Background. In most organic text books the words "chiral" and "optically active" is used interchangeably, but I have never seen a good justification of this. To me, these two concepts are defined in such a way that there is no obvious equivalence between them at all:
Definition 1. A compound is
chiral if its molecules are non-superimposable to their mirrior image (possibly after some rotation around σ-bonds).
Definition 2. A compound is
optically active if it rotates linearly polarized light.
So... what's going on here? Are the two concepts really equivalent to each other? I.e. are all chiral molecules optically active, and vice versa? And if so, why?
My own thoughts. I may be able to argue for an implication from optically active to chiral. If a molecule was
not chiral, a solution of it would, statistically speaking, be symmetric in all directions. Hence, there would be no way for the photons of the polarized light to tell right from lefter, and hence, its difficult to imagine any net rotation taking place.
As for the implication in the other direction, I have no idea. Are all chiral molecules necessarily optically active? I guess it all comes down to the quantum physical phenomenon that causes the rotation in the first place, but I haven't been able to understand how that works. I found a lot of text about it at
wikipedia, but I immediately get lost in all the physics. Maybe someone here knows about any simpler introduction to this topic, or knows a way to explain this themselves?
All input is much appreciated!
Edit: This may actually be more of a physchem question than one about orgo. If a moderator agrees, feel free to move the thread