Sciencemadness Discussion Board

Dissolution of a copper salt in mineral turpentine

weiming1998 - 3-8-2012 at 01:12

A few days ago, I added some NaOH solution to cooking oil and waited. A few days later, I obtained some bits of crude soap. I then dropped the bits of crude soap in hot sodium bisulfate solution, and it formed an oily layer at the top, which is the newly-formed fatty acid (since it reacts instantly with Na2CO3). The fatty acid was extracted by a pipette, about 10ml of it, and copper (II) oxide was added in to form a "copper soap", the copper salt of a fatty acid. After a while of careful heating, the mix slowly turned a deep turquoise colour, with water vapour bubbling out. After heating and leaving it reacting at room temperature overnight, I added mineral turpentine to the mix to see if the copper soap would dissolve in it. It did, forming a bright green- blue solution that was immiscible in water. I extracted the solution with the pipette again to a small conical flask.

The question is: Is the copper salt of this fatty acid a mostly ionic compound? If it is mostly covalent, then it will make much more sense, but if it is mostly ionic, then what form is it in when in water? Water splits ionic compound to their separate ions, but mineral turpentine is non polar and aprotic, and an ionic compound is made of an endless "stack" of ions in a particular shape, for example in a cube shape for sodium chloride, but if this solvent doesn't separate the ionic compound into its corresponding ions, how can it dissolve?

Vargouille - 3-8-2012 at 04:11

Perhaps it is a mixture of long and short-length anion-form fatty acids, as as far as I know, cooking oil does not have a specific chemical structure, thus the soaps from it will have carbon backbones of varying lengths. The shorter copper soaps will be more ionic, dissolving more completely (Cu+2>Cu(FACB)>>Cu(FACB)2), while the longer soaps will be less polar, meaning that completely dissociation will not occur as readily. In turpentine, the entire molecule is dissolved. The longer-chains are more soluble in the non-polar turpentine, so the colored solution will tend to remain in the organic layer.

weiming1998 - 3-8-2012 at 06:07

Quote: Originally posted by Vargouille  
Perhaps it is a mixture of long and short-length anion-form fatty acids, as as far as I know, cooking oil does not have a specific chemical structure, thus the soaps from it will have carbon backbones of varying lengths. The shorter copper soaps will be more ionic, dissolving more completely (Cu+2>Cu(FACB)>>Cu(FACB)2), while the longer soaps will be less polar, meaning that completely dissociation will not occur as readily. In turpentine, the entire molecule is dissolved. The longer-chains are more soluble in the non-polar turpentine, so the colored solution will tend to remain in the organic layer.


Everything dissolved in turpentine, though. There were virtually nothing that dissolved in the water. Shaking the solution with sodium carbonate solution will cause the copper to exit the organic layer as copper basic carbonate.

Anyway, are you saying that the shorter-chained copper soap dissociated into ions? Doesn't non-polar, aprotic solvents not dissociate solutes into ions because there are no proton that is attracted enough to the negative ion, and because no ligands are in solution (well, in the case of mineral turpentine, I can't see any potential ligands around).

Vargouille - 3-8-2012 at 06:46

In water, small amounts of both forms will become solvated and dissociate into ions, and the shorter-chained copper soap will do so more completely, leaving fewer ions with one fatty acid conjugate base (FACB), and fewer with both. This is because, and correct me if I am mistaken, the shorter tail means that the molecule overall is more ionic that the longer tailed variants, resulting in increased solubility, albeit slightly increased. The longer-tailed variants are still ionic, but less so, meaning that there will be more ions of Cu(FACB) and more solvated molecules of Cu(FACB)2. After all, even toluene is very slightly soluble in water.

In turpentine, the opposite holds true, as the longer chains are more readily solvated by the turpentine in the form of the entire molecule. The color is due to the copper, regardless of the FACB "ligands" on it.

EDIT: The precipitation of basic copper carbonate, if that is indeed what the compounds is, may be a combination of the carbonate ion reacting with the small amount water present in the turpentine to form bicarbonate and hydroxide. The bicarbonate reacts with the copper soap, acting as an acid to the FACB to jettison a molecule of the fatty acid which dissolves in the turpentine, and precipitating a mixture of copper carbonate and copper hdyroxide.

[Edited on 3-8-2012 by Vargouille]

Nicodem - 3-8-2012 at 07:01

Quote: Originally posted by weiming1998  
Anyway, are you saying that the shorter-chained copper soap dissociated into ions? Doesn't non-polar, aprotic solvents not dissociate solutes into ions because there are no proton that is attracted enough to the negative ion, and because no ligands are in solution (well, in the case of mineral turpentine, I can't see any potential ligands around).

Copper carboxylates are generally not "ionic salts" in the common sense of the term. They are coordination compounds. The Cu is bound to the carboxylate ligand via one or two coordination bonds (aka dative covalent bonds). The carboxylate can even act as a bridging ligand. There are plenty examples of determined structures of copper carboxylate complexes in the literature. I suggest you to read a few articles on the topic, if you are interested in the nature of this interaction, but in essence, the copper carboxylates do not need to dissociate in order to dissolve, and with a solubility handle like the long alkyl chains of the fatty acids they can dissolve even in turpentine.

The solubility of copper(II) fatty acid salts in non-polar solvents is known. The cobalt and other transition metal 2-ethylhexanoates are common pigments used as an admixtures in oils. For example, on the can of a blue color that I have, it says it contains cobalt 2-ethylhexanoate.

See also
https://www.sciencemadness.org/whisper/viewthread.php?tid=10...

weiming1998 - 3-8-2012 at 17:05

Quote: Originally posted by Nicodem  
Quote: Originally posted by weiming1998  
Anyway, are you saying that the shorter-chained copper soap dissociated into ions? Doesn't non-polar, aprotic solvents not dissociate solutes into ions because there are no proton that is attracted enough to the negative ion, and because no ligands are in solution (well, in the case of mineral turpentine, I can't see any potential ligands around).

Copper carboxylates are generally not "ionic salts" in the common sense of the term. They are coordination compounds. The Cu is bound to the carboxylate ligand via one or two coordination bonds (aka dative covalent bonds). The carboxylate can even act as a bridging ligand. There are plenty examples of determined structures of copper carboxylate complexes in the literature. I suggest you to read a few articles on the topic, if you are interested in the nature of this interaction, but in essence, the copper carboxylates do not need to dissociate in order to dissolve, and with a solubility handle like the long alkyl chains of the fatty acids they can dissolve even in turpentine.

The solubility of copper(II) fatty acid salts in non-polar solvents is known. The cobalt and other transition metal 2-ethylhexanoates are common pigments used as an admixtures in oils. For example, on the can of a blue color that I have, it says it contains cobalt 2-ethylhexanoate.

See also
https://www.sciencemadness.org/whisper/viewthread.php?tid=10...


Oh, so the carboxylates acts as the ligand. I get it now. Thanks. I'll look up more info about it.