Hoop stress

The ring tension is an additional chemical binding energy that is stored in a ring-shaped molecule (e.g. cyclobutane ) made up of several atoms or has to be used beforehand for its ring closure. In most cases, the smaller the ring, the larger it is. They are caused by various stereochemical effects, especially the angular stress .

Occur

Cycloalkanes

The graph shows the percentage yield as a function of the number of ring members for the Thorpe-Ziegler reaction and the Dieckmann condensation . The drop for rings with 9 to 13 links can be clearly seen. This decrease cannot be found in the acyloin condensation , since this is a radical reaction .

For alicyclic compounds , three and four-membered rings are very strained, since the strain-free tetrahedral angle of 109.5 ° is not achieved. Cyclopropane , which is flat due to its triangular structure, deviates the furthest from it with bond angles of 60 ° and therefore has the greatest ring tension.

Five-membered and six-membered rings (e.g .: cyclopentane , cyclohexane  - cycloalkanes ) show little to no ring tension. The tetrahedron angle is maintained, which - as in the chain-like aliphatic compounds - leads to a conformation with a non-planar molecular structure. In terms of energy, they are therefore more favorable and arise preferentially (for example, the cyclic hemiacetal formation of carbohydrates ).

In medium-sized rings with eight to twelve carbon atoms , ring tension occurs again; by conformation there is a repulsion of hydrogen atoms . Large rings only have significantly lower ring stresses.

Ring strains in alicyclic compounds are referred to as Baeyer strain .

The opposite repulsion of substituents in the 1,5-position in cyclohexanes, n- pentane, etc., can lead to what is known as Newman stress , depending on whether they are in the equatorial or axial position .

Epoxies

Epoxides have very large hoop stresses due to their three-membered ring.

effect

Molecules with a high ring tension such as epoxides are energetically very unfavorable. They are therefore unstable, the ring opens very easily with a strongly exothermic (heat-producing) reaction.

On the other hand, molecules with low ring tensions are energetically more favorable, so they are formed preferentially. In the case of aromatic ring molecules, such as benzene , the ring energies are negligible in relation to the mesomeric energy .

See also

• Baeyer tension
• Newman tension
• Pitzer tension
• Prelog tension

literature

• Manussawee Hengsuwan, Ann-Christin Pöppler: Synthesis of five, six and seven rings . Georg-August University of Göttingen , 2007.

Web links

• Calculation and tabular representation of the ring strain in cycloalkanes at ChemGaroo , ChemgaPedia
• Representation of the conformation of normal cycloalkanes at the Friedrich-Alexander-Universität Erlangen-Nürnberg
• Cycloalkanes and stereoisomerism  - ring tension and structure of the cycloalkanes ( Memento from February 5, 2010 in the Internet Archive ) at the Justus Liebig University in Giessen
• Essentials 4: Cycloalkanes (PDF; 128 kB) at Heinrich Heine University Düsseldorf

Individual evidence

1. ↑ Peptides and Proteins, page 10 (PDF; 2.8 MB) online-media.uni-marburg.de. Archived from the original on July 20, 2007. Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved April 23, 2009.