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Author: Subject: The chemistry of Sorbic acid
Boffis
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[*] posted on 28-5-2020 at 14:10
The chemistry of Sorbic acid


Two threads have appeared recently on the esterification of sorbic acid (1,2) and previously I have posted questions about possible Diels Alder type condensations of sorbic acid (3). These threads apart I would like this thread to become the resting place for all things sorbic.

Sorbic acid is an interesting substance having two adjacent double bonds, one of which is adjacent to a carboxylic acid group and this affects its reactivity. I have already experimented with adding bromine across the double bonds in a chlorinated solvent and I am currently experimenting with adding chlorine in an aqueous medium. Being a conjugate diene sorbic acid readily undergoes Diels Alder type addition reactions (e.g. (4), but numerous other references exist)

There are numerous papers out there concerned with the reaction of sorbic acid or sorbate ions in food with other food component and preservatives such as sulphite (7), thiols (7), nitrite (7,8,9,10) and ammonia(7). While these are mostly dealing with very dilute solutions they point to some interesting chemistry that I feel is worth following up at preparative concentrations as sorbic acid or its potassium salt are so readily available and cheap.

The reaction I alluded to above concerning the addition of chlorine to sorbic acid in aqueous solution is based on a method of analysing sorbates in food (5). The fact that the reaction is practically quantitative and carried out using very simple, OTC chemicals, makes it attractive. Initial results look promising, the resulting compound is claimed to be 2,5-dichloro-3-hexenoic acid. The 2,5-addition with migration of the remaining double bond to the 3 position seems to be a common theme in aqueous solutions of sorbic acid (7). The rapid addition of bromine in carbon tetrachloride is supposed to be 4,5 addition followed more slowly and with forcing conditions the 2,3 positions, ultimately giving hard glassy crystals of 2,3,4,5-tetrabromohexanoic acid.

The reaction of sulphite ions and thiols with sorbic acid give rather unstable addition products that tend to undergo reversible condensations (7). Nitrite ions or perhaps nitrous acid react with sorbic acid in a complex fashion producing an array of weird compounds such as ethyl nitrolic acid, a furoxan derivative and a dinitropyrrole. These reactions tend to have been investigated in rather dilute solutions by food scientists but may still merit further investigation as the reagents are cheap and available.

I found on paper that claims that the oxidation of sorbic acid with dichromate and acid yields malondialdehyde (11). The reaction of the latter with thiobarbituric acid produces a red polymethine dye that is the basis of the photometric measurement for sorbic acid. However, there is no explanation of the mechanism of malondialdehyde formation by this route and I must admit that I find it hard to believe. I have been unable to track down other references to this method or the underlying reaction.

The references below are only a few of the considerable number of papers I have tracked down related to the chemistry of sorbic acid and potassium sorbate. The Diels Alder addition type reaction seems a particularly fruitful avenue for amateur chemists.

1) Sorbic acid esters: http://www.sciencemadness.org/talk/viewthread.php?tid=155312
2) Synthesis of methyl sorbate: http://www.sciencemadness.org/talk/viewthread.php?tid=155365...
3) Diels Alder reactions of sorbic acid http://www.sciencemadness.org/talk/viewthread.php?tid=154454...
4) Craig & Shipman; 1952; Maleic anhydride adducts of sorbic acid and methyl sorbate; JACS; v74; p2905.
5) Spacu & Dumitrescu; 1967; Determination of sorbic acid with sodium chlorite, Talanta, v14, p981.
6) Farmer & Healey; 1927; Properties of conjugate diene compounds Part II Addition to Butadiene Esters; JCS p1060.
7) Khandelwal & Wedzicha; 1990; Nucleophilic reactions of sorbic acid; JFoodAdditives&Contam; v7; p685.
8) Namiki & Kada; 1975; Formation of ethylnitrolic acid by the reaction of sorbic acid with sodium nitrite; Agri. & Biol. Chem.; V39; p1335-1336.
9) Kito et al; 1978; A new N-nitropyrrole, 1,4-Dinitro-2-methylpyrrole; Tetrahedron; v34, p505-508.
10) Osawa et al; 1979; A new furoxan derivative and its precursors formed by the reaction of sorbic acid with sodium nitrite; Tetrahedron Letts; No.45; p4399-4402.
11) Molina et al; 1999; Spectrophotometric flow-injection method for determining sorbic acid in wines; LRA; v11, 299-303.
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[*] posted on 28-5-2020 at 16:13


