optical isomer synthesis and seperation

[aj47]

If optical isomers have the same physical and chemical properties how can they be seperated or syntheised with out creating a racemic mixture. I know the 2001 noble prize for chemistry was involved in this but can any one give me a link or explain how it works??

[budullewraagh]

take the racemate, react to form a salt, then use the different solubilities of each isomer to fractionally crystallize out one isomer

[akcapr]

whats a racemic mixture?

[Bluenoise]

A racemic mixture contains equal amounts of both optical isomers.

 

To seperate them you a reactive species that's present in only on enantiomer. This will give you two different products that you can seperate. Then you reverse the reaction.

[woelen]

take the racemate, react to form a salt, then use the different solubilities of each isomer to fractionally crystallize out one isomer

I do not believe this works. Enantiomers have exactly the same physical and chemical properties, except when they are brough in contact with other enantiomers and when the change of direction of polarization is involved. In living animals, plants and humans, of many compounds only one enantiomer is used, but chemically speaking enantiomers cannot be distinguished. The solubility in any non-optically active solvent will also be the same for two enantiomers.

[Yggdrasil]

I do not believe this works. Enantiomers have exactly the same physical and chemical properties, except when they are brough in contact with other enantiomers and when the change of direction of polarization is involved. In living animals, plants and humans, of many compounds only one enantiomer is used, but chemically speaking enantiomers cannot be distinguished. The solubility in any non-optically active solvent will also be the same for two enantiomers.

 

This is true if you react with a non-chiral acid/base to form an enantiomeric salt. However, usually people will reach a racemic mixture with a chiral acid/base to form a diastereomeric salt. Since diastereomers exhibit different chemical and physical properties, it is possible to fractionally crystalize diastereomeric salts and recover one enantiomer from a racemic mixture. For example (R,R)-diaminocylclohexane can be isolated from a commerical mixture of 1,2-diaminocylcohexane by forming a relatively insoluble salt with L-(+)-tartaric acid (see Walsh, et al. J. Chem. Edu. 1998 75(11): 1459).

[budullewraagh]

i did not mention any solvent in particular. i should have mentioned use of, say, methanol. while enantiomers may have the same physical and chemical properties, diastereometric salt complexes from each enantiomer of a chemical do not have the same physical and chemical properties. after forming these salts, fractional crystallization can occur

[latentheat]

whats a racemic mixture?

 

An equal mixture of the two optical isomers of a substance: dextrorotary and levorotary. When the optical isomer is important in a particular situation, the name of the chem will include the bolded part.

 

The dextro and levo rotary refer the the direction the molecules rotate plane polarized light.

[gothchemist]

While dextrorotary can be used to designate the (+)-isomer and laevorotary the (-)-isomer of a particular compound, compounds labeled D (dextro) and L (laevo) are not necessarily the + and - isomers repectively. The D and L terminology is from the early days of chemistry, in a time before X-ray crystallography when proof of configuration of a compound was carried out via degradation from a isomerically pure source typically the two isomers of glyceraldehyde with the + isomer being designated D and the - L. Unfortunately when D-glyceraldehyde is degradated to the structurally related D-lactic acid the isomer is the - one not the +. The D and L terminology are use to related molecules as being structurally related to either D or L gyceraldehyde not the direction of rotation of plain polarised light. This designation is not used today except in areas estabalished by tradition such as sugars and amino acids. Today enantiomers are designated either +/- if the configuration is unknown or R/S if it is known. A full explaination of this is found here : http://www.cem.msu.edu/~reusch/VirtualText/sterism3.htm#isom13.

[gothchemist]

Methods of Separation:

 

Chiral Chromatography: Pass a solution of the racemic mixture through a column containing a chiral solid matrix (one isomer of a chiral solid) and the different enantiomers will have different elution times.

 

Enzymatic resolution: Take for example a racemic mixture of chiral alcohol i.e 50:50 add enzyme that only oxidises one enantimer to the ketone (you then get 50% of one enantiomer of the chiral alcohol and 50% of the corresponding non chiral Ketone) and then reduce the ketone back to the alcohols using standard chemistry e.g borohydride (which will turn the 50% ketone component into a 50:50 mixture of the two enantiomers of the chiral alcohol, so you end up with a 75:25 mixture of the two alcohol enantiomers overall). If you then repeat the oxidation and reduction steps again you get a 87.5:12.5 mixture, and again 93.75:6.25. If you repeat above procedure six times you will end up with greater than 99% of the enantiomer of the chiral alcohol that isn't oxidsed by the enzyme.

[John Cuthber]

There is another way of separating racemic mixtures. It doesn't always work but it has historical importance because it's the first method that got reported (By Pasteur iirc).

With some racemic mixtures the molecules prefer to associate with others of the same type so, if you crystalise the stuf slowly, you get 2 sorts of crystals- one set are right handed and the others left handed. You can then sort them out with a pair of tweezers (Of course this only works because you can tell left from right and that, in turn, means that you are chiral).

[gothchemist]

As an addition to the previous post: the molecule Pasteur separated was tartaric acid. Here is a link: http://en.wikipedia.org/wiki/Louis_Pasteur

[Tetrahedrite]

Interestingly, the infamous drug Thalidomide is a chiral compound and was originally sold as a racemic mixture (the results of which are now well known). However, only one of the enantiomers has the horrible effects, and the non-"toxic" enantiomer is now being used again to treat a variety of diseases.

 

The Thalidomide experience actually changed the testing regime for new drugs. For chiral drugs to be approved, each enantiomer must be trialed separately, as well as in a racemic mixture.

[aj47]

There is another way of separating racemic mixtures. It doesn't always work but it has historical importance because it's the first method that got reported (By Pasteur iirc).

With some racemic mixtures the molecules prefer to associate with others of the same type so' date=' if you crystalise the stuf slowly, you get 2 sorts of crystals- one set are right handed and the others left handed. You can then sort them out with a pair of tweezers (Of course this only works because you can tell left from right and that, in turn, means that you are chiral).[/quote']

 

If I remember rightly, the methods aptly named crystal picking