Application of H-NMR for the Quantitative Analysis of Mixtures of Oleate Esters of Short Chain Alcohols (C1-C4) Ronald P. D’Amelia, Brandon Khanyan, Joseph Mancuso
Chemistry Department, Hofstra University, Hempstead, NY 11549-0151
Quantitative nuclear magnetic resonance spectroscopy (qNMR) is a technique used to determine the concentration of one or more analyte within a mixture. Although NMR spectroscopy is typically used to qualitatively determine molecular structure, the quantitative application of NMR extends to concentration determinations and purity assessments. The experiment is designed to increase awareness of both the qualitative and quantitative applications of NMR spectroscopy and to be integrated into undergraduate analytical and instrumental chemistry laboratory course curriculums. The experiment entails the quantitative analysis of binary long-chain monounsaturated fatty acid mixtures ranging from 0% to 100% in 20% intervals of methyl oleate (MeOl), ethyl oleate (EtOl), propyl oleate (PrOl) and butyl oleate (BuOl) using proton NMR. The goal of the experiment is to determine the structure and weight percent composition of both analytes in each of the mixtures. The results show a strong, linear correlation between the gravimetric compositions and the weight percent compositions found using proton NMR. The experiment supports qNMR as a tool for determining weight percent compositions of mixtures and can be incorporated at the undergraduate chemistry laboratory level.
Results
Experimental
Introduction
C) Analysis: The proton NMR profiles of the short chain fatty acid oleate ester mixtures were obtained on a JEOL 400 MHz model ECS-400 NMR. The JEOL Delta NMR processing and control software version 5.0.2 (Windows) was used to analyze the individual spectra. Each sample was run as a single pulse, 1D proton NMR, no solvent, acquisition time of 4-5 sec and a resolution of 0.25 Hz. No carbon spectra were obtained.
Results
Figure 1: Proton NMR of 100% Methyl Oleate
Figures 1, 2, 3 & 4 show the proton NMR spectrum, without an internal reference standard, of 100% methyl oleate, ethyl oleate, propyl oleate and butyl oleate respectively along with the corresponding peak assignments showing the chemical shift, multiplicity, & normalized integration values summarized in table 2. One can see that the methyl protons (A) on the carbon atom attached to the oxygen are singlets and are shifting up-field or lower frequency as the molar mass of the short chain alcohols increases. These protons are shielded & sense a smaller effective magnetic field as the number of methylene groups are added to the ester. Therefore, these protons come into resonance at a lower frequency. These protons are in a more electron dense environment as the compound increase the # of methylene units. Figures 5, 6, 7, & 8 are the proton spectrum, without an internal reference standard, of 4:1 mixtures of methyl, ethyl, propyl and butyl oleates. The normalized integration of the appropriate peaks correspond to the % composition of the mixtures. Figures 9, 10, 11 & 12 are the linear plots of the qNMR determined weight % of MeOl or EtOl in mixtures versus experimental weight % of the same component.
Figure 2: Proton NMR of 100% Ethyl Oleate
qNMR wt. % MeOl vs. Gravimetric wt. % MeOl in MeOl:EtOl Mixture
Experimental
qNMR wt. % MeOl vs. Gravimetric wt. % MeOl in MeOl:PrOl Mixture
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A) Materials and Chemicals: The NMR sample tubes were Wilmad 5mm thin wall 7” Pyrex tubes having a round bottom and a sandblasted marking spot for labeling samples. They have a concentricity of 51 um and a camber of 25 um. Methyl, ethyl, propyl and butyl oleate were purchased from various vendors as anhydrous liquids with greater than 90% purity. All reagents were used without purification.
