Stereochemistry

The stereochemistry is a branch of chemistry that deals in two aspects:

• The doctrine of the three-dimensional structure of molecules having the same chemical bonding and composition but a different arrangement of the atoms, the constitution, configuration and conformation determine the three-dimensional structure of the molecule ( stereochemical isomerism )
• The doctrine of the spatial sequence of chemical reactions of stereoisomeric molecules ( stereochemical dynamics).

The study of stereochemical phenomena extends to the whole field of organic, inorganic, physical and supramolecular chemistry and biochemistry.

Historical Development

After development of the Atomic Theory by John Dalton, the considerations went by how the atoms could be arranged spatially. Already in 1808 postulated William Hyde Wollaston for compounds of the type AB4 tetrahedral arrangement. Verbose also André -Marie Ampère in 1814 dealt with the three-dimensional arrangement of atoms in molecules, especially in crystals, where he explained various forms by the telescoping of tetrahedra and octahedra. Leopold Gmelin developed in the 1840s, a core theory for the structure of organic compounds. After that was " ethene " the root core with a cubic structure, from which ableiteten other compounds. At least he proposed his theory to more serious clashes with the spatial structure of organic compounds.

Louis Pasteur succeeded in 1848, the first resolution by sorting enantiomeric crystals of tartar. Pasteur also suspected first that the phenomenon of optical rotation, which was discovered by Jean -Baptiste Biot in 1813, was due to the existence of mirror-image molecules.

Both Archibald Scott Couper and Friedrich August Kekulé postulated in 1858 that carbon atoms could be linked to each other and put on appropriate formulas that were present constitutional formulas very similar.

The onset after 1865 discussion on the formula of benzene was stereochemical nature, because it also makes three-dimensional structures have been considered. John Wislicenus employed in the 1860s with Milchsäureisomeren whose optical activity and three-dimensional structure.

The real lesson of the spatial arrangement of atoms was suggested in 1874 by van't Hoff and Joseph Le Bel and is based on three developed in the 19th century, knowledge of the atomic theory and the tetrahedral bonding of carbon, the structure theory of chemical bonding and optical rotation. So van't Hoff continued the run by Emil Erlenmeyer and John Wislicenus discussion on the constitution of lactic acid. Furthermore, he suggested that different structural formulas of isomeric compounds ( hydrocarbons, alcohols, organic acids) with asymmetric carbon atoms, the tetrahedral model this was an important basis. He also developed a concept of the double bond between carbon atoms, which have a common edge in two tetrahedrons.

Another milestone finish the works of Emil Fischer on the structure of carbohydrates and the description of their stereochemistry by the Fischer projection represents the work Fischer were awarded the 1902 Nobel Prize in Chemistry.

Alfred Werner's studies on the stereochemistry of coordination compounds of cobalt, which in 1913 also received the Nobel Prize in Chemistry, can also be viewed as the beginning of complex chemistry.

Even Vladimir Prelog research into the stereochemistry of organic molecules and reactions were awarded the 1975 Nobel Prize. Co-developed by him Cahn -Ingold -Prelog convention is used to stereochemical description of organic molecules.

Isomerism

Isomerism occurs when molecules may have different spatial structures with the same molecular formula. The conformational due to the rotation about a single bond of a molecule, such that the substituent of the linked via a single bond, the atoms may take mutually different positions. Molecules which are differ only in this specific arrangement of atoms called conformers. Spiegelbildisomerie occurs when chemical compounds that behave to a counterpart as its mirror image. The corresponding chemical compounds are called enantiomers or optical antipodes. This type of isomerism may be based on a stereo center, a chiral axis or planar or helical chirality. From the group theory shows that the absence of a rotating mirror axis is the necessary and sufficient condition for the occurrence of enantiomers. Diastereoisomerism occurs when multiple stereocenters present in these molecules partly in the same and partly in various configurations.

Funktionsisomerie occurs when molecules have different functional groups with the same molecular formula. As Skelettisomerie refers to the existence of different scaffolds. Positional occurs in molecules, in which the same functional group may occur at various positions in the framework. The cis -trans isomerism or (Z ) - ( E) isomerism is a special form of positional isomerism. They occur in compounds in which different two or more with respect to the position of the substituents with respect to a reference plane. As linkage isomerism or valence isomerization is defined as the occurrence of different numbers σ - and π - bonds in molecules.

Stereochemical terms

Symmetry properties and operations

The stereochemistry of treating the symmetry properties of the molecules. The molecular symmetry may have axes of symmetry, a center of symmetry or a plane of symmetry. There are four fundamental operations of symmetry, mirroring the rotation and inversion, as well as solids translation.

Symmetry axis Cn

The symmetry, or axis of rotation axis describes a molecule in which, by rotation of the molecule by the rotation angle 360 ° / n, the new arrangement of the atoms in the molecule with the previous one is identical. An example of a molecule of the C2 point group is water, an example of the point group C3 is ammonia. Benzene has both a C2 -axis and a C6 -axis.

Plane of symmetry σ

Symmetry or mirror plane describes a level in the molecule, which divides the molecule into two halves symmetrically matching. Depending on where the symmetry plane lies in the molecule differentiates planes along the main axis of the molecule, referred to as σv (from vertical). Passing the plane perpendicular to the major axis of the molecule it is referred to as σh (from horizontal), planes, which extend diagonally, are called σd.

Inversion center i

An inversion center of symmetry or transferred all atoms by reflection at a central point in symmetry- equivalent atoms. Molecules with an inversion center are non-polar. Molecules with even the inversion center is not located on an atom of the molecule, for example in benzene, in the odd-numbered molecules inversion center, the carbon atom is incident on one atom, for example carbon dioxide.

Rotating mirror axis Sn

A rotating mirror axis converting the atoms in a molecule by a rotation through an angle of 360 ° / n, and then in mirror symmetry equivalent atoms. The mirror plane is perpendicular to the axis of rotation. The symmetry element S1 corresponds to a plane of symmetry σ, the symmetry element S2 corresponds to an inversion center i