nimgoldman
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Gallic Acid and Pyrogallol from EGCG/Tannin ?
I was looking into green and OTC sources of gallic acid (GA), pyrogallol and other simple phenolic compounds until I stumbled upon a common food
suplement called EGCG (epigallocatechin gallate).
I already have about 700 g of EGCG, a.k.a. "green tea extract" - the label states it contains 50% polyphenols.
So even if it contained - say - 250 g pure EGCG, that is an equivalent of 92 g gallic acid, which is an interesting amount.
EGCG is one of the many esters of gallic acid (GA) found in plants:
A simple base-catalysed hydrolysis (e.g. refluxing with aq. NaOH) should release GA as sodium gallate. The rxn mixture can be washed with solvents to
remove nonpolar organics, acidified and steam-distilled to obtain even more GA. GA might readily crystallize from water or it can be further
fractionated, extracted etc. if needed, before crystallization.
It seems that the pyrogallol part attached to EGCG might also be avaiable.
A similar compound - catechin - has catechol attached at that site, where gallocatechin has pyrogallol attached.
As catechol has been obtained from catechin in 1832 by desctructive distillation, maybe pyrogallol can be obtain in a similar way from gallocatechin.
All in all, these seem to be green sources of simple phenols.
What pushed my interest even further were tannins. These are abudnant in many plants, particularly walnuts and acacias.
Many vendors sell tannin extracts from these plants, that have high concentration (even 98% !) of tannic acid:
Note that 1 molecule of tannic acid holds 4 molecules of GA - this seems to be an excellent source.
I am looking into any extraction/isolation methods for EGCG and tannins.
So far I found only articles about tannin analysis from certain plants without more details provided about isolation of the compounds.
When they isolated them, they used chromatography, but I doubt this is how it's done in the industrial production of phenols from natural sources.
My best bet so far is to try the base hydrolysis of EGCG and tannin extracts including liquid tannin (pharmaceutical) and powdered tannin extract from
walnut.
Of course, GA can be purchased pure as a niche photographic developer, but this is quite expensive. It's also not worth the shipping when I only need
small amount (say 50 g) of GA or pyrogallol.
I am mapping green/OTC sources of phenols, here is where I am so far:
phenol - decarboxylation of salicylic acid; various synthetic routes from basic/OTC chemicals (sodium benzoate, benzoic acid,
benzaldehyde, benzene...)
catechol - distillation of catechu (catechin-rich extract from acacia); oxidation of salicylaldehyde from salicylic aicd acid; synth.
from phenol
gallic acid - hydrolysis of gallocatechins (from EGCG or tannin extract rich in tannic acid - walnut, acacias etc.)
pyrogallol - decarboxylation of gallic acid; distillation of gallocatechins (freed from GA?); synth. from catechol
Here is the proposed procedure for EGCG:
1. boil/reflux in aq. NaOH, using 2-3 mol. eq. of NaOH, enough water to easily hold EGCG and products (gallocatechin, gallic acid, sodium gallate)
2. wash with non-polar solvent to remove gallocatechin and other organics
3. evap. solvent, heat residue to obtain sublimated pyrogallol; purify by resublimation
4. acidify the aqueous mixture, extract gallic acid
5. evap. solvent, re-extract if needed (acid-base extraction), purify gallic acid by recrystallization from water
Now ideally this will produce gallic acid and pyrogallol. More pyrogallol can be produced by from the gallic acid.
The other way is also possible (gallic acid from pyrogallil), but probably uneconomical - requires halogenation or formylation followed by oxidation
to carboxylic acid either directly or in the Grignard way.
[Edited on 24-4-2020 by nimgoldman]
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Boffis
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Hi Nimgoldman, the example you give as a component of green tea extract will give you a mixture of pyrogallol and phloroglucinol on destructive
distillation. I have a done a fair bit of work on tannin and catechin type natural materials and their dry distillation gives terrible yield in most
cases. You can also use distillation with an alkali/alkaline earth oxide or hydroxide, traditionally KOH was used. This may give modest yields but I
am not sure how you separate the products. Be prepared to do a lot of experimentation! There are a couple of threads on SM about these types of
processes and I started one looking at chemical tests to characterise the type of tannin present.
Acid or aqueous alkali hydrolysis may be a better root to obtain the gallic acid first and then process the residue by what ever means. Aqueous
alkalis may open the pyrane ring (the one with the oxygen in it) as well as cleaving the gallic acid. The residue can then be destructively distilled
while still alkaline.