Experimental weight % Methyl Oleate 0 19.714 41.471 60.907 81.356 100 Experimental weight % Methyl Oleate 0 20.013 41.017 61.316 79.872 100 Experimental weight % Methyl Oleate 0 19.737 39.826 61.946 82.517 100 Experimental weight % Ethyl Oleate 0 20.466 39.635 60.342 79.483 100
y = 0.9812x + 0.9331 R² = 0.9992 40
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Figure 3: Proton NMR of 100% Propyl Oleate
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Figure 4: Proton NMR of 100% Butyl Oleate
Figure 6: Proton NMR of 4:1 MeOl:PrOl mixture
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Gravimetric wt. % MeOl Figure 10. qNMR wt. % MeOl vs. gravimetric wt. % MeOl in MeOl:PrOl mixture
qNMR wt. % EtOl vs. Gravimetric wt. % EtOl in EtOl:PrOl Mixture
qNMR wt. % MeOl vs. Gravimetric wt. % MeOl in MeOl:BuOl Mixture
Figure 5: Proton NMR of 4:1 MeOl:EtOl mixture
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Figure 9. qNMR wt. % MeOl vs. gravimetric wt. % MeOl in MeOl:EtOl mixture
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60 y = 0.9342x + 2.6435 R² = 0.9993 40
60 y = 0.9957x + 1.1293 R² = 0.9984 40
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Table 1. % Composition of MeOl-EtOl-PrOl-BuOl Mixtures Volume ratio Methyl Oleate: Ethyl Oleate 0ml:5ml MeOl:EtOl 1ml:4ml MeOl:EtOl 2ml:3ml MeOl:EtOl 3ml:2ml MeOl:EtOl 4ml:1ml MeOl:EtOl 5ml:0ml MeOl:EtOl Methyl Oleate: Propyl Oleate 0ml:5ml MeOl:PrOl 1ml:4ml MeOl:PrOl 2ml:3ml MeOl:PrOl 3ml:2ml MeOl:PrOl 4ml:1ml MeOl:PrOl 5ml:0ml MeOl:PrOl Methyl Oleate: Butyl Oleate 0ml:5ml MeOl:BuOl 1ml:4ml MeOl:BuOl 2ml:3ml MeOl:BuOl 3ml:2ml MeOl:BuOl 4ml:1ml MeOl:BuOl 5ml:0ml MeOl:BuOl Ethyl Oleate: Propyl Oleate 0ml:5ml EtOl:PrOl 1ml:4ml EtOl:PrOl 2ml:3ml EtOl:PrOl 3ml:2ml EtOl:PrOl 4ml:1ml EtOl:PrOl 5ml:0ml EtOl:PrOl
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qNMR wt. % MeOl
B) Methods: Fifteen 5.0 mL binary mixtures of methyl oleate (MeOl), ethyl oleate (EtOl), propyl oleate (PrOl) and butyl oleate (BuOl) were prepared as shown in Table 1. Each reagent was added using a Gilson classic model P1000 pipette and 1 mL of each mixture was added to NMR tubes for qNMR analysis. All of the 7 mL vials and NMR tubes were labeled with the volumetric ratio of reagents in the mixture. Following each addition, the mass of the vial was recorded using a Mettler analytical balance having a precision of 0.1 mg. The weight percent composition of the mixtures were determined using these masses. All mixtures were analyzed without an internal reference standard.
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Gravimetric wt. % MeOl Figure 11. qNMR wt. % MeOl vs. gravimetric wt. % MeOl in MeOl:BuOl mixture
Figure 12. qNMR wt. % EtOl vs. gravimetric wt. % EtOl in EtOl:PrOl mixture
Conclusions
Figure 7: Proton NMR of 4:1 MeOl:BuOl mixture
Figure 8: Proton NMR of 4:1 EtOl:PrOl mixture
Table 2. Summary of Oleate Ester NMR critical values
There is a strong, linear correlation between the gravimetric compositions and the weight percent compositions found using proton NMR. The calibration curves can be used to accurately determine the % composition of an analyte in binary mixtures involving oleate esters. The experiment corroborates the quantitative use of proton NMR to determine the composition of binary mixtures.
References D’Amelia, R.P., * Kimura, W. M. Nirode, W. article published entitled “Application of Quantitative Proton Nuclear Magnetic Resonance Spectroscopy for the Compositional Analysis of Short-Chain Fatty AcidEthyl Ester Mixtures" in the World Journal of Chemical Education, vol 9, # 1, pp.8-13 (2021). D’Amelia, R.P., * Mancuso, J., Wachter N. article published entitled “Application of Quantitative Proton Nuclear Magnetic Resonance Spectroscopy for the Compositional Analysis of Short Chain Fatty Acid Benzyl Ester Mixtures”, World Journal of Chemical Education, vol. 7, #3, pp. 189-195, (2019).
Support We acknowledge the support from a Hofstra HCLAS Faculty Research & Development Grant printed by www.postersession.com