Absolute asymmetric synthesis
The absolute asymmetric synthesis refers to a reaction in which one enantiomer is preferentially formed, d. H. Enantiomeric excess ee > 0%, without an existing chiral center and without the induction of stereochemical information due to other optically active substances present in the system.
This synthesis is not possible according to the previous understanding, but there are a variety of possible theories. Thus, the synthesis of optically active products, i. H. ee > 0%, always a previous chirality information z. B. in the form of an auxiliary or chiral crystallization surfaces.
Few compounds crystallize as a conglomerate , i.e. That is, crystals are formed with the pure enantiomers in the crystal lattice. If the formation of a conglomerate is viewed as an absolute asymmetric synthesis, this is possible for special substances because of the chirality of a large number of the crystallographic space groups. However, it is not a chemical, but a physical process and thus a variant of the racemate separation.
Modern research tries to form conglomerates in a targeted manner by adding auxiliary substances. According to the definition given above, this procedure contradicts an absolute asymmetric synthesis.
Physical aspects
The physical approach to the question of whether there can be an absolute asymmetric synthesis leads to the fact that enantiomers differ in a very small energy difference that violates the parity rules.
This means that the thermodynamic reaction characteristics also differ and an absolute asymmetric synthesis should therefore be possible. The order of magnitude of the energy difference is around 10 −14 J / mol.
Theories of Absolute Asymmetric Synthesis
Nature shows a homochriality in almost all possible substance classes, which can be seen in the examples D- carbohydrates and L- amino acids . This chirality information must have been created by an absolute asymmetrical induction. There are many discussions about what this looks like.
Possible aspects are magnetic fields , radiation with linearly polarized light or the preferred formation of an enantiomer on specific surfaces, e.g. B. Quartz crystals .
Individual evidence
- ^ Siegfried Hauptmann, Gerhard Mann, Stereochemie, Spektrum Akademischer Verlag, 1996.