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Author: Subject: Weird Color in Esterification Reaction
DionSukhram6
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[*] posted on 30-12-2017 at 12:28
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?


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DraconicAcid
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[*] posted on 30-12-2017 at 12:32


That happens all the time- there are some intermediates and byproducts which are very strongly coloured. If it's not that shade of purple, it's orange.



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DionSukhram6
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[*] posted on 30-12-2017 at 12:35


Ah ok thanks, I’ve never seen purple before like this
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UC235
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[*] posted on 30-12-2017 at 13:36


Iron-salicylic acid complex. It's strongly purple, probably a little iron in your acid. Reaction should work fine anyway.
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[*] posted on 30-12-2017 at 13:46


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!
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[*] posted on 30-12-2017 at 14:01


My students do this every year, and the purple is very common- it might be from iron impurities, or just air oxidation under strongly acidic conditions.



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[*] posted on 30-12-2017 at 15:15


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" ;)

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DionSukhram6
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[*] posted on 31-12-2017 at 13:37


Well it went pretty normally and the color faded almost completely after refluxing around 100C all day yesterday. After distilling off the excess methanol the methyl salicylate came out of solution and was slightly orange as usual. Not sure how the yield was though since I ended up making the rookie mistake of leaving the stopcock on my sep funnel open when I was pouring in the methyl salicylate to wash it and lost a bit. Happens to even the best of us sometimes I suppose.
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Antiswat
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[*] posted on 9-1-2018 at 12:21


almost offtopic: an forgotten brand of duct tape that got into contact with a standard nitration mix created a strongly yellow coloured grease, upon basifying with ammonia it turned into a bright orange, both had some fluoroscent quality to them
despite all scientific dogmas my intuition tells me that the compound that caused this colour is not uncommon to be found in duct tape, likely the adhesive.




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