What makes a molecule optically active?

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Molecules are optically active when they react differently to left or right circularly polarized light. This is because they have a screw-oriented structure which is itself either left-handed or right-handed, making propagation preferential for one polarisation, rather than for the other.

That's all well good, and how is it that molecules become structured in this way? Is there a good reason to become structured in this way, or is it just a fluke of evolution?

I say this because at the beginning of the 20th century, a text is written containing the following statement:

Now I maintain that the original segregation of levo-molecules, or molecules with a left-handed twist, from dextro-molecules, or molecules with a right-handed twist, is absolutely incapable of mechanical explanation.
This argument is based on a point raised earlier in the text. Apparently, at the moment the text was written, it was believed that
All the ordinary physical properties of the right-handed and left-handed modifications are identical. Only certain faces of their crystals, often very minute, are differently placed. No chemical process can ever transmute the one modification into the other. And their ordinary chemical behavior is absolutely the same, so that no strictly chemical process can separate them if they are once mixed.
So, I could ask a more concrete question :

2a) Is it possible to separate levo-molecules from dextro-molecules, by some operation? What is this operation?
2b) What property does such a mixture of molecules require, for such a separation to occur? In other words, what are the constituents that carry such a preferential handedness? What is the property of these constituents related to such handedness?
2c) Can every molecule (or every constituent in a molecule) somehow be adapted to alter this property, thereby changing its handness? What is an operation that can make this happen?

More questions in this line of thought could be asked, as I'm sure you can guess.

So what is to be said about designing optical activity?
 

Answers and Replies

  • #2
Molecules are optically active when they react differently to left or right circularly polarized light. This is because they have a screw-oriented structure which is itself either left-handed or right-handed, making propagation preferential for one polarisation, rather than for the other.

That's all well good, and how is it that molecules become structured in this way? Is there a good reason to become structured in this way, or is it just a fluke of evolution?

I say this because at the beginning of the 20th century, a text is written containing the following statement:



This argument is based on a point raised earlier in the text. Apparently, at the moment the text was written, it was believed that


So, I could ask a more concrete question :

2a) Is it possible to separate levo-molecules from dextro-molecules, by some operation? What is this operation?
2b) What property does such a mixture of molecules require, for such a separation to occur? In other words, what are the constituents that carry such a preferential handedness? What is the property of these constituents related to such handedness?
2c) Can every molecule (or every constituent in a molecule) somehow be adapted to alter this property, thereby changing its handness? What is an operation that can make this happen?

More questions in this line of thought could be asked, as I'm sure you can guess.

So what is to be said about designing optical activity?

this is a fascinating, and unresolved, question.

one hypothesis is that pre-biotic molecules formed in the presence of a strongly polarizing field, and that once the initial "seed" biomolecules formed in this way their method of assembly dictated that all new molecules reproduced would inherit this "handedness". Who knows, it may turn out that chirality is requisite for life, as we know it, to exist.

one of the science experiments aboard the failed Beagle 2 Mars probe was to detect enantiomeric excess of organic compounds - any excess over 50% would be strong evidence of life.

as far as seperation goes, yes enantiomers can be seperated - and it is very difficult. pharma companies are always trying to have enantiomerically pure compounds since (a) usually one form is vastly more active than the other and (b) the other enantiomer may have unknown side-effects (a la the thalidomide disaster).

stereospecific seperations may be as simple as reacting with tartaric acid, or as difficult as doing a very very sensitive distillation (since infact there IS a slight difference in their condensed phase interactions that is revealed as a very small difference in boiling point).
 
  • #3
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Hey, thank you very much for your reply!

Do you have any reference that describe some of the stuff you said? I would be very interested in reading some papers that discuss the condensed phase interactions you mentioned.
 
  • #4
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2a) Is it possible to separate levo-molecules from dextro-molecules, by some operation? What is this operation?
Certainly. Many enzymes only accept one of two enantiomers. Getting this to do something useful for you on an industrial scale though, is the tricky bit, as quetzal correctly elucidated.
 
  • #5
Do you have any reference that describe some of the stuff you said? I would be very interested in reading some papers that discuss the condensed phase interactions you mentioned.
i don't have any references but imagine a fluidic mixture of enantiomers - their intermolecular force interactions will be different than an enantiomerically pure fluid because of the mixed symmetry. In terms of liquid structure, the two radial distribution functions will be slightly different since the geometrical distrance between atoms of neighboring molecules will be different on average. This leads to a difference (albeit very small) in boiling point over an entantiomerically pure fluid.

According to this reasoning, I believe that if you had a fluid of pure L and a seperate fluid of pure D enantiomers, they would both have exactly the same boiling point (im not entirely sure about this though).
 

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