Stereochemistry

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Mirror image isomerism in lactic acid

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

  • The theory of the three-dimensional structure of molecules that have the same chemical bond and composition but a different arrangement of the atoms , whereby the constitution , configuration and conformation determine the three-dimensional structure of the molecule (stereochemical isomerism)
  • the study of the spatial sequence of chemical reactions of stereoisomeric molecules (stereochemical dynamics).

The study of stereochemical phenomena covers the entire field of organic , inorganic , physical and supramolecular chemistry as well as biochemistry .

historical development

Louis Pasteur

After the development of the atomic theory by John Dalton , the considerations went to how the atoms could be spatially arranged. As early as 1808, William Hyde Wollaston postulated a tetrahedral arrangement for connections of the type AB 4 . In 1814 , André-Marie Ampère also dealt extensively with the three-dimensional arrangement of atoms in molecules, especially in crystals, explaining different shapes by pushing tetrahedra and octahedra into one another. Leopold Gmelin developed a core theory for the structure of organic compounds in the 1840s. According to this, “ethene” was the stem core with a cubic structure from which other compounds were derived. At least with his theory he stimulated further serious discussions about the spatial structure of organic compounds.

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

Both Archibald Scott Couper and Friedrich August Kekulé postulated in 1858 that carbon atoms could also be linked to one another and set up corresponding formulas that were very similar to today's constitutional formulas .

The discussion about the formula of benzene that began after 1865 was of a stereochemical nature because three-dimensional structures were also taken into account. Johannes Wislicenus dealt in the 1860s with lactic acid isomers , their optical activity and three-dimensional structure.

The actual theory of the spatial arrangement of atoms was suggested by van't Hoff and Joseph Le Bel in 1874 and is based on three findings of the atomic theory and the tetrahedral bond of carbon, the structural theory of chemical bond and optical rotation developed in the 19th century. So van't Hoff followed up on the discussion led by Emil Erlenmeyer and Johannes Wislicenus on the constitution of lactic acid. Furthermore, he proposed different structural formulas for isomeric compounds ( hydrocarbons , alcohols , organic acids ) with asymmetric carbon atoms, whereby the tetrahedron model was an important basis for this. He also developed a concept of the double bond between carbon atoms, in which two tetrahedra share a common edge.

Emil Fischer's work on the structure of carbohydrates and the description of their stereochemistry using the Fischer projection represent a further milestone . Fischer's work was awarded the Nobel Prize in Chemistry in 1902 .

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

Even Vladimir Prelog research into the stereochemistry of organic molecules and reactions were awarded the Nobel Prize 1975th The Cahn-Ingold-Prelog Convention , which he helped to develop, is used for the stereochemical description of organic molecules.

Isomerism

Classification of different types of isomerism

Isomerism occurs when molecules with the same empirical formula can have different spatial structures. The conformational isomerism is based on the rotation around a single bond of a molecule so that the substituents of the atoms linked by the single bond can assume different positions with respect to one another. Molecules that differ only in this specific arrangement of the atoms are called conformers . Mirror image isomerism occurs in chemical compounds that are related to a counterpart like its mirror image. The corresponding chemical compounds are called enantiomers or optical antipodes. This type of isomerism can be based on a stereocenter , a chiral axis, or planar or helical chirality. From group theory it follows that the absence of a rotating mirror axis is the necessary and sufficient condition for the occurrence of enantiomers. Diastereomerism occurs when molecules with several stereocenters are partly in the same and partly in different configurations.

Functional isomerism occurs when molecules with the same empirical formula have different functional groups . As Skelettisomerie refers to the existence of different frameworks. Positional isomerism occurs in molecules in which the same functional group occurs at different positions in the framework. The cis-trans isomerism or ( Z ) - ( E ) isomerism is a special form of positional isomerism. It occurs in compounds in which two or more differ with regard to the position of substituents with respect to a reference plane. As linkage isomerism or valence isomerization is defined as the presence of different numbers σ - and π bonds in molecules.

Stereochemical terms

Symmetry properties and operations

Stereochemistry deals with the symmetry properties of molecules. The molecular symmetry can have axes of symmetry, a center of symmetry or a plane of symmetry. There are four fundamental symmetry operations, mirroring, rotation and inversion, as well as translation for solids.

Axis of symmetry C n

The axis of symmetry or rotation describes an axis in the molecule in which the new arrangement of the atoms in the molecule is congruent with the previous one by rotating the molecule through the rotation angle 360 ​​° / n. An example of a molecule of the point group C 2 is water, an example of the point group C 3 is ammonia. The benzene has both a C 2 axis and a C 6 axis.

Plane of symmetry σ

The plane of symmetry or mirror describes a plane in the molecule which divides the molecule into two symmetrically matching halves. Depending on where the plane of symmetry lies in the molecule, a distinction is made between planes along the main axis of the molecule, referred to as σ v (from vertical). If the plane runs perpendicular to the main axis of the molecule, it is called σ h (from horizontal), planes that run diagonally are called σ d .

Inversion center i

An inversion or symmetry center converts all atoms into symmetry-equivalent atoms by mirroring them at a central point. Molecules with a center of inversion are non-polar. In the case of even-numbered molecules, the inversion center does not lie on an atom of the molecule, for example in benzene , in the case of odd-numbered molecules the inversion center falls on an atom, for example the carbon atom in the case of carbon dioxide .

Rotating mirror axis Sn

A rotating mirror axis converts the atoms in a molecule into symmetry-equivalent atoms by rotating them through an angle of 360 ° / n and then mirroring them. The mirror plane is perpendicular to the axis of rotation. The symmetry element S 1 corresponds to a plane of symmetry σ, the symmetry element S 2 corresponds to a center of inversion i.

See also

Individual evidence

  1. ^ Kurt Hermann: Stereochemistry before van't Hoff and Le Bel . In: Chemistry in Our Time . tape 8 , no. 5 , 1974, p. 129-134 , doi : 10.1002 / ciuz.19740080502 .
  2. ^ Otto Krätz: The portrait: Jacobus Henricus van't Hoff 1852-1911 . In: Chemistry in Our Time . tape 8 , no. 5 , 1974, p. 135-142 , doi : 10.1002 / ciuz.19740080503 .
  3. ↑ Nobel Prize Lecture by Prelog (PDF; 663 kB)
  4. ↑ Rotary axes Cn
  5. ^ Karl-Heinz Hellwich: Stereochemistry: Basic Concepts , p. 96 ( limited preview in Google book search).

literature

Web links