This experiment involves the synthesis and characterization of a simple porphyrin: 5,10,15,20-tetraphenylporphyrin. This product is a structural analog of important biomolecules, such as the heme portion of the oxygen-transport molecule hemoglobin and chlorophyll of plants.
The carbon skeleton of the porphyrin ring is prepared by eight electrophilic aromatic substitution reactions between four equivalents of pyrrole and benzaldehyde. Traditionally, porphyrin syntheses have been carried out in corrosive, high-boiling solvents. Toxic reagents have been used to oxidize the intermediate porphyrinogen ring to porphyrin.
In this experiment, porphyrinogen is prepared by the solvent-less reaction between pyrrole and benzaldehyde in the gas phase, and then oxidized to porphyrin by air oxygen. The product is identified by thin-layer chromatography, purified by column chromatography, and further characterized by ultraviolet-visible spectroscopy.
This gas-phase reaction eliminates the need for hazardous solvents, avoids the use of corrosive reagents, and uses air as a mild and safe oxidant for converting porphyrinogen to porphyrin. The link to the laboratory procedure includes pre- and post-lab questions.
Summary prepared July 2005 by Don T. Fujito, Chemistry Department, La Roche College.
Doxsee, K. M.; Hutchison, J. E. Green Organic Chemistry - Strategies, Tools, and Laboratory Experiments, Print 2004; pp 152-158.