Sciencemadness Discussion Board

Phenol from Salicylic acid

Melkor333 - 30-8-2022 at 09:54

I’m currently trying to make phenol by decarboxylating salicylic acid (based from a NileRed video
https://youtu.be/Q_f1hRmhUX8). I added 50g of salicylic acid into a round bottom flask, and set up the distillation the same way as seen in the video. I began heating it and soon the salicylic acid melted, though I’m having some problems. It’s been molten for the last almost 2 hrs and nothing is coming over. It’s bubbling slightly but the temperature has stayed steady at ~40C so evidently nothing really is reaching the still head. I insulated the apparatus well in aluminum foil, it’s not cold or windy outside and I know my nothing is wrong with my hot plate. Idk if I’m being impatient and that this reaction is supposed to take quite a while or if there’s something wrong.

Pumukli - 30-8-2022 at 10:09

Needs hard heating, the temperature of the molten acid should be (in my opinion) around 150-200 C at least!
You should see quite a heavy bubbling in the flask!

When you reach the right temperature the reaction is fairly quick.

Melkor333 - 30-8-2022 at 10:43

Oh it’s quite hot I think. I just checked and the molten SA is at 220C, and still a little bubbling but not much. It also went from quite clear to a yellow colour

DraconicAcid - 30-8-2022 at 12:11

Watching this carefully, as I'd like to do the same with p-anisic acid. I know it's harder, because of the lack of ortho-participation....

paulll - 30-8-2022 at 12:23

Apparently I didn't write down the temp the phenol came over at when I did this but I'm pretty sure it raced over well shy of 200C. By 220 it should be boiling off even if it doesn't decomp. , I'd think. Did you make the acid yourself - and test it?

[Edited on 30-8-2022 by paulll]

Melkor333 - 30-8-2022 at 12:48

No I bought it online. The first time it did it using SA I made myself, and I was running into the same problem.

j_sum1 - 30-8-2022 at 20:26

Sounds like a temperature problem.
IIRC, Doug's lab has a video on this that is better and clearer than Nile. You might want to search for that and watch it.

Pumukli - 10-9-2022 at 10:29

Ladies and gentlemen!

I think that the root of the problem lies elsewhere... :)

By accident I came across a 130 years old (!) article from 1892 and it describes fairly well what one can experience when salicylic acid is heated.

Spoiler: it seems it is not a high yielding method to obtain phenol... :) But a good one to obtain something else and then "transmutate" it into another "else"... :)
Good, old, amateur-friendly chemistry project, which was at its time cutting-edge science, but today we can repeat it in our kitchen-laboratories as well.

Enclosed is an English translation of the original German paper, translated by Google Translate under my "supervision". Mind you, I don't speak German at all... :) Editing this translation was a nice mental puzzle-solving game as well! ;)

Attachment: salol_translated_ENG.odt (26kB)
This file has been downloaded 192 times

[Edited on 10-9-2022 by Pumukli]

Attachment: salol_translated_ENG.docx (6kB)
This file has been downloaded 217 times

[Edited on 10-9-2022 by Pumukli]

Attachment: salol_orig_GER.pdf (138kB)
This file has been downloaded 243 times

Tsjerk - 10-9-2022 at 10:48

My Android phone doesn't support .odt and when I try to convert using Google documents I get a conversion error. I would be interested in reading the article though!

SplendidAcylation - 10-9-2022 at 14:11

I feel like I'm stating the obvious, but have you seen this excellent write-up?
http://www.sciencemadness.org/talk/viewthread.php?tid=27617
UnintentionalChaos reports a 66.8% yield.

I have done this myself, many years ago, and it was quite successful, but maybe I was just fortunate!

paulll - 10-9-2022 at 20:14

I'd agree with J's suggestion that this is likely a temperature problem. It seems very improbable that a bunch of salicylic acid is just hanging out at 220C. Even if Melkor333 had only worked his own product and even if we considered, for the sake of argument, that that product might not be exactly reagent-grade, for it to go above 180-ish and do nothing beyond a bit of bubbling, just doesn't sit right.

Diachrynic - 11-9-2022 at 15:03

Quote: Originally posted by Pumukli  
I think that the root of the problem lies elsewhere... :)

By accident I came across a 130 years old (!) article from 1892 and it describes fairly well what one can experience when salicylic acid is heated.

