RadicallyStabilized
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Synthesis: Making phenol by catalytically decarboxylating salicylic acid in benzoic acid
Description:
Phenol was made by decarboxylating salicylic acid in benzoic acid with magnesium as a catalyst according to the procedure laid out in [1].
Overview:
A flask was charged with 50 g of benzoic acid and 950 mg of magnesium oxide. The mass was heated to 215 °C and over the course of about four hours 79
g (0.5 mol) of salicylic acid were added with stirring. Phenol vapor was driven over with CO2 through an air condenser. The product was distilled
twice and yielded 31 g (0.33 mol) of phenol (66%). The remaining reaction mixture was recovered and saved for a future run.
Discussion:
This reaction is easy to perform. Cleanup is simple, too, as all materials are more or less water-soluble. An advantage is that theoretically
unlimited amounts of phenol can be made by just adding more salicylic acid. Addition speed doesn't seem too critical (if you add too much you may get
slight bumping), or it may be interrupted for some time. The benzoic acid can be cooled down and heated up again and the reaction can then continue.
Care should be taken to ensure good ventilation as every time the stopper is opened fumes of salicylic acid and phenol escape. A disadvantage compared
to the plain decarboxylation of salicylic acid is the necessity to use benzoic acid. This chemical is cheap and OTC, though, and it can be mostly
recovered and used for the same reaction or perhaps be worked up. Another disadvantage is the use of CO2 as a means to drive over the phenol. This
might be omitted; however, I felt that using CO2 greatly eased the coming over of the phenol. Of course, other inert gases such as N2 or a mixture of
argon and CO2 (used for welding) could also be used. Using a vacuum is not feasible with this reaction due to the need of continuously adding starting
material.
The yield was not great but acceptable considering that conditions were certainly not optimal, and it was comparable to those of the decarboxylation
of salicylic acid alone which is apparently a lot more messy.
Experimental:
A 250 ml three-necked round bottom flask was charged with a small stir bar and 50 g of benzoic acid (Merck, p. a. > 99%). On top of this were added
950 mg of magnesium oxide (96.4%). The flask was equipped with a thermometer, a three-way adapter and a gas inlet tube which did not extend into the
material. The straight end of the three-way adapter was stoppered. The crooked end of the three-way adapter was connected to a 30 cm air condenser, a
vacuum adapter and a 100 ml receiving flask. The vacuum adapter was connected by a hose to a gas bubbler dipped into water (to help determine gas flow
rate and to absorb any phenol fumes that might come over). The gas inlet tube was connected to a CO2 bottle.
Heating was started and the benzoic acid soon melted. When the magnesium oxide dissolved there was gas evolution. Moderate stirring was started.
Everything was dissolved when the mixture reached about 180 °C. Heating was continued up to 215 - 220 °C and the mixture was held at this
temperature throughout the reaction.
The little water that had formed was driven out of the apparatus' parts with a heat gun. The three-way adapter and the upper half of the condenser
were wrapped in aluminium foil. 34.53 g (0.25 mol) Salicylic acid (Merck, > 99%) was introduced by removing the stopper and adding spatulas of the
salicylic acid one by one. White fumes appeared on addition and the mixture began to bubble. The CO2 flow was regulated to about five bubbles a
second. In the beginning the addition was done very slowly and it took about one hour for the first drop of phenol to appear. Addition was sped up
later, however. The whole 0.25 mol of salicylic acid were added over the course of about three hours during which time a steady flow of phenol drops
came over. When the phenol solidified in the condenser it was gently heated with a heat gun. When the condenser became too hot the aluminium foil
around it was removed and it was cooled with a moistened rag.
After three hours it was decided to add 0.25 mol more of salicylic acid. This portion was added considerably faster during the course of half an hour.
This didn't seem to affect the reaction negatively, as the rate of bubbling remained relatively constant even when more salicylic acid was added.
After everything had been added heating was continued for about half an hour by which time bubbling had subsided. The aluminium foil was taken off and
the apparatus was heated with a heat gun to help drive over remaining phenol. The flow of CO2 was increased, but this turned out to be detrimental
because fumes of salicylic acid started to become visible and subsequently solidified in the condenser.
At this time heating and stirring was stopped. The remainders of phenol in the vacuum adapter that were not obviously salicylic acid were melted with
a heat gun. The vaccum adapter's hose was disconnected and CO2 flow was stopped. The apparatus was then left to cool. A little less than half of the
100 ml receiving flask's volume of a clear liquid with a solid chunk of a crystalline substance was collected.
Cleanup was straightforward. The contents of the reaction flask had solidified into a hard mass (the thermometer had been taken out before that). They
were quickly melted with a heat gun and poured on a sheet of aluminium foil, then broken up while they solidified. The flask and the condenser could
be cleaned from the remaining solids with hot water. There was no smell of phenol from the flask with the scrubber water.