Gallic acid is very difficult to prepare from pyrogallol. Most routes to carboxylic acid tend to convert pyrogallol into pyrogallol-4-carboxylic acid
where as gallic acid is pyrogallol-5-carboxylic acid.
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SWIM
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Reminds me of the production of heliotropin from...
Was it gambial pine sap?
Some pulp-mill type byproduct was used.
Whatever it was, it used to be a lynchpin of industry. (Well, at least the heliotropin industry)
Some glucoside or something was hydrolized.
Got to look that up again.
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nimgoldman
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Quote: Originally posted by Boffis  | Hi Nimgoldman, the example you give as a component of green tea extract will give you a mixture of pyrogallol and phloroglucinol on destructive
distillation. I have a done a fair bit of work on tannin and catechin type natural materials and their dry distillation gives terrible yield in most
cases. You can also use distillation with an alkali/alkaline earth oxide or hydroxide, traditionally KOH was used. This may give modest yields but I
am not sure how you separate the products. Be prepared to do a lot of experimentation! There are a couple of threads on SM about these types of
processes and I started one looking at chemical tests to characterise the type of tannin present.
Acid or aqueous alkali hydrolysis may be a better root to obtain the gallic acid first and then process the residue by what ever means. Aqueous
alkalis may open the pyrane ring (the one with the oxygen in it) as well as cleaving the gallic acid. The residue can then be destructively distilled
while still alkaline.
Gallic acid is very difficult to prepare from pyrogallol. Most routes to carboxylic acid tend to convert pyrogallol into pyrogallol-4-carboxylic acid
where as gallic acid is pyrogallol-5-carboxylic acid. |
Thanks for the input. It may be difficult, but there are relatively cheap commercial extracts claiming 98-99% gallic acid, see this for example:
This means they have a way to separate the compound from tannins very well. Wikipedia also notes that gallic acid and pyrogallol were obtained back in
1800s so the process must not be too difficult...
Perhaps the compound is separated by complexing with metals or sth. like that?
Distillation of these phenols is doable by an amateur, note the melting and boiling points (in degC):
phenol: 40.5 / 181.7
catechol: 105 / 245.5
pyrogallol: 134 / 309
Phenol can be distilled with a Liebig condenser running hot water to keep it melted. This has been done by Doug's Lab and Nile Red, who worked with
phenol.
Catechol and pyrogallol require significantly higher temperatures so one would have to use an externally heated air condenser and perhaps vacuum as
well.
Another option is resublimation.
I found patents on purification of the compounds, this is not difficult, but so far I found very little information on the actual extraction and
isolation from root barks and tannins - this is quite frustrating since it's done so commonly in the industry. Maybe I am just missing the right
keywords...
I found some articles, like this one:
| Quote: | | Bazinet, Laurent, David Labbé, and Angelo Tremblay. "Production of green tea EGC-and EGCG-enriched fractions by a two-step extraction procedure."
Separation and Purification Technology 56.1 (2007): 53-56. |
They use simple two-step extraction with water to obtain catechin-rich extract. It however contains lots of other stuff, like caffeine. This should be
somehow removed to get the EGC and EGCG concentration high enough to be usable in further separation of the phenolics.
If I understand it correctly, the raw plant material (e.g. pulverised root bark) is autoclaved, freeze-dried, sonicated or treated in similar ways to
break it down on cell level. Then it's extracted with water to obtain so called "hydrolyzable tannins". From these a subgroup called "gallotanins" is
separated as this is the one desired. Gallotanins are then source of gallic acid and pyrogallol.
Another subgroup of tannins called "catechins", is a source for catechol.
It seems to be a huge topic and even after hours of searching I get bascially nowehere. So I will probably just purchase the already-purified gallic
acid / pyrogallol directly, perhaps from China, and postpone this project...
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SWIM
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| Quote: Originally posted by SWIM | Reminds me of the production of heliotropin from...
Was it gambial pine sap?
Some pulp-mill type byproduct was used.
Whatever it was, it used to be a lynchpin of industry. (Well, at least the heliotropin industry)
Some glucoside or something was hydrolized.
Got to look that up again.
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yeah, that was vanillin. Oops.
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nimgoldman
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Heliotropin (piperonyl aldehyde) is made industrially from catechol by methylenation and formylation (depending on the order of reactions, it can be
via 1,3-benzodioxole or protocatechuic aldehyde).
I am looking into gallic acid and pyrogallol as these are important starting materials for 1,2,3-trimethoxybenzene and 3,4,5-trimethoxybeznaldehyde
(pharmaceutical precursor).
Phenol and catechol are also important. Either can be purchased, but may become unavailable, so I am looking for ways to extract/isolate/synthese them
from OTC products.
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