What is the mechanism of the fluorescein synthesis performed by von Baeyer?
This reaction is considered a modified Friedel-crafts acylation. The reaction is formally a series of electrophilic aromatic substitutions, with hydroxyl leaving groups picking up protons (1), however the lewis acid catalysis obviously plays a role in the mechanism. I think it increases the electrophilicity of the pthalic anhydride in the first step, and facilitates the elimination of the hydroxyl groups. For the mechanism without the involvment of the Zinc (II) chloride, see (1). Here is a mechanism emphasizing the role the lewis acid might play in catalyzing the reaction:
This reaction works in a similar way to the Nencki reaction (2)
In order to get the form of the molecule shown as the product of your reaction, the pH needs to be increased in order to open the lactone ring. (3) This is fluorescein-A, the form of the molecule that actually fluoresces.
- Mohan, Jag. Organic Anlaytical Chemistry: Theory and Practice. 2003, Alpha Science ltd., Rohtak, India, p. 497
- I. M. Nencki, N. Sieber. J. Prakt. Chem., 1881, 23, p. 147
- Stephenson, C. J.; Shimizu, K. D. Org. Biomol. Chem, 2010, 8, p. 1027-1032