1. Synthesizing Loras College Purple from p-Dichlorobenzene
Chris Rock and David Oostendorp
Loras College Division of Molecular and Life Science
ABSTRACT
After using the newly developed synthesis reaction for 6,6’-dichloroindigo, I discovered that it is not as simple as it seems. The solvent, conditions, and temperature for the Friedel-Crafts acetylation reaction were difficult to identify.
RESULTS
During the distillation, at the end of the acetylation, there was no product left in the reaction flask at 270˚ C, the boiling point of 2′,4′-dichloroacetophenone. To overcome this, I doubled the reactants but to no avail. Next, I tried grinding the p-dichlorobenzene, but this did not work. Then, I attempted to use different solvents. However, each time I ended up with a waxy substance appearing at around 170 ˚ C in the condenser of the distillation apparatus.
INTRODUCTION
Loras College purple, or, more formally, Tyrian purple, dates back to ancient history. During this time, the only way to obtain this color as a dye was to painstakingly crush tens of thousands of murexes, tropical sea snails. It has been reported that 12,000 murexes were used to obtain a mere 1.4g of dye [1]. The amount of labor and time it took to collect this dye are what made Tyrian purple only for the rich and royal.
It was not until the early 20th century that the first chemical synthesis of Tyrian purple was reported. Since then, there have been multiple different schemes developed to synthesize this dye. However, almost all of these schemes require very dangerous chemicals and environmental conditions that are hard to create. There has, however, been a series or reactions, developed by Wolk and Frimer, that can synthesize Tyrian purple using relatively common chemicals in easy to control reactions [2]. My project was to start with a chemical slightly different (see figure 3) and see if this would yield Loras College purple.
Figure 1. Murex shell [3]
Figure 2. 6, 6-dibromo derivative of indigo [4]
DISCUSSION
The acetylation reaction was very difficult to run due to the anhydrous aluminium chloride needed as a catalyst. I believe the p-dichlorobenze being used was not driy which might have caused it to interact with the anhydrous aluminium chloride.
I learned that there is a lot more to laboratory work than what a textbook often will reveal. There is a lot of information, such as temperature, concentrations, and length of reaction, that are often overlooked.
Figure 3. Different starting molecules
METHODS
Acetylate p-dichlorobenzene into 2′,4′-dichloroacetophenone
Oxidize 2′,4′-dichloroacetophenone to 2,4-dichlorobenzoic acid
Condense 2,4-dichlorobenzoic acid with glycine to give chlorocarboxyphenylglycine
Perform a Claisen condensation to yield chlorodiacetylindoxyl
Hydrolyze and oxidize chlorodiacetylindoxyl to yield 6,6’-dichloroindigo
SOURCES
[1] Friedländer, P. Über den Farbstoff des antiken Purpurs aus Murex brandaris. Ber. 1909, 42, 765.
[2] Wolk, Joel L., and Aryeh A. Frimer. "Preparation of Tyrian Purple (6,6′-Dibromoindigo): Past and Present." Molecules 15 (2010):  Molecules - An Open Access Journal from MDPI. 10 Aug Web. <
[3]
[4]
Figure 4. My attempted reaction