Porphyrins are polyheterocyclic constituents of hemoglobin and myoglobin, the molecules that transport molecular oxygen in living systems, of the cytochromes, which also play central roles in biological redox processes, and of chlorophyll (Chemical Highlight 24-2), which mediates photosynthesis in all green plants. 

Porphyrins are the products of a remarkable reaction between pyrrole and an aldehyde in the presence of acid: 

This reaction is complicated and has many steps. The simpler condensation of one molecule of benzaldehyde and two of pyrrole to give the product shown below, a dipyrrylmethane, is illustrative of the first stage in porphyrin formation. Propose a step-by-step mechanism for this process. 

Transcribed Image Text:

R :0: H. H+ R R R H H R A porphyrin