Article pubs.acs.org/JAFC
Osage Orange (Maclura pomifera L.) Seed Oil Poly(αhydroxydibutylamine) Triglycerides: Synthesis and Characterization Rogers E. Harry-O’kuru,*,† Brent Tisserat,‡ Sherald H. Gordon,# and Alan Gravett§ †
Bio-Oils Research Unit, ‡Functional Food Research Unit, and #Plant Polymer Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, U.S. Department of Agriculture, 1815 North University Street, Peoria, Illinois 61604, United States § Hedge Apple Biotech, 211 Woodrig Road, Bloomington, Illinois 61704, United States ABSTRACT: Milled Osage orange seeds (Maclura pomifera (Raf.) Schneid) were Soxhlet extracted with hexane, and portions of the extract were treated with activated carbon before solvent removal. The crude oil was winterized and degummed by centrifugation at low temperature. Decantation of the centrifugate gave an admixture of the triglycerides and free fatty acids. The free fatty acid content of the oil was removed when portions of the admixture were diluted with hexane and shaken with cold aqueous ammonium hydroxide (0.1 M) solution. The desiccant-dried organic phase was concentrated under reduced pressure to give the cleaned Osage orange triglyceride after solvent removal by rotary evaporation at 67 °C. Epoxidation of the resulting cleaned triglyceride was effected by reaction with in situ generated peroxy performic acid in H2O2. The oxirane rings of the derivatized oil were then opened using N,N-dibutylamine catalyzed by anhydrous ZnCl2 to afford the poly(αhydroxydibutylamine) triglyceride. The purpose of this work was to derivatize and thereby stabilize this highly unsaturated tree oil for its eventual use in lubrication applications. KEYWORDS: Osage orange, seed oil, epoxidation, amination, poly(α-hydroxydibutylamine) triglyceride
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to the tree bark.13−15 A recent study of methyl esters of the seed oil suggests the potential of this biobased material in biodiesel fuel.16 In the current study, however, our interest is investigating conversion of the highly unsaturated Osage orange seed triglyceride into a variety of new products starting from developing the oxirane from its olefinic moieties as a platform material for synthesis of new products. Thus, from the polyepoxy derivative ring-opening, α-hydroxyamines of Osage orange oil have been synthesized and characterized spectrometrically using FTIR and 1H and 13C NMR. Industrial applications for the product are being explored in lubrication and other uses.
INTRODUCTION In exploring alternative vegetable oils for nonfood industrial applications, especially in temperate climates, tree seed oils that are not commonly viewed as competitors to soybean, peanut, canola, and corn oils can become valuable sources of new oils. Many trees produce edible fruits and seeds, whereas others produce fruit and pods containing seeds not considered edible for human consumption in terms of the “food versus fuel” debate. Oils from the seeds of such plants can serve some of the need for renewable non-food industrial products. Examples of such plants include the Osage orange, Maclura pomifera (Raf.), or hedge apple, the black locust (Robinia pseudoacacia L), the mulberry (Morus alba L), and many other seed-producing deciduous trees. Commercial cultivation of such trees has not been practiced traditionally other than establishment of Osage orange hedge rows in the prairies and elsewhere before wire fences were developed. However, the recent call for environmental stewardship has led people to engage in a tree-planting culture, especially Osage orange, to partly sequester some of the excessive amounts of CO2 in the present atmosphere and in this process also generate some economic value as well. Maclura pomifera is a member of the Moraceae family; its present occurrence stretches from southern Canada to Texas by introduction. Its history and many botanical studies and investigations of chemical constituents of the wood, bark, and fruit have been published.1−10 It is a plant built for all-purpose use ranging from bow-making in Native American tribes to hedge row establishment by farmers to many implement components manufacture because of the beauty, rot resistance, and hardness of the wood. The fruit chemical components have been reported to exhibit pesticidal properties, and the bark is a pigment source.11−14 Medicinal characteristics are also ascribed © 2015 American Chemical Society
