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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: PoetryInMotion on July 24, 2016, 07:52:30 PM
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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 (https://en.wikipedia.org/wiki/Optical_rotation), 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 :)
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Optical activity is a direct result of chirality.
There are conformationally mobile systems that may seem like chiral compounds. For instance, (cis)-1,2-dimethylcyclohexane exhibits an non-superimposable enantiomer through its ring flip. However it is not chiral because the ring flip rapidly occurs at room temperature, rendering it by definition achiral.
I'm not sure what you mean when you say a not chiral solution would be symmetric in all directions, nor can I provide a rationale for why this occurs other than by definition.
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Thanks for your reply!
As for my argument about "symmetry in all directions", that is very much an abuse of the word symmetry, and when I think about it I'm not even sure what I mean any more.
[...] nor can I provide a rationale for why this occurs other than by definition.
What do you mean follows by what definition?
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I wonder if the type of chirality that the BINAP-ligands give optical activity?
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It's called axial chirality.
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Thanks for your reply!
As for my argument about "symmetry in all directions", that is very much an abuse of the word symmetry, and when I think about it I'm not even sure what I mean any more.
[...] nor can I provide a rationale for why this occurs other than by definition.
What do you mean follows by what definition?
As in, optical activity is a defining measure of chirality.
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As in, optical activity is a defining measure of chirality.
I don't follow. Is your point that my mirror-image-based definition of chirality (see definition 1 in my original post) is wrong? Do you mean that chirality instead should be defined through the concept of optical activity?
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Hi PoetryInMotion,
I think you're making this way more complex than it needs to be. Your two definitions are correct. And then, you need to know that yes all solutions of chiral compounds (when enantioenriched or enantiopure) will rotate the plane of polarized light (except, obviously, for some exceptional cases). Since solutions of achiral compounds won't, then optical activity is a property exclusive to chiral compounds. And so by abuse of language, "chiral" and "optically active" are often taken to mean the same thing (even if they don't.)
If you want a more approachable discussion than the one on Wikipedia, look at the one in March's Advanced Organic Chemistry. It's very good and succinct, and that's where I direct my students to introduce them to the concepts.
Also, note that there is a very real historical reason behind the fact that "chiral" and "optically active" are often taken to be the same: Before the advent of modern chromatographic methods using a chiral stationary phase, the only method that was widely used and usable to characterize chiral compounds and determine their enantiopurity was optical activity studies. This has now largely been supplanted by "chiral GC" and "chiral HPLC" for a variety of reasons.
As in, optical activity is a defining measure of chirality.
Hi j.k.11,
I understand what you mean by this but you have to word it more carefully. Chirality is not a property that can be measured: an object is either chiral or it's not. The degree of chirality (which is a concept that does not exist) doesn't increase when the optical activity increases.
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Thanks for a great reply -- I'll definitely have a look in March!
except, obviously, for some exceptional cases
Aha! So there are exceptions? Do you have any particular compound in mind?
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Well, I think only Woodward and Hoffman had the chutzpah to suggest that a given chemistry rule can have no exception...
If you are looking for a more extended discussion, you should look into the bible of stereochemistry: Eliel's masterful Stereochemistry of Organic Compounds (1994). You'll find exceptions in there.
In any case, one example that's well documented can be found in Tetrahedron 1974, 1923/Tetrahedron Lett. 1969, 3121. In this, Horeau shows that the value of the specific rotation of solutions of the enantiomer of a specific compound can have a positive or negative value (or even zero!) depending on the concentration, even though specific rotations are supposedly independent of concentration. If you can, go and dig up these articles: there is a beautiful hand-drawn (!) plot of the specific rotation values as a function of concentration.
But make no mistake: Exceptions are few and it's generally a safe assumption that solutions of a nonracemic chiral compound will rotate the plane of polarized light.
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@lb2: Thank you so much! Your clear, detalied and well-written answers are very much appreciated!
Also, great references! I looked up the Horeau articles right away, and I must say, few things beat finding the answer to a question in a French, partially typewritten article :D
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No problem, happy to help.
If you like the exoticism of older papers, you may like the 3D-plots-before-the-advent-of-computers, for example on p. 340, of Chem. Rev. 1977, 313. They're a thing of beauty!
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Consider looking into substitute allenes. This should help grow your understanding.