Pentacyanopropene, acid strength

These acids (51) are organic molecules that contain a plurality of cyano groups and are readily ionized to hydrogen ions and resonance-stabilized anions. Typical cyanocarbon acids are cyanoform, methanetricarbonitrile (5) 1,1,3,3-tetracyanopropene [32019-26-4] l-propene-l,l,3,3-tetracarbonitrile (52) 1,1,2,3,3-pentacyanopropene [45078-17-9], l-propene-l,l,2,3,3-pentacarbonitrile (51) l,l,2,6,7,7-hexacyano-l,3,5-heptatriene [69239-39-0] (53) 2-dicyanomethylene-l,l,3,3-tetracyanopropane [32019-27-5] (51) and l,3-cyclopentadiene-l,2,3,4,5-pentacarbonitrile [69239-40-3] (54,55). Many of these acids rival mineral acids in strength (56) and are usually isolable only as salts with metal or ammonium ions. The remarkable strength of these acids results from resonance stabilization in the anions that is not possible in the protonated forms. 

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