RESONANCE 



79 



the formulas, it ought to be possible to synthesize the two pyrromethenes 

 as indicated in Figure 1(). Manj' such syntheses have been carried out by 

 Fischer and collaborators, but Ijoth syntheses give one and the same pyr- 

 romethene. This does not, in itself, prove resonance, since one of the two 

 tautomeric isomerides may be unstable. On the first glance, resonance 



CHO H 



■N- 



H 



H 



a " I 



H d 



b<^ ^ ^^ >e h 



^H 



H OHC 



rv' 



W^ f 



c H d 



a % ^" f B 



V 



T 



y 



b< V V >e 



-N N- 



H 



f 



Fig. 16. Nonexistence of pyrromethene isomerides. 



between A and B appears even impossible, since in these formulas the hydro- 

 gen atom on the pyrrole nitrogen is bound in A to the left, and in li to the 

 right pyrrole ring; it is well known that resonance is only possible when no 



D, 



+N= 

 H f 



=N+ 

 H 



/>2 



Fig. 17. Pyrromethene cations as resonance structure. 



atoms, but only electrons, cliange their places. It will be seen later, however, 

 that there is evidence in such compounds of hydrogen linkage l)etween the 

 two nitrogen atoms, which permits the assumption of resonance (Figure 16C).* 



*Resonance forms with separated charges have been left out of consideration; they 

 contribute less to the resonance state than the uncharged forms. 



