The Violurate Issue: Understanding the Unique Behavior of Barium
Analysis of Barium Violurate via Sodium Violurate
University of Connecticut
Department of Chemistry
GENERALIZED REACTION AND RELATED DISCUSSION
As described by the user “RustyShackleford” on the sciencemadness forums, the procedure begins from Barbituric acid and via NaNO2 forms Sodium Violurate. There is mention of heating the solution of acid when adding the NaNO2. Additionally, the use of Sodium Violurate to form the special Barium and Ethylene Diamine compounds is mentioned separately than that of the Violuric acid for the other compounds. There is also mention of pH and buffer usage in order to complete the crystallization of the products. Common ion effect seems to also be of importance.
Another important note is that even with the limited information given, it is most likely that a ratio of 1:1 is sufficient, and hydration number is important to recognize. The author clearly states, “Note: if you touch the damp precipitate with a metal spatula, blue-violet spots will appear at the point of contact after a short time. Obviously, even the smallest traces of metal lead to a salt / complex formation.” This leads me to believe that the reaction proceeds very easily, however it does not speak to how strong the bond is over time. My expectation is that although the reaction proceeds rapidly in excess, it may need coaxing to stay complexed. The author mentions the usage of a common ion carrier as well as pH control. A nice graph from a research article on Ketoximes and their pKa, shows some compelling evidence on the nature of why the reaction is so nuanced.
Figure 3 from the literature: 1HNMR vs pH graph of acetamidoxime
Something to keep in mind here as you look at the graph, is that different bases have conjugates of differing pKa. An important example is the catalytic nature of H2SO4, which has a diprotic nature, but can instead use a single proton catalytically. Therefore, the pH must be manipulated to afford a chemical environment that feasibly gives you the ability to conduct the reaction. Interestingly enough, the user states that a special procedure was used to form Barium and Ethylene Diamine compounds, but did not state that they couldn’t be formed from the previous methods. But, they did state that they used the leftover Sodium Violurate from their first procedure to make the Barium Violurate. Although not stated, it is likely they used an ion exchange technique involving Barium Monochloride as a way to get the more electropositive barium onto the Ketoxime.
Going back to the part of the procedure talking about precipitating out the Sodium Violurate, there are some interesting notes. Firstly, the barbituric acid reacted immediately. The haste of the reaction means that very little side reactions were formed and was partially backed up by a user-defined calculation stating a yield of 84.5%. If this calculation is correct, it means the rest of the compound lost was most likely due to procedure and incomplete precipitation. We also can confidently say that no sodium ions reacted with any other amino group on the barbituric backbone, due to known chemistry. The addition of sodium or other alkali metals onto an amine such as ammonia actually requires a lot of energy, and is usually done with electrochemistry.
In conclusion of this section, we can say for sure that we are only concerned with pH, relative pKa’s and ion strength.
pKa AND COMMON IONS: THEIR EFFECTS
Figure 6: pKa values in water using theoretical methods corrected using verified experimental results
From the literature, we can see two major outcomes of this research. The first recognition is the pKa ofs various oximes, including a cyclic imidazole oxime (No. 17 - Figure 6). Appended to the figure No. 6, was a statement, “A recently published study demonstrated that the metal cation affinity of the amidoxime group can be enhanced by increasing electron donation to the oxime group.” This statement is the second important recognition, as it states directly that these compounds indeed have an affinity for metals and are rightly labelled as chelators. They further stated that the cyclic imidazoline oxime had a greater basicity at the oxime oxygen rather than the other amino groups in the imidazoline ring. Inversely, if the amines within the ring are coupled to carbons that are electropositive due to electron withdrawing groups, then the acidity of the oxime oxygen goes up. This further bolsters our hypothesis that metal ion stability is forced by controlling pH and using common ion techniques. Our Barbituric acid derivative, Violuric Acid, has amino moieties that are blocked by the carbonyls present and act as a method of increasing the acidity of the ketoxime.
In the image above, one can see the partial-charge distribution of the violuric acid. Notably the stability leads to a high acidity of the oxime oxygen, and thus freely forms metal salts. However, if one was to use the commercially available Chemicalize (ChemAxon), you would find a different theory. The reason this happens is because the machine does not understand Oxime behavior, and literature seems to be the only road leading understanding.
With all of this information, we now can start to understand exactly why the Barium and Ethylene Diamine Violurates require such odd methods to achieve.
CONCLUSION: AN UPDATED METHODOLOGY
Barium, unlike many other salts, behaves oddly since it is so electropositive. The only similar atom is Potassium, with a pauling-scale electronegativity lower than that of Barium. A lot of this is important seeing as electronegativity is highly correlated with pKa and conjugate pKa. Unlike KNO2 or similar , it is easier than Barium, this is probably due to the size of the atom of Potassium. Barium is much larger, and so electropositive, that it is far more protic than say hydrogen. Hydrogen having an electronegativity of 1.0 means that reacting Barium requires that the conjugate formations are stable and that the hydrogen goes to a base. Although one may expect barium to displace hydrogens on the ketoxime, it seems that the pKa doesn’t correlate with the stability in this case. The barium compounds are far more stable than using hydrogen, and bariums size makes it ideal. Instead you can get around using the Barium monochloride salt, which has no hydrogens, and can be used to displace a more electronegative atom such as sodium. A nice experiment that could prove this theory would be the testing of displacement of potassium ions with barium ions. If the barium displaced the potassium, then we’d know that our theory is not correct. If the potassium fails to be displaced, we would know that the reaction stability is based on electronegativity (or in this case electropositivity).
A revised method may be:
If you have attempted the production of violurates with anything but Barium chloride, a few things may happen. One occurrence could be the fading or decoloring of the achieved product upon drying. This is most likely due to the breakdown of the compound back into Violuric Acid. Since Violuric Acid has a slightly pink color, your precipitate may look like a fade between pink and the color gained upon successful reaction. More specifically, it is due to the sodium reforming a salt after drying, and because not all of it reformed, you will have an odd mixture of Violuric Acid (slightly pink-creme colored), Sodium Violurate (deep red colored), and Barium Violurate (orange-red colored) which in conjunction can form a nice hot-pink color.
Image Courtesy of: PoorMansChemist - Barium Violurate
Lastly, judging by the color of that precipitate, it seems that there was not enough Barium ions to overpower the Sodium (or any other metal ions present). Likely there was either contamination or a lack of effective stoichiometry or the usage of a Barium salt other than Barium Chloride.