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MATERIALS AND METHODS
Materials. Osage orange seeds were supplied by Hedge Apple Biotech (Bloomington, IL, USA). Chemicals. Hexane (HPLC grade) was from Thermo-Fisher Scientific (Chicago, IL, USA), whereas formic acid (90%), hydrogen peroxide (50%), Na2CO3, MgSO4, NaCl, N,N-dibutylamine, ZnCl2 anhydrous, and ethyl acetate were purchased from Sigma-Aldrich (St. Louis, MO, USA). Methods. Instrumentation. (a) Fourier Transform Infrared Spectrometry. Test samples (liquids) were pressed between two NaCl disks (25 × 5 mm) to give thin transparent oil films for analysis by FTIR on an Arid Zone FTIR spectrometer (ABB-MB series, Houston, TX, USA) equipped with a DTGS detector. Absorbance spectra were acquired at 4 cm−1 resolution and signal-averaged over 32 scans. Interferograms were Fourier-transformed using cosine apodizaReceived: Revised: Accepted: Published: 6588
November 3, 2014 June 18, 2015 June 29, 2015 July 20, 2015 DOI: 10.1021/acs.jafc.5b01625 J. Agric. Food Chem. 2015, 63, 6588−6595
Article
Journal of Agricultural and Food Chemistry tion for optimal linear response. Spectra were baseline corrected, adjusted for mass differences, and normalized to the methylene peak at 2927 cm−1. FTIR spectra of the oils (crude, refined, and products) are shown in Figures 1−3, 5, and 6. (b) Thin Layer Chromatography (TLC). TLC of the reaction mixtures was carried out using precoated silica gel plates (0.2 mm, 5 × 20 cm; EM Science, Darmstadt, Germany). The solvent system used was (hexane/ethyl acetate/acetic acid 10:5:2). The developed chromatogram was visualized by methanol−sulfuric acid spray and heated to 100−110 °C. (c) Viscosity Measurements. Kinematic viscosity was determined using a Cannon-Fenske viscometer for transparent liquids (Cannon Instrument Co., State College, PA, USA) in accordance with AOCS Official Method Tq 1a-64. The tube number was 400. The cleaned dry tube was loaded with the sample oil at room temperature and placed in its holder in the constant-temperature bath. The sample was allowed to equilibrate for 10 min at 40 °C or for 15 min at 100 °C before the sample was suctioned into the lower bulb until the meniscus just overshot the mark above the lower bulb. The suction was removed and the meniscus adjusted to the mark. The sample was allowed to flow at the same time the stop clock was started. The time (in seconds) it took for the meniscus to reach the mark below the bulb multiplied by the tube constant gave the viscosity of the sample. The measurement was replicated and averaged to obtain the viscosity. (d) Nuclear Magnetic Resonance Spectroscopy. 13C NMR spectra were acquired on a Bruker AV-500 MHz spectrometer with a dual 5 mm proton/carbon probe (Bruker, Ballerica, MA, USA). The internal standard was tetramethylsilane. (e) Density. Sample density measurements were carried out using a pycnometer or Kimble specific gravity bottle (10 mL) equipped with a thermometer and a side arm cap (Kimble Glass Inc., Vineland, NJ, USA). The pycnometer was cleaned with hexane and acetone and dried with an air jet. The reassembled unit was weighed empty and then carefully loaded with the sample, cleaned of excess sample, and reweighed. The process was replicated so that sample masses could be averaged. (f) Oil Extraction and Degumming. Osage orange oil was extracted from 3 kg of ground seed using a Retsch mill. The seeds were preconditioned in powdered dry ice for several minutes before milling to facilitate particle size reduction. The obtained seed meal was extracted in approximately 1 kg batches in a Soxhlet extractor using hexane. The hexane extract was decolorized with activated carbon before solvent removal to yield the crude oil. The oil content of the seed was calculated to be 30−35% by weight of the dry seed. Portions of the crude oil were chilled (5 °C) and centrifuged (10000g) at the same temperature for 30 min to remove phospholipids and waxy content. The free fatty acid component was removed from the degummed oil (Figure 1) by dissolving it in hexane, and the solution
