Aromatic and Saturated Heterocycles
In contrast to cyclopropene, the heterocyclopropenes are extremely instable. The strong ring strain cannot account for this extreme instability, as the ring strain's strength in cyclopropene is similar to that in the heterocyclopropenes. The question is, what is the cause of the heterocyclopropenes' extreme instability? The decisive difference between cyclopropene and the heterocyclopropenes is the lone electron pair(s) of the heteroatoms (nitrogen, oxygen, and sulfur).
By applying the Frost circle, it is illustrated that in contrast to cyclopropene the additional electron pair of the heteroatom occupies an antibonding orbital. As a result, the molecule is additionally destabilized.