[walkerke]

i am very confused about what constitutes an optical isomer. I know about the rotation of polarized light but where does the chirality come in?
 
Here is the main issue. I have to draw structures from names like 3-chloro-2,2,5-trimethylhexane and figure out whether it is optically active. Where do I start?

[movies]

Any chiral molecule will rotate plane polarized light to some extent as long as it is optically pure.  Racemic mixtures don't rotate plane polarized light because the two enantiomers "cancel" each other out.  Achiral compounds don't have a chiral center to rotate the light.  Meso compounds are not chiral, so they won't rotate plane polarized light either.

That should be enough to get you started.  Try drawing out all the possible stereoisomers.

[Donaldson Tan]

chiral molecules lack the plane of symmetry

[Mitch]

More specifically chiral molecules lack an improper rotation which involves a plane of symmetry and then a rotation.

I should probably write a group theory article at some point.