Salicylaldehyde to 2,5-dihydroxybenzaldehyde

[Lego]

Oxidation of 2-hydroxybenzaldehyde (salicylaldehyde) to 2,5-dihydroxbenzaldehyde (gentisin aldehyde) with K₂S₂O₈
Z. physiol. Chem.; 53; 375-98; 1907


Abstract: In this article the authors report an anusual oxidation of salicylaldehyde (2-hydroxybenzaldehyde, o-hydroxybenzaldehyde) to gentisinaldehyde (2,5-dihydroxybenzaldehyde) with K₂S₂O₈ in aqueous NaOH.

Keywords: Salicylaldehyde, gentisinaldehyde, 2-hydroxybenzaldhyde, 2,5-dihydroxybenzaldehyde, K2S2O8, oxidation



Original article:

62 g Salicylaldehyd wurden in 400 cm Wasser + 25 g NaOH aufgelöst.
Dieser Lösungen wurden in je zehn Portionen 200 ccm 40%ige Natronlauge und 1500 ccm 10%ige Kaliumpersulfatlösung wechselweise zugesetzt, beginnend mit dem Laugezusatz.
Man läßt die Temperatur nicht über 35° steigen, weshalb diese Manipulation etwa 3/4 Stunde dauert. Äußere Kühlung wendet man zweckmäßig nicht an, da sie die Reaktion unnötig verlangsamt. Nach erfolgtem Zusatz der letzten Portion Kaliumpersulfatlösung läßt man die gebräunte Lösung 24 Stunden stehen.
Am nächsten Tage säuert man mit konzentrierter Salzsäure is zur deutlichen Bläuung von Kongopapier an, filtriert vom ölig ausfallenden unveränderten Salicylaldehyd und äthert dessen letzte Reste noch 2-3 mal aus. (Filter und Ätherrückstand liefern nach erfolgter Wasserdampfdestillation etwa 23 g zurückgewonnenen Salicylaldehyd.)
Die von unverändertem Salicylaldehyd befreite, den Gentisinaldehyd enthaltende Lösung wird nunmehr mit 1/6 ihres Volumens rauchender Salzsäure (spez. Gew. 1,19) versetzt und zur Spaltung des obenerwähnten Gentisinaldehydmonoschwefelsäureesters langsam auf dem Wasserbade bis auf 70° erhitzt, und dann freiwillig erkalten gelassen.
Ein amorpher schwarzer Niederschlag wird abfiltriert, das Filtrat wiederholt mit Äther extrahiert. Nach dem Verdampfen des Äthers hinterbleibt ein brauner öliger Rückstand, der von den letzten Spuren des Äthers auf siedendem Wasserbad befreit, meist kristallinisch erstarrt. Er stellt den rohen Gentisinaldehyd dar, der beim Lösen in heißem Benzol alle Verunreinigungen zurückläßt, um bei freiwilligen Verdunsten des Benzols in prächtigen benzolhaltigen Prismen anzukrystallisieren. An der Luft verliert er das Krystallbenzol und erweist sich mit dem von Tiemann dargestellten Produkt als völlig identisch.
Ausbeute 17 g.

Translation by Lego:
62 g salicylaldehyde (I) are dissolved in 400 ml H₂O containing 25 g NaOH.
To this solution 200 ml of a 40% NaOH solution and 1500 ml of a 10% potassiumperoxodisulfate (K₂S₂O₈) solution are added in 10 portions each, starting with addition of the base.
Because temperature should not raise above 35°C the additions takes about 45 minutes. External cooling is not used because the reaction would be slowed down unnecessarily.
After addition of the last portion the brown mixture is left for 24 hours.
Concentrated HCl is added until congo paper turns blue¹. The oily precipitate, mainly unreacted salicylaldehyde, is filtered. The filtrate is extracted with Et₂O 2-3 times.
(Ether residue and filter residue yield after steam destillation about 23 g recovered, unchanged salicylaldehyde.)
To the solution which is now free of salicylaldehyde fuming HCl is added (about 1/6 of the volume). To decompose the gentisinaldehyde monosulfuric acid ester the mixture is heated on a waterbath slowly to 70°C. Afterwards the reaction is allowed to cool to RT without external cooling.
An amorphous, black preciptitate is filtered, the filtrate extracted with Et₂O several times. Evaporation of the Et₂O yields a brown oily residue. After removing the last traces of Et₂O on a boling water bath the crude gentisinaldehyde crystallizes. After recrystallization from benzene all impurities are removed. After air drying 17 g² of pure 2,5-dihydroxybenzaldehyde (II) are obtained which are identical with the product prepared from the product prepared from hydroquinone and CHCl₃ by Tiemann.


