78 



III. PORPHYRIN CHEMISTRY 



groups, which no longer allow conjugation of double bonds and 

 resonance (c/. Chapter IV). On reoxidation, they are reconvertible 

 to the porphyrins from which they were formed by reduction, but 

 also may give rise to some urobilinoid substances. The dipyrryl- 

 methane linkage is rather easily broken, as will be seen in the chapter 

 on bile pigments. 



6. STEREOCHEMISTRY AND FINE STRUCTURE 

 OF PORPHYRINS 



6.1. Resonance 



It is now known that benzene and other aromatic compounds 

 do not have alternating double and single linkages as ascribed to 



them in the Kekule formulas ^1 and iTj , but that all six bonds 



are equal and of a character and length between a single and a 

 double bond. This state of such "aromatic" molecules, which cannot 

 be expressed adequately by our present formulas, is called a resonance 

 state, and is characterized by the fact that the energy content of the 

 molecule is lower than that of any of the forms with definitely placed 

 double and single bonds. From the infrared spectrum of pyrrole, 

 Pauling {212Ii) has concluded that pyrrole contains a resonance 

 structure between the "normal" formula (Fig. 15a), with a pair of 



H 



H 



Hrr^^H 



•N. 

 ■ H 



H 



H 



H 



H 



H 



Jn hW/^h hI^^h 



H 



H 



H 



.N 

 H 



'I 



Fig. 15. Resonance formulas of pyrrole, according to Pauling {2125). 



free electrons on the nitrogen atom, and four other formulas (Fig. 

 \5h-e), which accounts for the fact that pyrrole is rather a proton 

 donator (acid) than a proton acceptor (base). 



One might therefore expect even more evidence of resonance in 

 structures like pyrromethenes and porphyrins. There is, indeed, a 

 good (leal of evidence in favor of all these compounds having resonance 

 structure. The aromatic character of pyrromethenes and porphyrins 

 is confirmed by their heats of combustion (2635). 



If pyrromethenes of the structures A and li were isomers as indicated by 



