Azo dye: p-aminophenol and 4-chloro-2-nitrophenol

So, I've decided to try my hand at making an azo dye between these two compounds, just because I never seen it photographed at all. It also serves as a sort of project introduction, as I'm planning to characterize additional ""obscure"" azo dyes in the future (I'm going to have multiple dyes in one or two posts as to not spam topics, though). Now, into the introduction of the synthesis:

Because of the extremely low efficiency of the S_NAr reaction of 2,5-dichloronitrobenzene with NaOH, I only had less than 100mg of the 4-chloro-2-nitrophenol (in the form of the sodium salt), and this was the smallest scale I've ever worked on.

To start, I weighed out 25mg of the red-colored sodium 4-chloro-2-nitrophenoxide. In a separate container, I weighed 330mg of sodium hydroxide and added it to 3.5mL of water. After it dissolved, I added the solution to the sodium 4-chloro-2-nitrophenoxide and used a pipette to try and dissolve it. I ended up having to add more water (6.5mL) to get it all to dissolve. At the end, the solution was a yellow-orange. I poured it into a flask and set this aside.

After this, I weighed out 160mg of p-aminophenol (obtained from the acid hydrolysis of acetaminophen). With stirring, I added this to 5mL of water. I dripped in 31% HCl until the solution cleared (quite a bit of bubbling occured, which I believe to be from a small amount of sodium bicarbonate impurity). At this point, the solution was a green-ish. I put this in an ice bath with stirring.

I then weighed out 100mg of sodium nitrite and 1mL of water to it. Once it all dissolved, I waited until the p-aminophenol solution was cooled. It leveled out at about 7°C, so I started adding sodium nitrite slowly, keeping the temperature below 10°C. The solution color dulled and turned (what I would consider to be) "grayish-green".

I then took this out of the ice bath and took out the stir bar. I put the sodium 4-chloro-2-nitrophenoxide flask into the ice bath and then put in the previous stir bar (after rinsing it). Once the temperature got to 7°C, I started to add the formed 'p-hydroxybenzenediazonium chloride' solution at about a drop a second, keeping the temperature below 10°C. The solution took on a deep red color and had a rubbery smell (which I believe to be small amounts of p-benzoquinone formed from the substitution of the diazonium group in the basic solution to hydroquinone, followed by oxidation by the air).

After several minutes of stirring in the ice bath, I took it out to warm it up to room temperature while continuing to stir. The color started to turn a bit brown, and I got slightly concerned and corrected for pH (I knew it would be very basic, but I was concerned that it was degrading the product. Plus, I knew the phenoxide groups would increase solubility). So, I added 31% HCl dropwise until the pH was very acidic. At this point, I noticed a precipitate. So, I filtered it off, soaked up excess water with a paper towel underneath, then dried it fully with a fan.

The yield I measured was 11mg, which represents a yield of 26%. While this isn't great, the only reason I made this compound was to characterize its optical properties, and there are quite a few things I was pretty lax on (such as getting the temperature fully down to around 0°C, sort of improvising the NaOH amount, etc.).

The end product bears a resemblance to matchbox red phosphorus. It's nearly black without strong light.


But, in the sunlight, you can see the purple-red color of the product (you might need to zoom in, but it's there to at least some degree).


Also, the color was very prominent on the filter paper remnants (most likely because the product that was trapped on/in the filter paper was already very fine.

Hi pumpkins, I should really try that. I thought that the one-pot would have been less work, but it gives a very impure product (I got nearly a 300% yield once). I'll redo it using 91% isopropanol, which I have more of, and with the pure acetaminophen, the p-aminophenol should be a lot purer than not.

I did have success recrystallizing the impure p-aminophenol with water and a very small amount of sodium bisulfite as an oxygen scavenger (it needs a hot filter before cooling). Once cooled and filtered, the water needs to be squeezed out as fast as possible (I used paper towels), but once pretty much all of it is gone, it doesn't oxidize nearly as fast and can be dried with a fan. I haven't tested its properties as of yet, but considering it hasn't gone a dark purple, it hasn't oxidized too much.

Here's my recrystallized p-aminophenol (still very slightly damp):


Also, thanks for the procedure Fery!

I went to the store and tried a solvent extraction from some acetaminophen pills that had the least amount of inactive ingredients. I got a good yield and the crystals looked very pure although I didn’t do any tests to verify purity. I got an 85% yield from just boiling the crushed pills in acetone then hot filtering and boiling off the solvent.