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Author: Subject: The chemistry of Sorbic acid
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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:
2) Synthesis of methyl sorbate:
3) Diels Alder reactions of sorbic acid
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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