was shaken with cold aqueous (0.1 M) ammonium hydroxide solution in a separatory funnel. The separated organic phase was then washed with deionized water until the washings were neutral to litmus. The organic layer was dried over MgSO4, filtered, and concentrated under reduced pressure at 67 °C to give the Osage orange triglyceride (Figure 2); density = 0.979 g mL−1 at 23 °C. The free fatty acids were
Figure 2. (A) FTIR spectrum of cleaned Osage orange triglyceride and (B) structural representation of the triglyceride molecule. recovered by acidulation of the combined aqueous layer and washings with dilute HCl followed by extraction with hexane. The hexane extract was dried over MgSO4 and concentrated to give the free fatty acids, which were set aside from the intact triglyceride. Synthesis of Osage Orange Oxirane. In a 1 L jacketed threenecked reaction flask equipped with an overhead stirrer at a starting temperature of 40 °C was placed Osage orange oil (280.90 g, 320.8 mmol) to which formic acid (88%, 30.50 g, 25.0 mL, 663 mmol, i.e., 0.4 equiv/−CC−) was added in one portion. On stirring to homogeneity, H2O2 (50%, 200 mL, 7.05 mol) was added dropwise, and at the end of peroxide addition the reaction temperature was raised to 70 °C. Progress of the reaction was monitored in 30 min intervals by FTIR spectral analysis of withdrawn samples of the reaction mixture for disappearance of the 3008 and 1648 cm−1 bands of the olefin with the concomitant formation of a doublet observable at 822−845 cm−1 for the oxirane. After 2 h, the reaction was judged complete; the heat source was then withdrawn and the product mixture allowed to cool to near room temperature, when it was transferred into a separatory funnel with ethyl acetate as diluent and allowed to separate into two layers. The aqueous layer was removed and discarded. The organic phase was washed with a mixture of saturated NaCl and Na2CO3 solution followed by drying over MgSO4 overnight. Filtering and concentration of the filtrate gave a quantitative yield, 316.0 g, of the oxirane of Osage orange oil, d22.3 1.067 g mL−1. FTIR spectral features of its film on NaCl disk are given in cm−1: 2928 vs (−CH2− sym stretch), 2859 s (−CH2− asym stretch), 1743 vs (−CO stretch), 1461 s (−CH2− deform), 1377 m-s (−CH3 deform), 1236 m-s (−OCO), 1160 s (−CHO− stretch), 1045 (−CH2O− stretch), 845−824 doublet m (−COC− oxirane asym stretch), 725 (−CH2− wag). 13C NMR (CDCl3): δ 173.17,
Figure 1. FTIR spectrum of degummed crude Osage orange oil. 6589
DOI: 10.1021/acs.jafc.5b01625 J. Agric. Food Chem. 2015, 63, 6588−6595
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Journal of Agricultural and Food Chemistry
Figure 3. (A) 1H NMR spectrum of the cleaned triglyceride and (B) its 13C NMR spectrum. 173.1, 172.7 (ester carbonyls); 68.9 (−HCO−) glyceryl, 62.0, 60.3 (−CH2O−) glyceryl; 57.2, 57.1, 56.9, 56.88, 56.7, 56.6, 54.3, 54.27, 54.1 (oxirane carbons); 34.1, 34.0, 33.9 (C-2), 31.8, 31.77, 31.6 (C11), 29.6, 29.57, 29.5 (C-8), 29.46, 29.45, 29.4 (C-14), 29.27, 29.25, 29.21 (C-3), 29.18, 29.13, 29.10 (C-7), 29.08, 29.0, 28.9, 28.87, 27.81, 27.8, 27.74, 27.72, 27.1, 26.8, 26.5, 26.5, 26.48, 26.4, 26.36, 26.33, 26.2, 26.1, 24.8, 24.73, 24.70 (−CH2−); 22.6 (C-18), 14. 0 (C-18). 1H NMR (CDCl3): δ 5.20 (m, HCO− glyceryl); 4.23 (m, CH2O glyceryl), 4.1 (m 3H, CH2O glyceryl); 3.05 (m, 3H), 2.91 (m, 6H, epoxy methine protons); 2.82 (bs, 1H), 2.26 (t, J = 7.4 Hz, 7.3 Hz, C11 protons), 1.6 (m, 7H), 1.56 (bs, 8H), 1.43 (m, 13H), 1.28 (bs, 7H), 1.26 (s 38H), 1.18 (s, 9H), 0.83 (t, 9H, terminal CH3). The kinematic viscosity was 131.6 cSt (40 °C) and 19.2 cSt (100 °C); that is, a viscosity index (VI) = 166. Synthesis of Osage Orange Poly(α-hydroxydibutylamine) Triglyceride. In a dry 250 mL round-bottom reaction flask equipped with an overhead stirrer as above was placed epoxy Osage orange oil (59.5 g, 62.26 mmol), and anhydrous ZnCl2 (1.0 g) was added followed with N,N-dibutylamine (43.41g, 56.6 mL, 336 mmol, i.e., 5.4 equiv)
