H. M. KALCKAR 



A group is said to display mesomerism if it can be described by 

 two or more symmetric (or very nearly symmetric) electronic formulas, 

 representing approximately the same potential energy. In that case, 

 the chances that the electrons will occupy one or the other position are 

 equally great. The electrons are therefore moving forth and back 

 between these equivalent positions or, to use a term of physics, oscillat- 

 ing between the positions. The mesomerisms of the carboxylate anion 

 and the amidine cation represent some of the simplest examples of sym- 

 metric mesomerism in a molecular group. 



In these two sets of alternative structures, one of the oxygen or 

 nitrogen atoms is surrounded by a complete set of eight electrons, the 

 so-called octet, and the other by only six electrons, the last pair of elec- 

 trons participating in the double bond. If this pair of electrons were 

 moved up to complete the octet, the opposite oxygen or nitrogen would 

 have to donate one pair of electrons from their octet in order to restore 

 the double bond. Structures a and b are completely equivalent and 

 indistinguishable . 



The well-known benzene mesomerism is usually illustrated by 

 the two structures shown in formula III, 



(a) (b) 



III 



Electronic mesomerism can also exist between two molecules. 

 This so-called intermolecular mesomerism can be illustated by the 

 two symmetric structures of formula IV. In structure a, the left A 



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