Ring strain

The hoop stress is a further chemical bond energy, which in a carrier formed from a plurality of annular atoms molecule ( for example: cyclobutane ) is stored, or must first be spent for the ring closure. It is all the greater, the smaller the ring in most cases. Your cause various stereochemical effects, especially the angle strain.

Occurrence

Cycloalkanes

For alicyclic compounds that three-and four-membered rings are very excited because the strain-free tetrahedral angle of 109.5 ° is not reached. Cyclopropane, which is due to its flat triangular structure differs with bonding angles of 60 ° at most away from, and therefore has the greatest hoop stress.

Five-membered rings and six-membered rings (eg, cyclohexane - cycloalkanes ) to have a low or no ring strain. The tetrahedral angle is observed, which - as in the chain-like aliphatic compounds - leads to the conformation with non- planar molecular structure. Energetically they are cheaper and therefore are preferentially formed ( for example, the cyclic hemiacetal of carbohydrates ).

In medium-sized rings with eight to twelve carbon atoms occurs again ring voltage; by conformation it comes to the rejection of hydrogen atoms. Large Rings only possess significantly lower hoop stresses.

Ring voltages at alicyclic compounds are called Baeyer- voltage.

The opposite side rejection ( repulsion ) of substituents in 1,5 - position in cyclohexanes, n- pentane and so can differ depending on whether they are in an equatorial or axial position leading to the so-called Newman - voltage.

Epoxides

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

Effect

Molecules with large ring strain such as epoxides are highly disfavored. They are therefore unstable, the ring opens easily under strongly exothermic (heat producing ) reaction.

In contrast, molecules with small hoop stresses are energetically favorable, they arise therefore preferred. Wherein aromatic ring molecules, such as benzene, the ring in relation to the energy Mesomerieenergie are negligible.