Conformational isomerism

The conformation of an organic molecule describes the spatial arrangement of the rotatable bonds of the carbon atoms. Through them, the three-dimensional spatial coordinates of all atoms of the molecule are fully described.

The conformation thus includes the information on the stereochemistry, ie, the configuration of all stereotopen atoms and the constitution of the molecule.

Molecules with the same configuration, but differ in the specific arrangement of atoms and are in an energy minimum, is referred to as conformers. Synonym for this is also the name rotamer common. For their presentation, the sawhorse and Newman projection are used.

Lying different conformers in equilibrium with each other before, the ratio is given by a Boltzmann statistics.

Conformations in linear molecules

In the example on the left is the staggered and the eclipsed conformation of ethane is right in Newman projection shown. Below each is a representation in the ball - and-stick model.

In butane, a distinction is further conformations in anti, eclipsed, and skewed depending on the torsion angle.

However, the conformers can not be isolated, but are spectroscopically detectable only at low temperatures.

Conformations in cyclic molecules

Cycloalkanes can often exist in different conformations which are separated from one another by a significant energy barrier, so that the conformations are detected next to each other.

The twist, the tubs and the chair conformation are conformations of cyclic molecules such as cyclohexane. Compared to the boat conformation or boat conformation the Twistkonformation is slightly twisted. Thereby, the interactions between the two H atoms that are located at the two well peaks, weaker and the Pitzer - voltage smaller ( the H atoms are not exactly eclipsed ). Although the Baeyer strain is slightly larger by the slight twisting of the ring, but makes up for this omission the end of van der Waals repulsion between the two seated in the tub peaks H atoms. The Twistkonformation is therefore more stable by about 1.4 kcal / mol than the boat form and presents as the chair form a conformer dar. Since the trough shape energy is also expressed as the chair conformation, compounds such as cyclohexane occur on almost exclusively in the nearly strain-free chair conformation. Because it is possible to rotate the molecule in two directions, there are two energetically equivalent Twistkonformationen.

Chair conformation of cyclohexane

Ring inversion

The ring inversion (English: Ring - flip ) is a phenomenon that takes place in which by rotation about single bonds conversion of cyclic conformers same ring shapes. The spatial positions of the substituents do not necessarily remain the same there.

Cyclohexane is the most prominent compound wherein the ring inversion is observed. The preferred conformation of this six-membered ring - alkane is the chair conformation. Contrary to expectations, shows the 1H - NMR spectrum of cyclohexane no two signals at different resonance frequencies for axial and equatorial hydrogen atoms, which are located in slightly different chemical environments. The eclipsed transition state, which is traversed in the conformational change is energetically slightly raised so that the isomerization occurs rapidly at room temperature and only one signal can be observed. This changes when the temperature is greatly lowered, that due to the energy barrier of the isomerization, the conversion takes place much more slowly. In the ring inversion of cyclohexane all axial substituents at equatorial and axial equatorial all are.

The ring inversion of cyclohexane begins with the transition from the chair on the half-chair to the twist conformation, wherein the energy maximum is at the half-chair conformation. Of the intermediary step of the boat conformation, the twist conformation converts to the reversible twist conformation. The boat conformation represents the energy maximum dar. About another half-chair the chair conformation is achieved by ring inversion.

Different substituents prevent the occurrence of the inversion ring, such as a second ring of attached as in the trans-decalin. Acyclic substituents can have this effect. The tert- butyl group in tert- butyl cyclohexane allows only one conformation, therefore, is referred to as the substituent Konformationsanker. Due to its large steric bulk of it causes in the axial position far greater 1,3 - diaxial voltage than the three hydrogen substituent when it is equatorial. The " bottom " does not change during the ring inversion of tert- butyl cyclohexane.

Tetrahydropyran in an oxygen atom replaces one carbon atom of the cyclohexane ring, allowing the Sesselkonformere are asymmetrical. This can be explain by a special notation. If we imagine a plane that is formed by three C atoms and the O atom, a carbon atom is located below and one above this level (see picture). When the ring atoms are numbered, the ring is formed by the atoms of C2-C3 - C5 -O-; C1 is below, C4 above that level. This is referred to then as the 4C1 conformation. The alternate chair (pictured right) then has the 1C4 conformation.

The substituted tetrahydropyran D -glucose preferentially occupies the 4C1 conformation in which the largest substituent, the hydroxymethyl group in the equatorial position exists (see image 1). Into the 1C4 conformation (2) so that, in this case β -D-glucose that interacts with two further substituents (1,3- syn - axial) but would be positioned axially. This leads to a large 1,3- diaxial voltage. Other hydroxy groups interact with each other axially. Overall, these interactions destabilize the 1C4 conformation.

In glucopyranoses the largest substituent is therefore in the equatorial position if doing as few steric interactions are trained to other substituents. This is generally the case of D - sugars by the 4C1 conformation of the case. In contrast, L- glucose, the hydroxymethyl group positioned like equatorial in the 1C4 conformation.

Metallocenes

Metallocenes such as ferrocene, can be present in two different conformations.