Why are chiral centers optically active?

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I was thinking about this now and realized that I don't know the answer.

I was under the impression that the presence of "different substituents caused like to bend about the molecule."
But considering GLYCINE is not optically active (has 3 different kinds of subs)
upload.wikimedia.org/wikipedia/commons/2/2c/Glycine-3D-balls.png

But a Chiral center is optically active (has only 2 different kinds of subs)
http://chm233.asu.edu/notes/chiralit...s/image090.png


How exactly does the "ability to be superimposable" allow a molecule to be optically active? Especially considering working with just one enantiomer.

The micro-physical properties by which chiral molecules interact with plane-polarized light are way, way out of the scope of the MCAT. Just know that if a substance has no ability to bend and spin until it overlaps with its mirror image, it's chiral, and a uniform sample of one enantiomer of a chiral molecule will bend plane-polarized light.

The example of chirality you gave, CH3CH=C=CHCH3, is a really weird case, but I guess if you understand that one, you'll understand them all! It cannot be superimposed on its mirror image because those two double bonds don't rotate, and hold their pi electrons at right angles to each other, causing one CH3CH pair to be in a plane at right angles to the other. But I cannot imagine the AAMC asking us about a case like that without giving us the wedge-and-dash image; and the wedge-and-dash image shows that right-angle relationship.

Upshot: take "chirality (non-superimposibility of mirror images) --> optical activity" as a given, unless you want to take, like, fourth-year physical chemistry to get the details (I haven't, it scared me off!).

great thanks

It's optically active because light is polarized differently when it passes through a certain configuration of electrons/protons/neutrons. This configuration would, on average, be the same across two mixtures of certain molecules if those certain molecules could rotate and otherwise spontaneously interchange between themselves.

If those molecules cannot spontaneously rotate and interchange, then obviously light would be bent into two different polarities when it passes through two such different mixtures.

Why light does that is a topic for a higher class of physics.

Most chiral molecules have a chiral center (that is what you describe as 4 different substituents), what you found with the consecutive double bonds is usually explained in the chapter of the O.Chem book that deals with dienes. Basically they are unique and have what is called a Chiral Axis, rather than a chiral center. This pretty much only occurs with consecutive double bounds; otherwise, just look for chiral centers.