Preparation of Phenol from Salicylic Acid

A practical and quantified method for the preparation of phenol from inexpensive, readily accessible starting materials is something that, to the best of my knowledge, does not yet exist on this forum. Methods such as the cumene process or direct oxidation of benzene require specialized catalysts and conditions that are not amenable to amateur experimentation. Diazotisation of aniline yields phenol upon warming the solution of diazonium salt, but aniline is itself a difficult to acquire or tedious to make chemical. Fusion of benzenesulfonates with molten alkali hydroxides furnishes phenol, but benzenesulfonic acid is typically prepared with oleum and the alkali fusion is unpleasant at best.

Anyone who has heated salicylic acid knows that it decarboxylates to phenol, as is evident from its powerful smell. However, conversion is far from quantitative through brief heating, so more controlled conditions are required for an isolatable yield. One such procedure comes from another amateur experimentation forum, versuchschemie.de Suspecting that the stated 40% yield could be improved, I did some of my own experimentation.

The salicylic acid used was prepared from acetylsalicylic acid (aspirin) recrystallized from acetone. The one-pot cleavage of the phenyl acetate ester and crystallization of the product was carried out in the following manner:

Conversion of Aspirin to Salicylic Acid

A 1L RBF is charged with a stirbar, 45.04g of acetylsalicylic acid (250mmol), 600ml of distilled water, and 10ml of 31.45% hydrochloric acid. A vertical condenser is attached and the reaction mixture heated to reflux. After 30 minutes (This may be excessive, but I know it works), heating is stopped and stirring turned off. The solution, which should be a clear, colorless liquid, is very nearly saturated with salicylic acid and after cooling for a few minutes should be agitated to induce crystallization or seeded with a small quantity of salicylic acid crystals. The flask is left to stand undisturbed for several hours while crystallization occurs. Solubility at 20^oC is nearly negligible (~2g/L), and cooling in an ice bath is unnecessary. The product is a mass of small, white needle-like crystals that fill the solution. The solution is agitated to break up the fragile crystal mass and vacuum filtered. A clean stopper is used to press the "fluffy" mass of crystals to aid in removal of the mother liquor. This cake is washed with two 50ml portions of distilled water. The product can be dried in a dessicator, at room temp, or at mildly elevated temperatures. The yield of salicylic acid is roughly 95% of the theoretical as glittering white needles melting at 156-157.5^oC (uncorr, lit m.p. 158.6^oC)

The filtrate can be reused several times to hydrolyze additional batches of salicylic acid. Discard 100ml of liquid, add 1-2ml of 31.45% hydrochloric acid, and an additional 45.04g of aspirin. The reflux procedure is repeated and the product collected may be obtained in nearly quantitative yield, as solubility losses are minimized and the only limiting factors are the purity of the aspirin and mechanical losses.

Thermal Decarboxylation of Salicylic Acid to Phenol

A 500ml RBF is charged with 69.06g (500mmol) of salicylic acid and a few boiling stones. A short path distillation head with thermometer is mounted atop this flask, though there is no reason that a normal 3-way adapter couldn’t be used. Since the product distilling is phenol (m.p. 40^oC) it is prudent to maintain the condenser water at ~45^oC. As I am working in an unheated lab, I found that the easiest way to do this was to load the condenser with hot water and not cycle it. The distilling phenol dissipates enough heat into the small condenser to keep the water at the desired temperature.



The flask is strongly heated with a hotplate and the salicylic acid melts into a bubbling liquid. Some fine crystals of salicylic acid will probably deposit on the roof of the flask but should not make it into the stillhead. After a few minutes, a vapor front of phenol/water will rise up the walls and knock down any crystals that have formed. If this does not make it into the stillhead and begin a slow distillation after several minutes, increase the heat supplied to the flask or add insulation.



Distillate should slowly pass over at the stillhead. It will initially be milky and biphasic from small quantities of water, but the stillhead temperature will rise and it will become monophasic dry phenol. Do not allow the stillhead to exceed 165^oC. The formation of carbon dioxide by decarboxylating salicylic acid serves to “blow” phenol vapor into the condenser below its normal boiling point. Increasing the stillhead temperature will only serve to carry additional vaporized salicylic acid over as well (some is inevitable). No crystals should form in the condenser. For the duration of the experiment, phenol passed over from 145^oC to 165^oC and collected as a clear, colorless to very faintly yellow, supercooled liquid in the receiving flask.

Heating will need to be increased eventually to maintain a reasonable distillation rate and the whole process may take a few hours. At the very end, a second liquid, suspected of being phenyl salicylate (and possibly xanthone per, Org. Synth. 1927, 7, 84) may begin refluxing in the lower part of the distillation flask.



