NMR Spectroscopy as a Viable Tool in Determining the Solubility of Organic Compounds in Non-aqueous Solvents


by Cedric Tchakounte



Abstract


This paper provides an account of the solubility measurements attained with the use of proton-NMR spectroscopy. Solubilties of various carboxylic acids were determined in different solvents. The acids studied were candidates to the acid component of a Ugi product. Therefore, these Ugi reagents were tested for their solubility in different solvents to ascertain if they were capable of making 2M solutions. which is a suggested concentration for proper synthesis of the Ugi product.
Solubility results were documented and reported on the Open Notebook Science Challenge (ONSC) wikipage, which created a very beneficial dynamic to the research studies. ONSC is an open-source science initiative, wherein collaborative efforts amongst researchers in the scientific community lead to the practice of better science (Alicia 2007).

Introduction


As defined by the CRC Handbook of Chemistry and Physics, solubility refers to the quantity expressing the maximum concentration some material solute, be it solid, liquid, or gaseous, can exist and dissolve into another liquid or solid material solvent at the thermodynamic equilibrium at a specified temperature and pressure (Tim 2008). Solubility data is fundamental to many scientific disciplines, wherein its influence is seen in many various practical applications. Studying the solubility of organic compounds is useful in medical applications, which can be seen as a goal of the open source science research.

The solubility of organic compounds can be determined with the use of nuclear magnetic resonance (NMR). This is accomplished be making a solution of the organic compound being studied in a non-aqueous solvent. A NMR spectra of the solution is obtained and the peaks of the spectra are attributed to being sourced from the solute and the solvents. Those peaks are integrated and the referenced to one another to ascertain a ratio, which is then used to determine the concentration of the solution.

Alicia Hosley in her master thesis attempted to synthesize diketopiperazines and investigated their possible role as inhibitors to enoyl-reductase, which is a protein that malaria uses to synthesize critical fatty acids. Synthesis of diketopiperazines can be accomplished via the Ugi/de-Boc/cyclize (UDC) method, wherein the first process involves the formulation of the Ugi product. The Ugi product is an assembly of four components or Ugi reagents, which are an isocyanide, an amine, a boc-protected carboxylic acid, and an aldehyde (Alicia 2007).

Various carboxylic acids were studied to ascertain their potential as Ugi reagents. The experimental procedure for doing so was constantly being adapted and changed, not only to acquire better results, but to stream-line the process for other researchers to easily follow and reproduce as well.


Experimental Procedure


At the onset of the research accounted in this paper, the procedure for determining the solubility of an organic compound was sufficient; but was quite vulnerable to error.
First, a saturated solution of the chosen organic compound solute in a non-aqueous solvent to be studied was drawn-up. The solution was made by adding and mixing together the solute in 1mL of solvent into a vial. Upon additions of the solute, the vial was vortexed for 3 minute stints. Once the solution visibly showed an excess of solute, the vial was vortexed for a 10 minute period. Next, the solution was left to equilibrate, possibly overnight. Next, the supernatant within the vial was decanted out into another vial. Then, 100μL of the supernatant was dissolved into 700μL of deuterated chloroform within a NMR tube with an internal standard. Lastly, a proton NMR spectra of the solution was obtained.
As stated previously in the definition of solubility, solubility is specified and is dependent on temperature and pressure. However, the account of the temperature within the laboratory was not consistently noted, if at all, within the experimental procedure hence stated. Furthermore, saturation of the solution was being performed in differing methodologies between researchers partaking in the open source science research at different laboratories. Therefore, the experimental procedure adapted and evolved to limit error and be stream-lined, so that researchers partaking in the open source initiative were comparable. The resulting experimental procedure was used for the experiments reported in this paper:

First, a solution of the chosen organic compound solute in a non-aqueous solvent to be studied was drawn-up and the temperature of the laboratory was noted. The solution was made by adding and mixing together the solute in 300μL of the solvent into a vial. With every addition of solute to the solution, the solution was vortexed briefly. This was done until the solution visibly showed a collection of excess solute within the vial. Next, the vial was sonicated for a period of 30 minutes, wherein the temperature of the water was noted before and after the process. The solution was considered to be saturated if solute remained collected within the vial. Next, the vial was centrifuged so that the excess solute could be separated from the saturated solution. Afterward, the solution was left to equilibrate. Then, 100μL of the supernatant from the vial was dissolved into a NMR tube containing 900μL of deuterated chloroform, at which point the temperature of the laboratory was noted. Lastly, a proton NMR spectra of the solution was obtained.



