Weird Color in Esterification Reaction

I decided to just do a regular basic Fischer esterification to make myself some more methyl salicylate. I used nearly pure salicylic acid (usually sold for cosmetics), 99% water, and sulfuric acid from drain cleaner. The drain cleaner was clear when I bought it instead of the normal colored stuff which was surprising but I decided to add some hydrogen peroxide and heat it up to destroy any organic contaminants anyway. Based on its density, it’s about 93% and colorless. I dissolved the salicylic acid in the methanol in a rbf and it was clear and colorless and attached a condenser. But, when I poured the sulfuric acid in through the condenser it immediately turned a bright purple color (image attached). It’s a pretty beautiful color but I have no idea what it is and it probably shouldn’t be like that if I’m just making some methyl salicylate. Right?

I have also never seen that shade of purple when performing this reaction. It darkens up, but not that beautiful color. At the end before work-up the ester is tan, and is clear after distillation. I am curious to what yours will look like. I would guess that it is from the sulfuric, or maybe the H2O2? I just mixed a pinch of SA, a few ml of methanol, and few drops of H2SO4 in a small beaker to check, I got no such color. Heated to boil in microwave, still clear. My sulfuric was from dark hardware-store acid that had been distilled. I distilled about 600 ml, run over about 24 hours through a glass-packed claisen adapter. I have since acquired a nice gallon of clear hardware sulfuric (Kleen-out), which I use for experiments and cleaning. I try to save the good stuff.

Half way through writing this post I realized that I ought to try the above experiment with my impure acid, it still came out clear. I added some H2O2 afterwards, too, still clear... I was hoping to see some purple and solve a mystery

You might want to look into distilling a small amount of sulfuric for just this purpose. A few ml can go a long way when you only need catalytic amounts. Let us know how the ester turns out!

Quote: Originally posted by UC235

Iron-salicylic acid complex. It's strongly purple, probably a little iron in your acid. Reaction should work fine anyway.

That part. I did the same experiment, again, but added some iron chloride solution. It took heating, but the purple color came through strong. I had to dilute it so you could see it in the picture.

DraconicAcid: Next year when your students' methyl salicylate turns purple, you can tell them something more useful than "sometime that just happens"