The residue is tarry and brown and consists of unreacted salicylic acid, polycondensed crap, and what is likely a reasonable amount of phenyl salicylate/xanthone. The crude product formed a slushy crystalline mass on standing and weighed 37.55g.

To purify the crude product, vacuum distillation is employed, though it should be possible to do it without vacuum. At ~30torr, phenol distills from 85-90^oC With vigorous stirring under vacuum the product is first distilled until the stillhead reaches 85^oC. At around 30^oC, water passes over, and a small amount of phenol is lost in the 30-85^oC range, but ensures the product will be dry. No cooling is necessary for this stage. If operating without vacuum, distill until the stillhead reaches 181^oC



The vacuum is broken and the stillhead, condenser, and reciever are cleaned and dried. The phenol is then distilled utilizing a similar condenser setup as the initial distillation and passes over as a monophasic colorless clear liquid, freezing in the receiving flask. When product ceases to distill in the appropriate temperature window (though there may be liquid or even solid in the stillpot), heating is stopped. The stillpot residue freezes into a solid mass well above phenol’s melting point and consists largely of salicylic acid. The product weighs 31.45g, a 66.8% yield from salicylic acid.



(Note: This image is of a previous smaller run, but the product is visually identical to that of the 500mmol scale.)

Smaller scale experiments using 20g of salicylic acid gave 57% and 71% yields of phenol, so 60-70% is probably a reasonable expectation of yield.

Notes:

-Attempts to convert all of the salicylic acid to phenol through extended reflux failed. After an hour and a half, roughly 30% was converted as determined by decrease in mass from CO₂ evolution. As the refluxing phenol lowers the melt’s temperature significantly, the rate of decarboxylation slows to a crawl, as indicated by a mineral oil bubbler.

-Attempts to catalyze the decarboxylation using copper chromite catalyst failed. Conversion based on loss of mass was identical to that of the uncatalyzed reaction after 1 ½ hour reflux. This therefore rules out copper powder and copper oxide as other likely catalysts. While using quinoline as a reaction medium may promote decarboxylation with these catalysts, this is not viable for as simple a compound as phenol with inexpensive starting material where uncatalyzed decarboxylation is fairly effective.

-No attempt was made to work up the tar from the initial distillation for its suspected phenyl salicylate content. If you wish to do so, I suggest adding in the stillpot residue from the vacuum distillation and dissolving everything you can into toluene, washing with bicarbonate solution to remove salicylic acid, and vacuum distilling after stripping the solvent. Any forerun will likely be phenol and if there is phenyl salicylate present, it should distill over near 200^oC at 30 torr. (lit. 172-173^oC at 12torr) Xanthone, boiling at 351^oC at atmospheric pressure may complicate this isolation, or may prove to be the major product.

[Edited on 12-1-13 by UnintentionalChaos]

Quote: Originally posted by Boffis

Nice work UC! An interesting thought on your final paragraph; The distillation of phenyl salicylate at atmospheric pressure yields phenol and xanthone. The latter compound only distills at about 350-360 C. The detailed preparation is covered in Org Synth Coll. Vol. 1 p552 (attached).

Phenol from acetylsalicylic acid - it is insanity......
Commercial salicylic acid is really cheap (~10 euros/kg), so phenol prepared in this way is not very expensive, however much more expensive than commercial one. But for experiment, why not.

Oxidation of benzoic acid to phenol can be conducted via (thermal) decomposition of its Cu(II) salt but your preparation seems to be better solution.

Quote: Originally posted by ParadoxChem126

A similar method of making phenol from acetylsalicylic acid was discussed here: http://www.sciencemadness.org/talk/viewthread.php?tid=23930

Quote: Originally posted by ParadoxChem126

I did not formulate this idea, I got it from page 128-129 of this book: http://books.google.com/books?id=y5mZrW1KB_AC&pg=PA129&a...

Quote: Originally posted by The_Davster

Do you have no heat transfer agent between your plate and flask? Like sand bath or...? Or was it removed for photos?

Quote: Originally posted by BlackDragon2712

around 20 dolars for the kilogram of salicilyc acid. convertion of a 70 percent. the kilogram of phenol costs like 220 dolars so 700 grams would have a price of 154 dollars giving us a profit of 134 dollars O:!!! nicely done btw, mister UnintentionalChaos I really apreciate your work

Quote: Originally posted by UnintentionalChaos

Salicylic acid is made from phenol industrially so the starting phenol in any sane universe should cost less. That being said, you may not be able to buy it at that price point.

