Heterocyclic compounds ring strain

In four-membered heterocycles, the ring strain is less than in the corresponding three-membered compounds and is approximately equal to that found in cyclobutane. Nevertheless, ring-opening reactions forming acyclic products predominate. At the same time, analogy with the reactivity of the corresponding aliphatic compounds (ethers, thioethers, secondary and tertiary amines, imines) becomes more evident. 

Calculation of group increments for oxygen, sulfur and nitrogen compounds has allowed the estimation of conventional ring-strain energies (CRSE) for saturated heterocycles from enthalpies of formation. For 1,3-dioxolane, CRSE is about 20 kJ mol . In 2,4-dialkyl-l,3-dioxolanes the cis form is always thermodynamically the more stable by approximately 1 kJ mol" . 

See Dinitrogen pentaoxide Strained ring heterocycles See other strained-ring compounds... 

Despite the fact that the history of hetarynes is older than that of the benzynes (cf. 2), physical data on these compounds are scarce. Numerous trapping experiments furnished evidence for the formation of brradicaloid intermediates in the field of five-membered heterocycles (didehydrofurans, -thiophenes, and -pyrroles)." Direct spectroscopic data on these species, however, do not exist, which may be attributable to the increased ring strain in the five-membered o-arynes, associated with a strong tendency to undergo ring-opening reactions. 

The chemistry of saturated heterocyclic compounds is characteristic of their functional group. For example, nitrogen compounds are amines, oxygen compounds arc ethers, sulfur compounds are sulfides. Differences in chemical reactivity are observed for three-membered rings, e.g., epoxides, whose enhanced reactivity is driven by the relief of their severe ring strain. This chapter discusses heterocycles that are aromatic and have unique chemical properties. 

---