introduced dropwise into the stirred reaction mixture. A reflux condenser was attached, and the reaction mixture was stirred and heated to gentle reflux. The reaction was monitored by sampling the mixture at 1 h intervals to observe diminution of the 820−845 cm−1 band of the oxirane in the FTIR spectrum as ring-opening progressed. The reaction was stopped when no trace of the 820−845 cm−1 band was visible, ca. 2.5 h. TLC (hexane/EtAc/AcOH 10:5:2) was a second mode of tracking reaction progress, confirming the absence of starting epoxide. The reaction product was cooled to room temperature, diluted with ethyl acetate, and washed with saturated NaCl solution. The organic phase was dried over MgSO4 and filtered, and the solvent was removed by rotary evaporation under reduced pressure, giving 93.3 g, (i.e., 93.3%) yield of the poly(α-hydroxydibutylamine); d22.6 = 0.9985 g mL−1. FTIR film on NaCl disk: ν cm−1 3431 (br OH stretch, second deriv 3586), 2952 s (−CH3 asym stretch, second deriv 2956), 2929 vs (−CH2− sym stretch, second deriv 2931), 2853 s (−CH2− asym stretch, second deriv 2876), 2857 (−CH3 asym stretch), 1739 vs (−CO stretch, second deriv 1741), 1667 w (−NC, 1575 vw), 1463 s (−CH2− deform, second deriv 1468), 1378 m-s (−CH3 6590
DOI: 10.1021/acs.jafc.5b01625 J. Agric. Food Chem. 2015, 63, 6588−6595
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Journal of Agricultural and Food Chemistry deform), 1244 m (−OCO), 1164 m-s (−CHO− stretch), 1101 m-s (−CHO− stretch), 1035 sh (−CH2O− stretch), 726 (−CH2− wag). The kinematic viscosity was 410.54 cSt (40 °C) and 38.57 cSt (100 °C); VI = 141.
could be reduced, especially in a seed of higher fat content. To maximize the particle surface area of the grind, experience suggested that chilling of the dry seed before milling would give the best results. Therefore, at near dry ice temperature the seed was easily ground to a reasonably fine powder without clumping, as would otherwise be experienced. This condition of the meal greatly facilitated hexane extraction of each kilogram batch of the seed meal. This was evidenced by the result that at the end of each 24 h extraction interval, the residual pool of solvent below the extraction thimble was colorless. Concentration of the resulting extract by rotary evaporation gave a crude dark Osage orange oil. Decolorization of the crude was by redissolution in hexane and treatment with activated carbon. The decolorized crude triglyceride obtained after solvent removal was then winterized followed by centrifugation at low temperature (5 °C) to separate the phospholipid and waxy components. Although purification of the degummed crude oil could have been achieved by fractional distillation, we chose the wet chemistry approach for economic reasons. Cold storage of the seed seemed to have slowed the metabolic release of free fatty acids in the oil as seen in Figure 1. The free fatty acid level in the extracted oil was reasonably low judging from the lower intensity of the 1702 cm−1 carbonyl absorption band of the free fatty acids compared to that of the intact ester functionality of the triglyceride at 1748 cm−1. Consequently, we opted for the wet chemistry approach, which successfully separated the intact triglycerides (Figure 2A) from the admixture in Figure1. Interpretation of the FTIR spectral features of the cleaned triglyceride (Figure 2A) in relation to the structural representation of the oil (Figure 2B) is as follows. The absorption band at 3010 cm−1 is the olefinic (HC) stretch, whereas the 2956 and 2925 cm−1 bands are respectively the terminal methyl (H3C−) and the methylene (−CH2−) groups asymmetric stretching modes of the alkyl chains. The symmetric stretching bands of the