Quote: Originally posted by UnintentionalChaos

Quote: Originally posted by BlackDragon2712   around 20 dolars for the kilogram of salicilyc acid. convertion of a 70 percent. the kilogram of phenol costs like 220 dolars so 700 grams would have a price of 154 dollars giving us a profit of 134 dollars O:!!! nicely done btw, mister UnintentionalChaos I really apreciate your work Don't get ahead of yourself there. 70% yield of phenol from a kilo of salicylic acid is 477g. I think you may also be comparing different grades or reagents there... Salicylic acid is made from phenol industrially so the starting phenol in any sane universe should cost less. That being said, you may not be able to buy it at that price point.

Well, now I am regretting not documenting my experiments more closely. Inspired by the Pyridine decarboxylation thread, I have been dabbling in this particular subject.

I would like to report some success, using copper chromite catalysts. The catalyst was prepared according to UnintentionalChaos's methodology, i.e. no barium involved, with some modifications. No ammonium dichromate was available, so sodium chromate was taken up in aqueous solution, acidified to an orange color with sulfuric acid, and an equimolar volume of ammonium chloride was added. This solution was used in place of the straight ammonium dichromate solution, and appeared to perform identically.

Yields at this time can only be estimated because the product is still in its crude form, as pictured below. The crude product there is the result of using 195g of salicylic acid.

I also redistilled the phenol product from the reaction flask, as UC did with the pyridine procedure, so I am unsure of any potential product loss there. It seems as though the product contained less water before the redistillation, as it solidified continuously as the reaction proceeded, and had to be warmed above its melting point (which took a surprising long time and a lot of heat) to be poured back into the reaction flask for redistillation. After redistillation I obtained a slush of needles in what is presumed to be water saturated with phenol, as reported by UC and pictured below.

I did the 195g in two separate reactions, one using a propane fueled bunsen burner and one using a hotplate brought slowly up to temp. I did not observe any qualitative difference between the two heating methods. I used an unusual short path set up (flask -> distilling adapter -> claisen adapter (used as an air cooled condenser) -> vacuum adapter -> receiving flask). This set up required constant close attention with a heat gun to prevent phenol from clogging the vacuum adapter, and thus was far from ideal.

I also attempted the decarboxylation of salicylic acid using an equal weight mixture of sodium hydroxide and calcium hydroxide, in a retort of iron pipe, and heated by a propane bunsen burner flame. On mixing the three reagents, they heated up considerably, which implies to me that quite a bit of moisture was present. The product from this was mostly water with oily droplets of phenol floating about. It was discarded.

Quote: Originally posted by Natures Natrium

Well, now I am regretting not documenting my experiments more closely. Inspired by the Pyridine decarboxylation thread, I have been dabbling in this particular subject. I would like to report some success, using copper chromite catalysts.

Quote: Originally posted by Natures Natrium

As an aside, I processed the aspirin in the following way. Refluxed briefly in a solution of 3x molar sodium hydroxide, precipitated with hydrochloric acid to pH 2. Filtered, and washed the filtrate with 3x 100 mL ice cold dH20. Dried on a glass plate on a hot plate set to barely warm.

Quote: Originally posted by UnintentionalChaos

Quote: Originally posted by Natures Natrium   As an aside, I processed the aspirin in the following way. Refluxed briefly in a solution of 3x molar sodium hydroxide, precipitated with hydrochloric acid to pH 2. Filtered, and washed the filtrate with 3x 100 mL ice cold dH20. Dried on a glass plate on a hot plate set to barely warm. I used to do it that way years ago. The material comes out off-white and smells slightly of phenol in my experience. The acid approach I worked out for this produces perfectly white odorless material, not that it should matter for the intended use. The lower reagent usage is nice though. In fact, about 13g of the SA to go into the 500mmol batch was old base-hydrolysis material with roughly the same m.p. The melt should initially have less color than it does in my pictures if using only acid hydrolysis SA.

Quote: Originally posted by thebean

Out of curiosity UC, how was the odor? I've heard that's rather indescribable but is sometimes equated to the smell of rotting fish.

Quote: Originally posted by DraconicAcid

Quote: Originally posted by thebean   Out of curiosity UC, how was the odor? I've heard that's rather indescribable but is sometimes equated to the smell of rotting fish. That sounds more like pyridine than phenol.

Quote: Originally posted by thebean

Quote: Originally posted by DraconicAcid   Quote: Originally posted by thebean   Out of curiosity UC, how was the odor? I've heard that's rather indescribable but is sometimes equated to the smell of rotting fish. That sounds more like pyridine than phenol. I've heard them both described that way and wanted to clear it up for myself.

Quote: Originally posted by lordcookies24

This is the type of chemistry that I like. To make useful chemicals from normal everyday household items so I don't have to buy from chemical sellers

Quote: Originally posted by XeonTheMGPony

I do recommend doing this stuff as it builds skill and experience...