Very interesting, I will be checking out the references. I am honoured to be referenced as well.
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[*] posted on 29-5-2020 at 04:48
ethylnitrolic acid


From one of the OP's references is the facile synthesis of the interesting compound ethylnitrolic acid

"A solution of
sodium nitrite was added to a partially suspended solution of sorbic acid, each 0.5 M in
distilled water, at room temperature and heated
in a water bath at 90°C for 1 hr. The mixture colored yellow instantaneously, it turned
red and finally became dark red with heating,
where pH of the mixture changed from 4.3 at
the beginning to 6.0 in the final stage. The
reaction mixture was extracted two times with
dichloromethane and then three times with
ether, and each of the extracts was concentrated
in vacuo to give a dark red oily product."

From

Attachment: Formation of Ethylnitrolic Acid by the Reaction of Sorbic Acid with Sodium Nitrite.pdf (349kB)
This file has been downloaded 48 times




i am wg48 but not on my usual pc hence the temp handle.

Thank goodness for Fleming and the fungi.

Old codger' lives matters, wear a mask and help save them.
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[*] posted on 30-5-2020 at 10:36
Aqueous Chlorination of Sorbic Acid


Enough of the intro stuff, now for some real chemistry.

Attempts at chlorinating Sorbic Acid with Sodium Chlorite

These experiments were developed from a paper by Spacu and Dumitrescu (1) on the determination of sorbic acid in foods by chlorinating the acid with excess sodium chlorite and then measuring the excess chlorine in the reaction mixture. Sodium chlorite is a fairly accessible bleaching /sterilizing agent and can sometimes be purchased on line from various suppliers. It comes as a white crystalline powder that consists of about 80% NaClO2 with the remainder being mainly NaCl (salt).

Sodium chlorite reacts with excess hydrochloric acid to give sodium chloride and chlorine, approximately according to the equation:

NaClO2 + 4HCl -> NaCl + 4Cl + 2H2O

The reaction with sorbic acid is, according to Spacu and Dumitrescu:

NaClO2 + 4HCl + 2C6H8O2 -> NaCl + 2C6H8O2Cl2 + 2H2O

chlorosorbic acid.gif - 3kB

In the original work they were looking at low concentrations and higher concentrations do not always favour the same reaction route, however, in order to make recovery of the resulting compound easier I used fairly concentrated solutions. One problem was that I had no data of the properties, such as melting point and solubility, of the target compound so the first few experiments were carried out very much in the dark.

Experimental
The first 4 experiments were carried out on the same basic scheme:

Approximately 4g of sodium chlorite (80%) and 10g of potassium sorbate were dissolved in 40ml of water. In a separate beaker 23ml of 30% hydrochloric acid were diluted with 50ml of cold water. The mixed salt solution was then added dropwise into the rapidly stirred acid over about 10 to 20 minutes. The addition was accompanied but a slightly exothermic reaction and the mixture turns slightly yellow though this colour fades after 10 to 20 minutes. Faster addition causes a slightly more pronounced exotherm and some chlorine is lost from the solution.

In Experiment 1 the beaker was stood in a water bath at room temperature, about 16 C, as soon as the exothermic reaction was noted. When the addition was complete and the colour discharged the precipitated solid was filtered off, washed with a little water and dried to give 2.295g. The white compound can be recrystallized from dilute ethanol, azeotropic isopropanol or water. The latter solvent proved the best but about 30ml per g are required. When alcohols are used for the recrystallization the product was sticky.

After the filtrate had been discarded the product was identified as un-reacted sorbic acid.

Experiment 2 was conducted in exactly the same way except than the water bath was omitted. The ppt was filtered off as before (1.405g of sticky unreacted sorbic acid) and the filtrate extracted with 40ml of ether and then two further portions of 30ml each. The extracts were combined and the ether recovered by distillation to give a small volume of colourless liquid that soon crystallised into small rosettes of blades, these were filtered from the small volume of slightly thick aqueous liquid; yield was 2.741g after drying. This material is quite distinct form sorbic acid being highly soluble in water.

