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Mesoionic compounds are dipolar five membered heterocyclic compounds in which both the negative and the positive charges are delocalized.[1][2] A completely uncharged structure cannot be written and mesoionic compounds cannot be represented satisfactorily by any one mesomeric structure.[2] Mesoionic compounds are a subclass of betaines.[2] Examples are sydnones and sydnone imines (e.g. the stimulant mesocarb), münchnones.[1][2][3] and mesoionic carbenes.

The formal positive charge is associated with the ring atoms and the formal negative charge is associated either with ring atoms or an exocyclic nitrogen atom.[4] These compounds are stable zwitterionic compounds[5] and belong to nonbenzenoid aromatics.[6]

See also[edit]


  1. ^ a b Bhosale, Sachin K.; Deshpande, Shreenivas R.; Wagh, Rajendra D. (2012). "Mesoionic sydnone derivatives: An overview" (PDF). Journal of Chemical and Pharmaceutical Research. 4 (2): 1185–99. 
  2. ^ a b c d IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "mesoionic compounds".
  3. ^ Ollis, W.David; Stanforth, Stepher P.; Ramsden, Christopher A. (1985). "Heterocyclic mesomeric betaines". Tetrahedron. 41 (12): 2239–2329. doi:10.1016/S0040-4020(01)96625-6. 
  4. ^ "SYDNONES" (PDF). [page needed]
  5. ^ "Seeking Mesoionic Compounds". 
  6. ^ Badami, Bharati V (2006). "Mesoionic compounds". Resonance. 11 (10): 40–48. doi:10.1007/BF02835674. 

Further reading[edit]