The Colourful Synthesis of Azobenzene

In this post, PhD student Angel Santorelli from Kurt Gothelf’s group at iNANO, Aarhus University, takes you through the colourful synthesis of Azobenzene in the organic chemistry lab.

Every synthesis starts with research and planning. Reading papers and figuring out protocols that works for your specific scenario is essential. (And a vertical monitor is awesome for reading papers)


Welcome to the organic synthesis laboratory: This is where the magic happens!


The second step is setting up the synthesis. On the left, we have a reduction of azobenzene esters using LiAlH4, on the right an azobenzene polymerization. Air and water are public enemies number 1 in organic synthesis. Therefore, the reaction runs in dry atmosphere of argon, hence the rubber balloons.


One of the most common processes for purification in organic synthesis is flash column chromatography. The name derives from chrōma (the Greek word for color) since it was initially used to separate pigments in plant extracts. Nowadays its use is widespread, and color does not play a role in it. Luckily, azobenzenes are deep orange red. This can be clearly seen in the some of the many columns I have performed.


Once the target product is isolated, the excess solvent is removed using a rotatory evaporator. The solvent-free leftover solid is then collected. Many of my products forms stable and beautiful orange foams.


Some compounds cannot be columned, and other techniques can be used, such as crystallizations, cycles of dissolution and precipitation and so forth. Here I show some products that were purified by these different methods. Once a product is purified, it is usually subjected to high vacuum, using a specialized pump connected to a liquid nitrogen trap. A Schlenk line (a mixed system that can alternate between vacuum and argon atmosphere) is shown. It is here that the products are fully dried.


The very last step is making sure that you got what you wanted. One of the most potent techniques is Nuclear Magnetic Resonance spectroscopy (NMR). This technique provides spectra filled with structural information and it is used to identify and confirm the identity of your product. On the left we can see the NMR equipment while being filled with liquid nitrogen, on the right we can see one example of a simple NMR spectrum.

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