Experiment 3
In order to reduce the amount of unreacted sorbic acid this experiment was conducted as in Exp. 2 but with 4.616g of sodium chlorite. Unfortunately this generated a stronger exotherm (circa 40° C) and the ppt became very sticky, it stuck to the stir bar, the beaker walls and the anything else it touched. The sticky white material was ether soluble so the entire mixture was extracted with 50ml of ether in the beaker before pouring it into the separating funnel. It was then extracted with two further 30ml portions of ether. The ether was gently distilled from the combined extracts and the very pale yellow oily residue poured into a bowl and allowed to cool and the last of the ether to evaporate. About 2.5-3g of sticky white crystals remained after a few hours. Recrystallization from azeotropic isopropanol (87%) gave 0.822g of slightly sticky unreacted sorbic acid. Foolishly I rejected the filtrate; this would have been better evaporated and leached with water.

Initial Observation:
1) Higher reaction temperatures favour increased consumption of sorbic acid.
2) Higher reaction temperatures increase the amount of sticky polymer like by-product.

From these observations the obvious next step is to try a lower temperature reaction.

Experiment 4 was run with the 3.830g of sodium chlorite and 10.005g of potassium sorbate dissolved in 40ml of water and then chilled in the fridge to about 4°. 23ml of 30% hydrochloric acid were diluted with 50ml of water and placed in the freezer until it had reached -16° C.
The acid was placed on the stirrer plate and stirred rapidly as the mixed salt solution was added. A dense white ppt formed that eventually stopped the stir bar from moving; this is almost certainly simply precipitated sorbic acid. The temperature on completion of the addition was -5° so little or no exothermic reaction had occurred. In fact no reaction apart from the liberation of free sorbic appeared to have taken place so the mixture was left to stand and warm up slowly to room temperature overnight. The following morning the slurry was less thick and was easily vacuum filtered to give 3.800g of washed and dried sorbic acid. The yellow filtrate was extracted with three x 50ml portions of dichloromethane (DCM) and then the combine extracts dried with magnesium sulphate. The DCM was distilled off until only about 10-15ml remained in the flask; this was poured out into a bowl and allowed to evaporate slowly. No crystals formed but eventually a small volume of bright yellow, pleasantly ethereal smelling liquid remained. The smell of the liquid is reminiscent of perchloroethylene and I suspect that decarboxylation has occurred to give a polychloro pentane or pentene.

Conclusions
The desired reaction product appears to be exceptionally soluble in water and this may account for the low recovery of the material in Exp.2, basically a poor partition coefficient between water and ether. If this is the case, five or six extractions with ether may be required and possibly the addition of KCl to assist extraction.

The optimum temperature is probably about 15-20° C since higher temperatures favour the formation of a sticky by-product. Lower temperatures involve longer reaction times and this seems to result in a completely different reaction path.

A slightly larger excess of sodium chlorite coupled with a cooler temperature may reduce losses through unreacted sorbic acid and a greater number of ether extractions cycles may improve the yield of product.

These findings point to my next set of experiments but I also intend to investigate the direction of mixing with an experiment where the acid is added to the mixed salt solution.

1) Spacu & Dumitrescu; 1967; Determination of sorbic acid with sodium chlorite, Talanta, v14, p981.
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[*] posted on 3-6-2020 at 02:06


Does any one have access to Reaxys or scifinder? I can't find any useful data on 2,5-dichloro-3-hexenoic acid and I would greatly appreciate a bit of help here with say melting point, solubilty and reactions in order to determine the possible identity of my colourless crystals.

I have run some more experiments now, essentially the same as those described above but with varying excesses of HCl and chlorite but most importantly temperature control. I have manage to get the yield of the ether extract up to 7.5g and the level of unreacted sorbic acid down to a small amount though how small is difficult to tell since a viscous oil forms that sticks to the stir bar and the beaker and any remaining sorbic acid is incorporated into this. The optimum temperature to maximise the yield of the water soluble acid is about 20 C but at this temperature the reaction take several hour to lose its yellow colour and this favours the formation of by products. Lower temperatures (<5 C) favour the formation of the heavy oil (S.G. about 1.4).

Out of curiousity I decided to investigated the chemical properties of the cream coloured stick oil that formed in some of these experiments. The oil is completely soluble in both ether and isopropanol. An isopropanol solution of about 2.5-3g was mixed with a strong solution of sodium nitrite, 4g on 10ml of water. To my surprise there was no precipitate formed and no obvious reaction. On gentle warming a reaction began, the solution turned golden yellow and an odourless gas was evolved (CO2?). After about 20 minutes at 40-45 C a pale brownish ppt had formed. I filtered this off and then continued to heat the mixture a little more strongly. The IPA began to evaporate and more, slightly darker ppt formed from the increasingly dark solution. I let the mixture cool and partially evaporate in a shallow bowl. When cold long slender colourless prisms had formed in amongst a flocculant ppt. The mixture was warmed and a little water added to dissolve the crystals, the pale ppt filtered off and combined with the earlier crop. The filtrate was allowed to crystallise and the cream coloured crystals recovered.