terminal methyl and that of the methylene groups are seen around 2855 cm−1. The 2855 cm−1 band is a composite of the more preponderant methylene groups at 2855 cm−1, whereas the smaller population of methyl symmetric absorption band is at 2865 cm−1. The other prominent diagnostic absorption band of the oil is the very strong carbonyl (−CO) of the ester groups at 1746 cm−1. It should be noted here that the 1740 cm−1 region of the spectrum is devoid of any free fatty acid absorption, hence the absence of the 1702−1713 cm−1 band as exists in the admixture (Figure 1). The methylene and the terminal methyl deformation bands of the chain are seen at 1460 and 1375 cm−1, respectively. The small band at 1655 cm−1 indicates the puckering or breathing mode of the vinylic (−CC−) bonds of the triglyceride. The FTIR absorbance at 1242 cm−1 represents the (−OCO) stretching mode of the ester head groups of the triglyceride, whereas the 1157 and 1102 cm−1 bands are those of the glyceryl (HCO). A feature that seems to be missing is the 1050 cm−1 band of the (CH2O) of the glyceryl absorbance, which is here overlapped by the 1102 cm−1 band. Finally, the IR band at 725 cm−1 is the wagging mode of the long methylene skeleton. The 1H and 13C NMR spectra of the cleaned triglyceride are shown in Figure 3, panels A and B, respectively. From the chemical shift values, the most downfield protons are the vinylic multiplet at 5.4 ppm (9− 10H), immediately followed by a multiplet at 5.3 ppm (1H), which is the glyceryl methine proton by virtue of coupling from the two neighboring methylene groups. A doublet of doublets at 4.3 ppm (J = 4.3 Hz, 2H) and multiplet centered at 4.1 (2H)
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RESULTS AND DISCUSSION Efficient solvent extraction of the fat content of the seed depends on the extent to which particle size of the ground seed Table 1. Fatty Acid Composition of Osage Orange Seed Oil16 fatty acid
area %
C 14:0 C 16:0 C 16:1 Δ9 C 18:0 C 18:1 Δ9 C 18:1 Δ11 C 18:2 Δ9,12 C 18:3 Δ9,12,15 C 20:0 unknown (sum) Σsaturated Σmonounsaturated Σpolyunsaturated
0.1 7.0 0.1 2.4 11.9 0.8 76.4 0.4 0.6 0.3 10.1 12.8 76.8
Figure 4. (A) Epoxidation scheme for the cleaned Osage orange oil; (B) FTIR spectrum of epoxidized Osage orange triglyceride. 6591
DOI: 10.1021/acs.jafc.5b01625 J. Agric. Food Chem. 2015, 63, 6588−6595
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Journal of Agricultural and Food Chemistry
Figure 5. (A) 1H and (B) 13C NMR spectra of the oxirane of Osage orange oil showing resonances of the epoxy methine protons around 3.3 ppm and of the methine carbons from 57.15 to 54.13 ppm.
contiguous to the ester carbonyl, whereas the multiplet at 2.30 ppm represents the 6H of C11 sandwiched between C10, C12, and C14 vinylic carbons of linoleic acid moieties. The multiplet at 2.03 ppm (11H) is the C3 and C4 protons, and the broad multiplet at 1.6 ppm (6H) accounts for C8 protons. The huge multiplet at 1.29 ppm is assigned to the remaining methylene protons, whereas the terminal methyl protons are the overlapping triplets (multiplet) at 0.87 ppm. A profile of the fatty acid methyl esters of Osage orange oil (Table 1) shows the fatty acid composition of this oil.16 Epoxidation of the cleaned triglyceride was smooth and high yielding.17−20 Figure 4A depicts the epoxidation reaction scheme transforming the olefinic units of this oil to the polyoxirane derivative, and Figure 4B is the FTIR spectrum of
Table 2. Densities and Kinematic Viscosities of Derivatives compound
density (g cm−3)
kinematic viscosity (cSt)
viscosity index
unmodified triglyceride
0.979 (23 °C)
oxirane
1.067 (22.3 °C)
131.6 (40 °C) 19.6 (100 °C)
166
poly(αhydroxydibutylamine)
0.998 (22.6 °C)
410.54 (40 °C) 38.57 (100 °C)
141
>200
are the glyceryl CH2s. The clean triplet centered at 2.75 ppm (J = 6.6 Hz, 6.5 Hz, 4H) represents the methylene groups 6592
DOI: 10.1021/acs.jafc.5b01625 J. Agric. Food Chem. 2015, 63, 6588−6595
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Figure 8. 1H NMR spectrum of poly(α-hydroxydibutylamine).
