Aromatic and Saturated Heterocycles
Three-Membered Heterocyclic Compounds
Analogous to cyclopropane, three-membered heterocyclic compounds display a higher reactivity in comparison to the corresponding open-chain compounds. The higher reactivity arises from the noticeably strong ring strain. The strong ring strain can be attributed to two causes.
- On the one hand, the three-membered ring's geometry causes particularly small bond angles in the ring. In cyclopropane, for instance, each carbon is connected to four other atoms, namely two carbons and two hydrogens. Therefore, from an energetical point of view, an hybridization and, thus, a tetrahedral arrangement of the four atoms surrounding each carbon would be the most suitable for each ring carbon. In this case, the carbon-carbon bond angles in the ring would amount to 109.5°. However, due to the three-membered ring's geometry, each bond angle in the ring actually only amounts to 60°! As a result, the carbons are not precisely -hybridized. If this would be the case, the cyclopropane ring would not exist, as the orbital overlappings would be too small to allow the formation of stable carbon-carbon bonds. The orbitals are rather deformed in comparison to orbitals (banana bonds).
- On the other hand, as a result of the three-membered ring's geometry and the strongly restricted rotation of the ring bonds, the hydrogens of cyclopropane are forced into an eclipsed conformation. That is, the torsion angle of the hydrogens at adjacent carbons is nearly 0°. In the corresponding open-chain compound, bond rotation is hardly restricted. Thus, the open-chain compound can easily attain a staggered conformation. The eclipsed conformation that resulted in the three-membered ring causes additional steric interactions, which destabilize the ring. This destabilizing effect is known as torsional strain. The strength of torsional strain depends on the size, or steric demand, and on the number of substituents. Cyclopropane's torsional strain, for instance, is stronger than that of ethylene oxide (oxirane, 1,2-epoxyethane), as cyclopropane contains six hydrogens, while ethylene oxide possesses only four hydrogens.
The high reactivity of three-membered heterocycles in regard to ring-opening reactions is caused by the gain in energy, which results from the loss of ring strain. Furthermore, the carbon-heteroatom bonds in heterocycles are polarized, and the heteroatom residue is typically a good leaving group. Thus, ring-opening through a nucleophilic attack is additionally promoted.
Ethylene oxide is the most frequently used three-membered heterocycle. It is a valuable starting product in many syntheses. For instance, nucleophilic ring-opening of ethylene oxide by an alkyl nucleophile, such as a Grignard compound, leads to a chain extension of the alkyl group by two carbons. In the large-scale production of ethylene oxide, ethylene is catalytically oxidized with oxygen. On a laboratory-scale, oxiranes may be synthesized, for instance, through the oxidation of alkenes with peroxy acids.