Aziridine, proton affinities

The basicities of saturated heterocycles are similar to those of analogous open chain systems, with the exception of three-membered heterocycles, in which the basicity is markedly reduced. Table 1 gives pvalues for the equilibria between free and monoprotonated heterocycles. As the ring size increases, the protonated species become more stable and the pKa values approach those of the open chain analogues. Increasing basicity (thiirane < oxirane < aziridine) prevails in gas phase proton affinities (Table 2) (80JA5151). 

The role of hybridization and basicity in the gas phase has been discussed further by Aue [25], who draws attention to the low basicity of aziridine as against dimethylamine when compared with the other pairs of cyclic and acyclic compounds, and points out that acetonitrile is about 34kcalmol lower in proton affinity than is ethylamine. Beauchamp [26] has considered the importance of hybridization in comparing the effects of alkyl substitution on phosphines compared to amines. 

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