resulting from the methine carbons (−COC−) of the ring. This spectral region is featureless in the parent triglyceride (Figure 3B) and in subsequent derivatives from the oxirane. What is also noticeable from this epoxy spectrum is the complete disappearance or absence of signals of the olefinic moieties (140−120 ppm) in the 13C. In other words, epoxidation of the olefin was complete. The first upfield methylene resonances from the oxirane methine carbons are the three methylene carbons that are proximal to the ester carbonyl at 34.1−33.9 ppm. These are followed by the C-11 carbons sandwiched between the oxirane C-10 and C-12 species at 31.8−31.6 ppm, whereas the six (−CH2−) contiguous to epoxy carbons are observed at 29.6−29.4 ppm. The remaining methylene carbon resonances gradually tend upfield as distance from heteroatoms increases. Thus, some 21 (−CH2−) invariantly resonate between 29.02 and 24.70 ppm as their location tends toward the alkyl termini. The terminal methyl groups are upfield around 14 ppm. The other spectral features of the 13C NMR are the low-field ester carbonyl resonances at 173.2, 173.1, and 172.7 ppm, whereas the glyceryl methine carbon (−CH−O−) is at 68.9 ppm and its methyleneoxy carbons (−CH2O−) are observed at 62.0 and 60.3 ppm, respectively. Aminolysis of the oxirane was more sluggish compared to the oxirane formation even as catalyzed by anhydrous ZnCl2 under gentle reflux irrespective of the lower scale of reactants used. The reaction was sometimes run overnight to give the poly-αhydroxydibutylamine triglyceride derivative in excellent yield.17,20 This was consistent with earlier observation in amidation ring-opening of other vegetable oxiranes (Milkglyde, Saliglyde, and Soyglyde).18,19,21 TLC monitoring of the ringopening of the oxirane showed several discrete spots (Rf = 0.56, 0.52, and 0.48) for the starting epoxide lane in relation to the solvent front when migration was stopped about halfway up the plate height. The polyhydroxydibutylamine tended to smear a bit on the stationary phase even though migrating more slowly with an Rf = 0.26 compared to the oxirane starting material. When development of the loaded plate was allowed to run close to the top of the 5 × 20 cm plate, the developed chromatogram shows up to eight discrete spots in the oxirane lane (Rf = 0.77, 0.74, 0.67, 0.55, 0.48, 0.35, 0.29, and 0.19) as
Figure 6. (A) FTIR spectrum of poly(α-hydroxy-N,N-dibutylamine) triglyceride of Osage orange oil and (B) proposed structure of this derivative.
Figure 7. FTIR spectral difference between the epoxy band region before and after reaction with N,N-dibutylamine.
the polyoxirane of Osage orange oil. The glaring diagnostic spectral feature of the oxirane is the characteristic doublet at 824−845 cm−1 for the (−COC−) asymmetric stretching mode of the three-membered ring. A not so obvious absorption band of the oxirane, however, is the methine (HCO CH) band of the three-membered ring that is buried under the strong −CH3 asymmetric stretching mode close to 3000 cm−1. This band is only obvious in the second derivative of the epoxy spectrum at 2994 cm−1. The 13C NMR of the oxirane is shown in Figure 5. The essential diagnostic features observed here are the 10 resonance lines starting from 57.2 to 54.1 ppm 6593
DOI: 10.1021/acs.jafc.5b01625 J. Agric. Food Chem. 2015, 63, 6588−6595
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Journal of Agricultural and Food Chemistry
Figure 9. 13C NMR DEPT spectrum poly(α-hydroxydibutylamine).