Results


Exp073: Determination of the solubility of 4-chlorophenylacetic acid in methanol and THF using 2 methodologies by NMR


4-chlorophenylacetic acid


4_chlorophenylacetic_acid_in_methanol.jpg


4-chlorophenylacetic_acid_in_THF.jpg


Exp079: Determination of the solubility of 2-phenylbutyric acid in ethanol and acetonitrile by NMR spectroscopy
Exp083: Determination of the solubility of 2-phenylbutyric acid in toluene, THF, and DMSO by NMR spectroscopy



2-phenylbutyric acid

2-phenylbutyric_acid.jpg



Exp086: Determination of the solubility of phenylacetic acid in acetonitrile and toluene


phenylacetic acid

phenylacetic_acid.jpg



Exp093: Determination of the solubility of Ugi product 216-4A in methanol, ethanol, and acetonitrile


Ugi 216-4A

Ugi_product_216-4A.jpg



Discussion


In Exp073, the solubility of 4-chlorophenylacetic acid was studied in methanol and THF. The experiment was conducted in an interesting manner, wherein two methodologies (refer to the experiment for a detailed run-through of both methodologies) were used to study the solubility of the solute in the solvents. The purpose being 1) a test to determine if the internal standard being used is at all having an effect on the formulated solutions. 2) to test the accuracy of the SAMS method. The results show that there is not a big difference between the use of both methodologies (there is a slightly significant difference in the methanol results, but this is most likely attributable to some experimental error and not the method per se). Therefore, the SAMS method is accurate in determining solubilities. Furthermore, upon comparison of the NMR spectra from both methods, it was observed that the NMR tube without the capillary produced a better quality spectra. Having the capillary within the NMR tube acts to slightly interfere with the spectra scan. Lastly, because the solubility of 4-chlorophenylacetice acid in THF was also tested in Exp075, an opportunity for comparing solubility results for 4-chlorophenylacetic acid in THF was created. Exp075 reports a concentration of 5.02M for the solute in THF, which did not duplicate the findings in Exp073 (4.1M for method 1 and 4.24M for method 2). Interestingly, when comparing both findings, some concerns arise, because methodology 1 in Exp073 is the stream-lined experimental procedure that has been used for solubility experiments in the ONS and was also utilized as the procedure in Exp075; and their results are far off. This difference does not suggest that the SAMS method is inaccurate, because, it was stated that some of the NMR spectra results from Exp075 had peak distortion.

2-phenylbutyric acid was studied as a solute in ethanol and acetonitrile (Exp079) and in toluene, THF, and DMSO (Exp083). The results indicate that 2-phenylbutyric acid is quite soluble in some of the non-aqueous solvents. Also, in completing this experiment, a possible trend was observed. 2-phenyl butyric acid and 4-chlorophenylacetic acid are both carboxylic acids that have phenyl groups, which may be the deciding factor to their high solubility in the tested solvents.

The solubility of phenylacetic acid was determined in Exp086 using acetonitrile and toluene solvents. Originally, the experiment was to ascertain the solubility the solute in methanol, ethanol, acetonitrile, toluene, THF, and DMSO. However, the methanol and THF sample were lost and the NMR spectra for the DMSO sample was not properly saved. The biggest issue encountered in the experiment was that of the ethanol sample. The result was omitted because upon looking at the spectra it was not easily discernable as to which peak corresponded to certain aspects of both the solute and the ethanol (there is a substantial amount of peak-overlap occurring at 1.150-1.350ppm, where the methyl groups of the solute and solvent are located in the spectra). This created a problem when using the SAMS method for determining the solubility of the solution, because the method can not discern the difference between the peaks. Therefore, another method needs to be created and employed when peak overlap occurs, and there are no other peaks to reference and use for the SAMS method (ethanol produces two peaks in the spectra, methyl group and methylene group). A method that was contemplated to use was peak-subtraction, but there was a lack of confidence in the numbers obtained. It was also concluded in the experiment that screw-cap vials were to be used for the sonication process, because the capped vials used were popping due to the sonication process; which was resulting in the loss of samples.

The results for Exp093 was included for review in this paper as further evidence to the accuracy of the SAMS method for determining solubility. In experiment Exp097, Matthew Federici was able to reproduce the solubility results of the Ugi- 216-4a product in acetonitrile solvent with excellent accuracy.


Conclusion


Open-Notebook Science is an amazing and beneficial platform for research. It creates opportunities and allows for research to be conducted in a special manner, that being the ability to have experimentation critiqued; which only allows for the advancement of the research and experiment. For example, the solubility results determined for the Ugi reagents in the non-aqueous solvents reviewed in this paper was accomplished with the help, comments, and critique of other researcher and scientists with the appropriate backgrounds. Also, the experimental procedure grew and adapted as a result of the Open-Notebook Science model, which has helped to stream-line the procedure so that other laboratories could experiment and test reported findings in the same manner.

NMR is a viable technique that can be used to determine the solubility of organic compounds in non-aqueous solvents, and is a great method to use when studying Ugi-reagents to determine if they are capable of being candidate for components for the Ugi-product.



References


Bohinski, T. (2008). A Review of Some of the Solubilities of Organic Compounds in Non-Aqeuous Solvents. Drexel University. UsefulChem.wikispaces.com.

Dumpala, S., & Mirza, K. (2009). To determine the solubility of 4-chlorophenylacetic acid in acetonitrile, toluene, THF and DMSO. Drexel University. ONSChallenge.wikispaces.com.

Federici, M. (2009). Test the solubility of Ugi Product 216-4A in THF, methanol, acetonitrile, DMSO, and toluene. Drexel University. ONSChallenge.wikispaces.com.

Holsey, A. (2007). Synthesis of Diketopiperazines, Possible Malaria Enoyl Reducatase Inhibitors Using Open Source Science. Drexel University. UsefulChem.wikispaces.com.