In the present experiment students prepare a porphyrin, tetraphenylporphyrin, from four molecules each of pyrrole and benzaldehyde. The carbon framework of the porphyrin is assembled through eight electrophilic substitutions between benzaldehyde and pyrrole to produce a porphyrinogen. Oxidation of the porphyrinogen during the reaction yields the porphyrin (an 18-electron aromatic compound).
Traditionally, porphyrin syntheses have been carried out in corrosive, high-boiling solvents, such as propionic acid or have been done in large volumes of a halogenated solvent containing a corrosive Lewis acid catalyst. In many cases, toxic oxidizing compounds are used to convert the porphyrinogen to porphyrin. In this experiment, students employ a method of porphyrin synthesis that is solventless-it is carried out on a solid support. The two reagents, pyrrole and benzaldehyde, react on the support under irradiation in a conventional microwave oven. The product is eluted off the solid support with a small amount of solvent. Usually one purifies porphyrins by column chromatography often employing chlorinated solvents such as methylene chloride or chloroform. In this experiment, a safer chromatography solvent mixture (hexanes and ethyl acetate) is used. The link to the laboratory procedures includes pre- and -post lab questions.
Summary prepared May 2005 by Julie A. Haack, Department of Chemistry at the University of Oregon.
Warner, M.; Succaw, G.; Doxsee, K. M.; Hutchison, J. E. Microwave Synthesis of Tetraphenylporphyrin Greener Approaches to Undergraduate Chemistry Experiments, Print Kirchhoff, M., Ryan, M., Eds.; American Chemical Society: Washington D.C., 2002; pp 27-31.