Pixel International Conferences

Since electrochemical processes inherently benefit from the increasing share of renewable energy sources for power generation in the grid, they can play a valuable role in future processes in chemical industry helping to make production more sustainable and efficient. However, due to often complex setups and the lack of striking experiments, examples for electrosynthesis are rarely discussed in science classes.
The experiment presented herein describes the electrosynthetic bromination of fluorescein to Eosin Y (tetrabromo fluorescein) via reactive bromine generated in-situ [1]. The developed experiment makes use of bromide ions which are oxidized at the anode to form a reactive intermediate and is therefore avoiding the direct handling of hazardous bromine by school students. The reaction of fluorescein with bromine is well-established since it can be monitored easily by changes in both adsorbed and fluorescent colour and is commonly used as a detection reaction for bromine.
Within several minutes at low voltages of around 3 V, a significant change in colour from yellow to red is observable and at the same time the intense yellow green fluorescence under UV light vanishes at the given slightly basic pH condition. The product can be precipitated by lowering the pH and subsequently collected by filtration. In comparison with fluorescein, the tetrabromo derivative shows a greatly increased solubility in water which can serve as a suitable example for the concept of structure-properties-relations. Furthermore, a semi-quantitative analysis of the Fluorescein/Eosin Y ratio can be performed by thin-film chromatography where both components are easily distinguishable by their respective fluorescence colour under UV irradiation.
This experiment can be expanded to a set of experiments by synthesizing the fluorescein in the lab course via reaction of resorcinol with phthalic anhydride as well as using the Eosin Y product as a photoredox organocatalyst [2, 3].

Keywords: Electrosynthesis, fluorescence, model experiment.

References:

1. Vasudevan, D. & Anantharaman, P. N. (1993). Electrochemical synthesis of eosin from fluorescein. J. Appl. Electrochem. 23, 808-812.
2. Bayer, A. (1871). Über eine neue Klasse von Farbstoffen. Ber. d. Dt. Chem. Ges. 4, 558.
3. Frontana-Uribe, B. A. et al. (2010). Organic electrosynthesis: a promising green methodology in organic chemistry. Green Chem. 12, 2099-2119.