The crude product was then vacuum distilled by setting up a similar apparatus. The stopper was replaced with a thermometer. The vacuum adapter was
connected to the scrubber flask with a three-way tube and a stopcock to allow for controlled breaking of the vacuum without suckback. The glassware
was dried under vacuum with a heat gun and left to cool. The flask with the product was put in place (a few boiling stones were added) and
distillation was started at around 30 mbar. As before, the upper parts of the device were wrapped in aluminium foil. Condensation started around 30
°C. Up until 85 °C a few drops of a turbid liquid, about 2.5 ml, were collected. (This liquid gave a dark blue color with a few drops of saturated
FeCl3 solution.) Heating was removed and the remaining drops of liquid in the glassware were driven over with a heat gun. The vacuum was carefully
broken (by clamping the hose to the scrubber flask and opening the stopcock, then switching off the vacuum pump) and the apparatus was allowed to cool
down again. The receiving flask was swapped with an empty one. Fresh boiling stones were added to the distillation flask and distillation was started
again by applying the vacuum and switching on the heating mantle.
The majority of the final product came over at around 90 °C with occasional bumping as a clear liquid that almost immediately crystallized. The
product that solidified in the condenser was liquefied using a heat gun. When necessary the condenser was cooled using a wet rag, and the receiving
250 ml flask was cooled by immersing it in a water bath.
The fraction up to 145 °C was collected and the apparatus was left to cool. The slightly pink residue which soon crystallized in the distillation
flask was taken out. The boiling stones were removed and the material added to the benzoic acid residue from the main reaction. Again no smell of
phenol was apparent from the scrubber water.
The apparatus was cleaned, re-assembled and dried under a vacuum. This time no scrubber flask was used for the following vacuum distillation at about
40 mbar.
At 30 °C traces of moisture could be detected but no drops collected. The temperature quickly rose to 90 °C. After some time liquid began to
condense. Unfortunately the mixture had a tendency to produce heavy bumps despite having added a few boiling stones, so it was quickly transferred to
a 500 ml two neck flask fitted with a capillary tube. This got rid of the bumping, however heating had to be reduced due to the increased contact area
of the bigger flask. At 90 °C there was a lot of reflux but nothing came over so heating was increased and the insulation was improved. After some
time at 95 °C distillate began to collect again. The vaccum adapter was heated to avoid clogging. Liquid was coming over fairly rapidly and the
condenser had to be cooled. When the temperature had risen to 100 °C distillation was stopped. The remains in the condenser were liquefied with a
heat gun. The contents of the receiving flask were also liquefied and transferred to a storage container.
The final yield of what was assumed to be fairly pure phenol was 31.06 g (0.33 mol). This represents a percent yield based on salicylic acid of 66%.
The weight of the recovered material (benzoic acid, unreacted salicylic acid and perhaps some undistilled phenol) was 46.53 g.
Notes:
- The scrubber flask during distillation is probably not necessary which makes the setup easier.
- I found boiling stones to be ineffective during these vacuum distillations. A capillary tube works much better.
- The low yield (the original paper states 96%!) might be explained by:
* salicylic acid fumes getting carried over during addition
* phenol being lost when opening the apparatus to add more acid
* salicylic acid having been added too fast, resulting in losses by distillation (perhaps the gas flow was too fast)
* not waiting long enough after adding the final portion of acid
* doing distillation twice when one might probably have been sufficient
* transfer losses
- Using more benzoic acid and magnesium will probably afford faster reaction time.
References:
[1] US Patent 3,061,651 (1962-10-30): W. W. Kaeding, Manufacture of phenols by decarboxylation of salicylic acids
Attachment: Manufacture Of Phenols - US3061651.pdf (313kB)
This file has been downloaded 237 times
[Edited on 17-10-2018 by RadicallyStabilized]
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walruslover69
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What is the point of adding the salicylic acid step wise instead of all at once in the beginning?
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RadicallyStabilized
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The patent does not mention that. I assume it's to avoid too much salicylic acid distilling over because the reaction is rather slow and the boiling
point of salicylic acid is 211 °C, very close to the reaction temperature.
Anyone have an idea how magnesium acts as a catalyst here?
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happyfooddance
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I would try to put in paragraph breaks if you can still edit, it would help a lot for readability.
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UC235
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Not to rain on your parade, but your yield here doesn't seem any better than from the direct, uncatalyzed decarboxylation of salicylic acid I detailed
here which doesn't need constant additions of salicylic acid: https://www.sciencemadness.org/whisper/viewthread.php?tid=27...
The 96% yield in the patent appears to refer to yield when operated as a continuous process and ignoring what might be left in the dead space of the
reaction setup at the end. Losses in batch reactions are sure to be higher. If the patent is true, though it should be possible to tweak the yield
higher in this process, particularly by just running it longer. In the direct decarboxylation, I suspect loss to phenyl salicylate/xanthone is the
major snag.
Another issue with this process is co-distillation of benzoic acid. With a stillpot temperature of 220C, benzoic acid's partial pressure (bp 250) will
be substantial. With an efficient column for a continuous process, all of this would be returned to the stillpot, but much less so for an amateur
batch process, after which some sort of washing procedure or additional fractionation is needed to remove it at the expense of yield.
Naphthoic acids might be a better "liquid phase" with substantially higher boiling point, but they are not all that readily available.
[Edited on 18-10-2018 by UC235]
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VSEPR_VOID
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Very good write up! But not very useful results
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RadicallyStabilized
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Thanks for the feedback. Of course I know that this is just another way of making phenol, not a substantial improvement.
OTOH, I think it's an interesting reaction and for me it was nice to do it this way because I don't like tar...
The co-distillation of the benzoic acid isn't a big issue because it will be separated when redistilling the phenol and can be kept with the
"leftovers". Separation of benzoic and salicylic acid will probably be unfeasible, however.
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