The filtrate was warmed again in a shallow bowl to assist evaporation. The colour darkened further, more gas was evolved and more ppt formed. The volume of solution was now <10ml, it was filtered hot to remove the small amount of light brown ppt and cooled. An abundance of brown crystals formed, obviously colourless but stained by residual liquor, 1.95g of slightly damp crystal after drying. These are awaiting further investigation.

The filtrate was acidified with dilute HCl (2M) but a mass of brown tar formed leaving and almost clear solution. The tar is insolulbe in ether or IPA. The solutions were tested at intervals and remained almost neutral through out. Both the crystals and the light brown ppt are mildly energetic and deflagrate vigorously when heated on foil.

My theory is that the cream coloured viscous oil is a mixture of unreacted sorbic acid and various chlorinated products that react with sodium nitrite causing decarboxylation and nitration, ether via a nitroso intermediate or via halogen replacement.

I tried preparing a copper salt from the crystals, the grass green ppt was not energetic but swelled to a foam like mass much like mercury thiocyanate.
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[*] posted on 21-6-2020 at 14:48


I checked Reaxys for 2,5-dichloro-3-hexenoic acid. Believe it or not, I wasn't able to find a single reference for this compound! (The Spacu and Dumitrescu paper you mentioned is not in the database of syntheses, apparently.) I was also unable to find any reactions of sorbic acid with either sodium chlorite, chlorine dioxide, or chlorous acid in the literature. I was however able to find two papers giving the reaction of aqueous chlorine/hypochlorous acid and sorbic acid:

https://sci-hub.tw/https://pubs.rsc.org/en/content/articlela...
https://sci-hub.tw/https://pubs.rsc.org/en/content/articlela...

Both papers suggest a series of chlorinated products are the outcome of the reaction, with hydrolysis to the alcohol and lactone formation being observed to a large degree.

I also find it somewhat interesting that the authors seem to believe that the reaction of chlorite ion with hydrochloric acid produces chlorine, when as far as I know the major product is chlorine dioxide. Maybe this would explain the lack of literature precedent.
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[*] posted on 22-6-2020 at 13:47


Hi Cryolite; those are two really interesting papers. They highlight the difference between the electrophilic attack of halogen which seem to give rise to 3,4 substitution in sorbic acid as against the nucleophilic reagents which according to some of the papers I referenced above give mainly 1,4 substitution. This raises the issue of what the actual reactive species is in aqueous HCl/chlorite mixtures. I was also interested in the difference the presence of water makes. I am just writing up the bromination of sorbic acid to both 4,5-dibromo-3-hexenoic and 2,3,4,5-tetrabromohexanoic acid for this thread. These were carried out in essential dry conditions in DCM and carbon tetrachloride respectively and the results match the published products. The chlorination in aqueous condition therefore may give entirely different products.

Incidentially, some years ago I did some experiments with caffeine. When dissolved in dilute HCl both sodium chlorite and sodium chlorate yield first of all a well crystallised chlorocaffeine and then proceed to degrade the 5-membered ring to give dimethylalloxan and methylurea. In the first stage of this reaction both sodium chlorite and chlorate seem to act as proxys for Cl2.
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[*] posted on 22-6-2020 at 14:33


Glad my search was of use to you-- let me know if you'd like me to search Reaxys for anything else.

I would definitely believe that the anhydrous halogenation gives the desired 2,5-dibromo and 2,3,4,5-tetrabromo products, as that would proceed by the standard 1,4 addition reaction standard to dienes. For the aqueous chlorination, my hypothesis is that the halonium ion forms as normal but the nucleophillic carbonyl attacks this ion as it forms and gives a lactone (which then may hydrolyze to the alcohol). This is an analogous mechanism to the reaction of amino acids with nitrous acid, which gives the alpha-hydroxyacid in dilute conditions and the alpha-haloacid in concentrated acids. This would explain the products in the papers I linked-- addition to the 4,5-double bond might be preferred for steric or electronic reasons.

I too am interested in what the true oxidizing species in chlorite + HCl mixtures is. You might very well be right that the main reactive species is chlorine, as evidenced by your chlorocaffeine synthesis.
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