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In chemistry, mesoionic compound is one in which a heterocyclic structure is dipolar and has 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 or other 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]