Propionic Acid

Pyrrole

Benzaldehyde

Methanol

Methylene Chloride

Pyrrole was freshly distilled. benzaldehyde and propionic acid, purchased recently, were used as is. Pyrrole (1.9mL, 2.69E-2mol) and benzaldehyde (3.0mL, 2.69E-2mol) were combined in a graduated cylinder then agitated until thoroughly mixed.  Propionic acid (100mL) was added to a 250 mL round bottom flask fixed with a cold water condenser and heated to reflux.   The benzaldehyde and pyrrole mixture was added to the refluxing propionic acid and allowed to react for 30 minutes.  Color change to red/brown occurred rapidly and by the end of the reaction the crude solution was opaque.  The crude solution was cooled to room temperature then filtered using a Buchner funnel.

The reaction flask and filtered solid were washed with methanol leaving behind shiny purple crystals.  The filtered solid as well as the solid remaining in the reaction flask were then washed with methylene chloride into an oven dried and pre-weighed round bottom flask. The solution was concentrated to dryness then weighed for yield determination.

Mass Collected: 716mg

%Yield: 17.3%

The reaction is simple and reliable.  The challenge is achieving optimum yield.  Reactant concentrations and reaction time are very important.  For optimal yield, reactant concentration should be approximately 0.27M.   Although the Adler synthesis of TPP constitutes a quick and reliable method for synthesizing tetraarylporphyrins, it is by nature a very low yielding reaction.  Inspection of the mechanism for porphyrin formation reveals why. 

If available, dry propionic acid gives greater yields.  Make sure both pyrrole and benzaldehyde are fresh and in high purity.  I recommend storing both in sealed containers under nitrogen and in a refrigerator.  Chromatography on the product is not necessary.  Chlorin side product can be oxidized to porphyrin by refluxing the reaction product with DDQ in toluene.

HNMR data in chloroform-d:

2H singlet -2.8ppm

12H multiplet 7.7ppm

8H doublet 8.1ppm

8H singlet 8.8ppm