waves that were vibrating "up-and-down," others "side-to-side," and a lot that were vibrating in all directions diagonally.

Suppose I put a polarizer in front of the flashlight. An example of polarizers are the lenses in a pair of Polaroid sunglasses. These lenses look "gray," but that's because they only pass light waves that are vibrating in one direction, say "up-and-down." Now you would see only light waves that are vibrating "up-and-down."

Now suppose I put a tube full of optically active liquid in front of the flashlight and the polarizer lens. It can be a liquid compound or a solution of a solid compound. Now you would see that the direction of vibration of light waves was changed from all "up-and-down" to some angle diagonally side-to-side. The compound has rotated the plane of polarization of plane-polarized light. You would write down the angle by which the vibrations of the light waves had changed.

The amount (angle) of rotation depends on the length of the tube, concentration of the compound in solution, temperature, and wavelength (color) of light. So you use an equation that takes allof these things into consideration and gives a specific rotation. So no matter how you change the experiment, as long as the compound is the same, it will always come out to have one specific rotation.