Spoiler: it seems it is not a high yielding method to obtain phenol... :)

This article has me thinking if the decomposition of phenyl salicylate to xanthone is actually the main phenol producing reaction when salicylic acid is thermally decarboxylated. This would explain why phenol only starts to distill from the reaction mixture under very strong heating (I found a plate temperature above 300 °C to be required at least, incidentally similar to what Orgsyn reports for the synthesis of xanthone) and why the yield is not very good usually - if this is the major reaction, one can only expect around half of the salicylic acid to be actually converted to phenol.

It has me also thinking about doing the phenol production more efficiently. What if salicylic acid is heated just enough to form phenyl salicylate, but not xanthone. Then saponification of the residue should provide a mixture of phenol and salicylic acid. Ideally, half the salicylic acid would be recovered and half would be converted to phenol per run. Of course, side reactions will lower the yield.

Pumukli - 11-9-2022 at 15:31

Hello Diachrynic! When I found the paper I thought roughly the same.

But Unintentional's phenol preparation got me thinking.
He achieved 60-70+ % yields of phenol BUT he reportedly used LESS heat! He distilled the phenol off around 150-160 C.
The german paper says that under 200 C the salicylic -> salol conversion is SLOW!

I think these two things are not contradicting but it's too late for me to start any deeper thinking about it now. :-)

Diachrynic - 11-9-2022 at 23:05

Quote: Originally posted by Pumukli  
He achieved 60-70+ % yields of phenol BUT he reportedly used LESS heat! He distilled the phenol off around 150-160 C.
The german paper says that under 200 C the salicylic -> salol conversion is SLOW!

While UnintentionalChaos does say to distill slowly, the 150-160 °C is referring to the temperature in the stillhead and not the main liquid, which likely is considerably hotter. On the third picture in the post, the main liquid is labled "Refluxing Phenyl Salicylate? >300C".

Pumukli - 12-9-2022 at 03:31

That's true, UC wrote that the main bulk of phenol was passing over between 145-165 C, measured in the stillhead...

I think what he thought about as refluxing phenyl-salicylate was rather refluxing xanthone. (Xanthos = yellow (greek), if memory serves me well.)

I found another related paper by Graebe, ("Ueber die Euxanthongruppe", Justus Liebigs Annalen der Chemie Volume 254, Issue 3 First published: 1889)

In this paper there is a three step scheme (page 282) which depicts how salol rearranges into xanthone and what else happens. This process goes like this:

1., Salol rearranges into o-phenoxy-benzoic acid at high temperature
2., o-phenoxy-benzoic acid cyclize intramolecularly into xanthone and releases H2O
3., the released H2O hydrolizes a salol molecule and the released "free" salicylic acid immediately decarboxylates (high temperature!)
The net result of step 3 is: salol + H2O -> 2 phenol + CO2

So it seems that in this funny salicylic acid - salol - xanthone - phenol - water - system everything is dynamically inter-related and the temperature program is the main deciding factor and this (with timing) determines the yield of the individual components.

There seems to be not even "easy route" to the more interesting compounds of this system either! In another (third) paper there was a note about trying various dehydrating agents and catalysators to modify the equilibrium (anhydrous ZnCl2, POCl3, acetic anhydride, etc) in vain! The authors could not improve the yield of xanthone apreciably. :)

This is exactly what I call amateur-friendly "challenge" chemistry. :)
Simple, not too toxic (OTC) starting material, simple "reagent" (heat, time), well-defined, characteristic product(s) which are easy to isolate and purify even for an amateur. A not too complicated theory behind the whole process and some obvious points that are "crying for" tinkering and experimenting with. :D
A process, that superficially looks easy but gets complicated once you touch it. Like a Rubik's cube. :D

Here's the Graebe paper, in German:


Attachment: xanthon_graebeGER.pdf (1.4MB)
This file has been downloaded 209 times

Junk_Enginerd - 12-9-2022 at 23:38

For what it's worth, I have tried the same synthesis and it worked just fine, except I used acetyl salicylic acid instead. I also heated via flame. I didn't pay much attention to the exact temperature and just increased the heat until phenol started to come over, so perhaps it's just a temperature issue?