cm−1, which is assignable to a −C−N absorption from the small amount of competing amidation process on the ester head groups. This band is assignable to an “amide I” C−N stretching mode especially as it is accompanied by an unresolved but much smaller band around 1575 cm−1 for the “amide II” absorption mode. This phenomenon has been observed in an earlier study of aminolysis ring-opening of new crop epoxy oils.21 Figure 8 is the 1H NMR spectrum of the poly(αhydroxydibutylamine). The theoretical total proton count in the product is 194, and the proton integration values when normalized to the spectral resonances of Figure 8 are consistent with characteristics of the desired compound. Figure 9 is the 13 C distortionless enhancement proton transfer (DEPT) spectrum of the product. The methylene carbons bound to nitrogen atoms in the product resonate downfield starting at 42.0, 47.1, 49.5, 51.4, and 65.0 ppm, whereas the methine oxygen-bonded carbons are farther downfield from those bonded to nitrogen atoms and are observed between 81.1 and 66.0 ppm. This derivatized vegetable oil is in the exploratory stage for a non-food use in lubricant and related applications, where oxidative stability of the oil is of value. We intend to further modify the hydroxyl units of this tree oil derivative to both lower its polarity and enhance its oxidative stability so as to potentiate its lubricity character.
contrasted to Rf = 0.13 for the product. For an understanding of the TLC data, the cleaned natural oil is a mixture of various C18 triglycerides and so the epoxidized oil should of necessity contain those permutations. Hence, the species are shown by the TLC spots on the developed plate. The aminolysis seemed to have tamped down the conformational differences into one through streamlining of the structures from the semi sp2 hybrid in the three-membered rings to full sp3 configurations that would allow better packing of the chains except for the disrupting influence of the N,N-dibutylamine substituents. Aminohydration of the chains introduces both higher polarity to the midsection of each acyl chain of the triglyceride and greater sphericity or bulkiness induced by branching of the triglyceride chains. One of the resulting effects of branching in the triglyceride caused by insertion of N,N-dibutylamine is lowering of the density of the derivatized triglyceride as observed in Table 2. This table also shows enhanced viscosity of the derivative relative to the oxirane. The corresponding FTIR spectrum of the product confirmed complete replacement of the characteristic 820−845 cm−1 band of the epoxide with the α-hydroxyl dibutylamine groups. Figure 7 is an overlay of the FTIR spectra of the starting oxirane over that of the poly-α-hydroxydibutyl amine product in the 800−870 cm−1 region, that is, before and after reaction of the starting polyoxirane. This convincingly shows the extent of the ring-opening aminolysis reaction. The broad IR band centered around 3428 cm−1 in Figure 6A is the stretching mode of the α-hydroxyl (O−H) groups of the hydroxydibutylamine triglyceride (Figure 6B). The tertiary polyamine generated in the epoxy ring-opening aminolysis reaction is spectroscopically transparent in this region under the product isolation conditions. Inspection of Figure 6A shows attenuation of the ester carbonyl absorption as compared to the strong carbonyl absorption band in the oxirane triglyceride. The explanation lies in the major contribution resulting from the mass effect of adding over 600 mass units of alkyl side chains to the alkyl component of the triglyceride molecule without a corresponding increase in carbonyl moiety contribution. A second but minor contributing factor also is the very weak IR band at 1655
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AUTHOR INFORMATION
Corresponding Author
*(R.E.H.-O.) Phone: (309) 681-6341. Fax: (309) 681-6524. Email:
[email protected]. Notes
Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We express gratitude to Dr. Karl Vermillion for the NMR spectra and to Mark Klokkenga for technical assistance. 6594
DOI: 10.1021/acs.jafc.5b01625 J. Agric. Food Chem. 2015, 63, 6588−6595
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DOI: 10.1021/acs.jafc.5b01625 J. Agric. Food Chem. 2015, 63, 6588−6595
