Anthraquinone Dyes: A Synthetic and Chemical Characterization Protocol for an Industrial Chemistry Laboratory Course

A fast, operationally simple, and reproducible synthetic protocol for anthraquinone dyes (AQDs), suitable for upper-division undergraduate industrial and organic chemistry laboratory courses, has been developed. This experiment is composed of three stages; the first step deals with the synthesis of AQDs, followed by a purification step, and finally a chemical characterization of the pure dyes. This can be completed within 2-3 laboratory sessions (3 h each). Synthesis is performed via Ullmann coupling reaction starting from inexpensive commercially available materials, 1-amino-4-bromoanthraquinone-2-sulfonic acid sodium salt (bromaminic acid), amine/aniline derivatives, phosphate buffer (pH 6-7), and a catalytic amount of elemental copper (Cu⁰) under microwave irradiation at 120 °C within a short reaction time (2-20 min). The crude blue dyes are purified by extraction with dichloromethane followed by column chromatography using reversed phase C-18 material. The clear difference in colors between starting material (orange and red), byproduct (dark red), and products (blue) allows for great ease in visual monitoring of the chromatographic separation. The target AQDs are fully characterized by spectroscopic methods.