[1] Congo paper is paper impregnated with congo red. The colour changes from blue-violet-->red-->orange (pH 3 to 5.2). So pH 3 should be adequate here.
[2] Yield: 24,2%, 38,5% based on only the unrecovered salicylaldehyde

I   2-hydroxybenzaldehyde

Molecule:

a1 ("c1(ccccc1O)C=O")



II  2,5-dihydroxybenzaldehyde

Molecule:

a2 ("c1(cc(ccc1O)O)C=O")



Comment by Lego:
The advantage of this reaction is that the unsuspicious and common chemical salicylaldehyde can be converted to a valuable precursor for 2C-X compounds with OTC chemicals, although yields are not great.
Salicylaldehyde can be either prepared from salicylic acid (which can be prepared by hydrolysis of acetylsalicylic acid) or from salicylaldoxime which is widely used in anorganic analytical chemistry. Salicylaldehyde is afaLk an unsuspicious chemical.
Introduction of the formyl-group to hydroquinone or 1,4-dimethoxybenzene is often low yielding and requires expensive or hard-to-get reagents, this method offers a novel route to 2,5-DMBA via oxidation and subsequent methylation of 2-hydroxybenaldehyde.
This reaction also works with salicylic acid instead of salicylaldehyde yielding 2,5-dihydroxybenzoic acid (gentisic acid).





Lego is not responsible for any translating errors nor any other errors, nor damages caused by any of these errors or otherwise.

[hest]

My personal eksp. with oxone (K2S2O8) is a lot of CO2 and that's it, but it sound's fun.

[Lego]

Oxone is a salt containing KHSO₅ aka potassium peroxomonosulfate.

K₂S₂O₈ is potassium persulfate.

It thence follows K₂S₂O₈ is not Oxone

[yellium]

I've performed a similar reaction with salicaldehyde and potassium persulfate, and all I ended up with was polymerous tar. But then again, that was one of the famous 'bake everything for 4 hours at much degrees, collect brown residue, distill and isolate 2nd fraction with 50% yield'-articles.

[GC_MS]

What's nice about Elbs is the fact that the target compounds can be obtained in a pure form, and that your precursors can often be recovered from the reaction mixture. I think that low yields is THE factor that keeps most bees from applying it regularly.

[GC_MS]

I've performed a similar reaction with salicaldehyde and potassium persulfate, and all I ended up with was polymerous tar. But then again, that was one of the famous 'bake everything for 4 hours at much degrees, collect brown residue, distill and isolate 2nd fraction with 50% yield'-articles.

You ended up with tar probably because your reaction conditions weren't monitored properly. Elbs should be performed at room temperature, and afaik, certainly not heated any higher. The Elbs reaction mechanism involves radicals which may lead to polymerization, usually by coupling of two substituted phenyl radicals to a biphenyl.

[hermanroempp]

Die von unverändertem Salicylaldehyd befreite, den Gentisinaldehyd enthaltende Lösung wird nunmehr mit 1/6 ihres Volumens rauchender Salpetersäure (spez. Gew. 1,19) versetzt und zur Spaltung des obenerwähnten Gentisinaldehydmonoschwefelsäureesters langsam auf dem Wasserbade bis auf 70° erhitzt, und dann freiwillig erkalten gelassen.
Ein amorpher schwarzer Niederschlag wird abfiltriert, das Filtrat wiederholt mit Äther extrahiert.

AFAIK fuming nitric acid has a specific gravity >1.42. The spec. gravity mentioned in the above cited article corresponds to about 23% nitric acid!
Now, which one to use, the fuming acid (= rauchende Salpetersäure), which is a very strong oxidizer (remember your're dealing with an aldehyde!), or the 23% acid?

Herman is somewhat confused....

[Lego]

This time it was no translating error, but Lego was too stupid to coordinate his eyes, brain and fingers.

It should read:
"...rauchender Salzsäure (spez.Gew. 1,19)..."

Sorry for the confusion

[hermanroempp]

"...rauchender Salzsäure (spez.Gew. 1,19)..."

means "...fuming hydrochloric acid, spec. gravity 1.19