2,4-dihydroxybenzoic acid from resorcinol

Kolbe-Schmitt synthesis - carboxylation of phenols (addition of CO2).

Phenol reacts with CO2 at high temperatures and high pressures forming salicylic acid.
In home lab such reaction runs well with more reactive phenols like resorcinol (1,3-benzenediol), phloroglucinol (1,3,5-benzenetriol), it works with NaHCO3 but KHCO3 should be better (RbHCO3, CsHCO3 too).

reactants in molar ratio 3:1
15,0 g KHCO3 (or 12,5 g NaHCO3) 0,150 mol
5,5 g resorcinol 0,050 mol
45 ml water (should fully dissolve KHCO3 even at 70 C).

5,5 g resorcinol (0,050 mol) + 15 g KHCO3 (0,150 mol) + 45 ml H2O was placed into 100 ml flask (the smaller volume the less of O2 to oxidize resorcinol) and boiling chips added.
1,5 hours heated in 95 C water bath during which a little of CO2 evolved, then 1 hour at reflux temperature (total reaction time 150 minutes, heating for longer than 2,5 h leads to product loses and purity degradation due to rearrangement into 2,6-dihydroxybenzoic acid).
Transferred into 250 ml flask while still hot + twice rinsed with little of H2O, cooled down to room temperature, dropwise added 17 ml 35% HCl (minimal amount for neutralization 0,150 mol 35% HCl = 14 ml) with massive CO2 evolution, acidic pH must be achieved with pH at least 3 (controlled with universal indicator paper after the whole HCl added).
Put into a fridge to 4 C for few hours, vacuum filtered.
Recrystallized from 20 ml of H2O.
Yield 1,9 g.
m.p. not determined due to allegedly decomposition - m.p. 208-211 C (dec.) (lit.)

maybe less than 45 ml of H2O could be used and a mixture of KHCO3 + NaHCO3 in which NaHCO3 dissolves later as the reaction proceeds ?
maybe different heating scheme could give better yield (maybe rfx temperature, but then HCO3 decomposes faster and may be missing for the reaction ?)
KHCO3 is better than NaHCO3 in industrial production but in home lab in H2O and reflux temperature and atmospheric pressure maybe both Na, K are similarly effective?


reactants - flask with KHCO3 on left, resorcinol in middle





heating in water bath, little of CO2 evolution





rfx





transferred into larger flask (on left) suitable for following massive CO2 evolution on acidification (HCl on right)





recrystallization

here a method with somewhat better yield (3,0 g instead of previous 1,9 g)

5,5 g resorcinol
25 g NaHCO3
60 ml H2O
250 ml flat bottom flask
3 boiling chips
a stopper just lightly inserted into the neck of flask to allow escaping CO2 gas

heated in boiling water bath for 2 hours, only very little of NaHCO3 (1-2 g) stayed undissolved at the end
removed from bath, very weak rfx 15 minutes with still the stopper in the neck (not to evaporate too much H2O otherwise later NaCl would crystallize)
cooled down to room temperature at which the whole content of the flask solidified
dropwise acidified with 30 ml 35% HCl (minimal amount 28 ml) with massive evolution of CO2, in the middle of the addition the content liquified, ugly frothing during the last 5 ml due to fine precipitated product, frothing managed by swirling the flask in hand vigorously,
then the content heated to dissolve the product, acidic pH must be achieved by previous HCl addition, at least pH 3 must be achieved (checked with universal indicator paper)
cooled down to room temperature very slowly and later put into fridge to 4 C to obtain crystals of the product rather than fine precipitate
vacuum filtered
recrystallized from 25 ml of water (here I didn't filter the hot solution as it was clear)
yield 3,0 g


15 minutes reflux, no need to mess/wet condenser, just do not boil out too much water (a little of evaporation does not matter)

Hi Boffis, using the baloon is great idea. I observed that bubbles of CO2 evolved slowly during the whole reaction for both K/NaHCO3.
Hi Pumukli, the KHCO3 seems to be better at industrial scale in which phenol is melted at 125 C, well not phenol but Na/K phenolate (phenol melts at 40 C) and brutal CO2 pressure 100 atm applied. In home lab we can't achieve these conditions so we are limited to perform the reaction in water solution and not to use phenol but more reactive resorcinol, phloroglucinol and others suggested by Boffis. Better yield 3,0 g vs. 1,9 g could be caused by doubling the concentration of HCO3-, not by cation itself, 3,0 g yield was with 25 g NaHCO3 which is 0,30 mol and 1,9 g yield was with 15 g KHCO3 which is 0,15 mol. Another difference was that whole KHCO3 dissolved at the beginning but NaHCO3 dissolved only partially and during 2 h of heating it dissolved slowly more and more so it could increase CO2 availability. Also different temperature scheme used for heating. I just wanted to improve yield as I found that the yield could be upto 50% and my initial experiment (KHCO3) yielded only 1,9 g = 12 mmol of the product from 50 mmol of resorcinol so the yield was only 24%. 3,0g = 19,4 mmol so the yield 39% which is much better.
Literature claims decomposition at m.p. 208-211 C so I do not know whether it is worth of trying (I have only capillaries and Thiele tubes for that, nothing more advanced).
According suggested chromatography I have only thin layer aluminium foils with silicagel layer, some of them with UV indicator and others without indicator so perhaps visualizing 2,4 and 2,6 acids and eventually other isomers could be done by spraying the foil with an acidobasic indicator? I have plenty of them, maybe 10 with different pH range color change. Foils are somewhat older, I saved them years ago while visiting biochem analysis lab in hospital which was just right shut down forever, they planed to trash them.



[Edited on 25-5-2020 by Fery]

I found this
Comprehensive Practical Organic Chemistry: Preparations And Quantitative Analysis
By V.K. Ahluwalia, R. Aggarwal, V.K. Ahluwalia