The imidazol-2-ylidenes are strong bases, having pK ≈ 24 for the conjugate acid in dimethyl sulfoxide (DMSO):
However, further work showed that diaminocarbenes will deprotonate the DMSO solvent, with the resulting anion reacting with the resulting amidinium salt.
Reaction of imidazol-2-ylidenes with 1-bromohexane gave 90% of the 2-substituted adduct, with only 10% of the corresponding alkene, indicating that these molecules are also reasonably nucleophilic.
pK values for the conjugate acids of several NHC families have been examined in aqueous solution. pKa values of triazolium ions lie in the range 16.5–17.8, around 3 pK units more acidic than related imidazolium ions.