3.2. NMR
Peak assignments of the triglyceride structure of soybean oil in the1H-NMR spectra are shown in Fig. 3a (each proton in a
different chemical environment has been numbered from 1-10 in blue),
corresponding to: 1) Methyl terminal group, 𝛿 (ppm) 0.8-0.9;2) Methyl terminal group for linolenic acid, 𝛿 (ppm) 0.9-1.0;3) Methylene protons
-CH2-CH2 -CH2-,
𝛿 (ppm) 1.2-1.4; 4) Methylene protons
–C(=O)-CH2-CH2 -CH2-
𝛿 1.5-1.7; 5) Methylene protons
-CH2-CH2 -CH=CH-, 𝛿 1.9-2.1;6) Methylene protons
–C(=O)-CH2 -CH2-CH2-𝛿
2.2-2.3; 7) Allylic group= CH-CH2 -CH=, 𝛿 (ppm) 2.7-2.9;8) Glycerol protons n-1 and n-3, doublet: 𝛿 (ppm) 4.0-4.3;9) Glycerol protons n-2, 𝛿 (ppm) 5.2-5.3; 10) Olefinic
protons CH2-CH=CH- CH2, 𝛿 (ppm)
5.3-5.4. As can be seen in Fig. 3a, untreated, and treated samples have
the same number of peaks with different intensities as well as the
partially hydrogenated sample of soybean oil (PHO). This result supports
the proposed mechanism shown in Fig. 3 where all the allylic protons
with a chemical shift at 2.7-2.9 ppm, have a decreasing tendency
concerning treatment time. Soybean oil-treated for 6h showed a reduction
of double bonds (peak 10) from 7.4% to 4.7% and an increasing
percentage of single bonds (peak 3) from 53.5% to 59.5%. This
reduction can be attributed not only to hydrogenation but also to
dimerization by the Diels Alder reaction (Koehler, Anseth, & Bowman,
2013; Smith, 2017) between the conjugated double bonds formed after the
HVACP treatment with the double bond of another triglyceride’s fatty
acid unit. So, each Diels Alder reaction (Smith, 2017) decrease two
double bonds. The proposed mechanism for Diels Alder reaction is
presented in Fig. 4. Ionescu and Petrovic patented the cationic
polymerization of soybean oil (Ionescu & Petrovic, 2009). They have
reported that the polymerization initiates with the formation of
conjugated double bonds on the hydrocarbon chain of linoleic and
linolenic acid units (Ionescu & Petrovic, 2009). Once the conjugated
double bonds are formed, the polymerization goes through a pericyclic
reaction like Diels-Alder (Aragonès et al., 2016; Ionescu & Petrovic,
2009; Smith, 2017). The results that we have obtained are consistent
with the results published by the authors aforementioned. We have
proposed a similar polymerization/dimerization reaction with one
difference, radical aided conjugated double bond formation, and
following Diels-Alder reactions (Fig. 2 and 4) (Smith, 2017).
Moreover, peaks 5 and 7 of the 1H-NMR spectra related
to single bonds close to double bonds will reduce if double bonds are
modified, as it occurs with treated samples. The reduction of peak
integration area for 5 and 7 may correlate with the increased amount of
peak 3, because of possible hydrogenated double bonds or Diels-Alder
reaction (Smith, 2017) proposed (Fig. 4). The glycerol moiety did not
show significant modifications, peaks 8 and 9. Suggesting that treated
samples maintain a triglyceride structure.
Figure 4 – about here
Oxidation products include hydroperoxides (chemical shift 𝛿 (ppm) 5.7,
8.3-8.9), aldehydes (chemical shift 𝛿 (ppm) 9.5-9.8), alcohols (chemical
shift 𝛿 (ppm) 3.43-3.62), epoxides (chemical shift 𝛿 (ppm) 2.63, 𝛿 (ppm)
2.88-2.90, 𝛿 3.1), ketones (chemical shift 𝛿 (ppm) 6.08, 𝛿 6.82), or
conjugated double bonds (chemical shift 𝛿 5.7-6.4) (Gunstone et al.,
2007). Treated samples showed an increased peak only for conjugated
double bonds, as can be seen in the expanded spectra in figure 3a,
corresponding to 𝛿 (ppm) 5.7-6.3. Moreover, spectra from 2D-NMR (HSQC)
of the treated sample was used to identify a cross-peaks between
J-coupled protons in the spin of the conjugated double bonds and each
carbon involved in this network. From this analysis, two points were
identified on treated samples: (𝛿 (ppm) 6.2;126.3) and (𝛿 (ppm)
5.9;128.7), as can be seen in figure 3c. These peaks are related to the
’double bonds’ region of the hydrocarbon chain in the13C-NMR spectra. These results suggest that double
bonds can migrate to a conjugated arrangement during HVACP treatment
(Fig. 2), forming a ring structure that can result in the dimerization
of triglycerides (Fig. 4) (Biswas et al., 2007).
The assignments of carbon resonances of the triglyceride molecules of
soybean oil are characterized by four main signals: carbonyl carbons (𝛿
(ppm) 170-174), olefinic carbons (𝛿 (ppm) 120-140), glyceride backbone
carbons (𝛿(ppm) 60-80), aliphatic and methyl carbons (𝛿 (ppm) 10-40), as
shown in Fig. 3b. The carbon spectra for untreated and treated samples
(6h, gel fraction) showed the same number of peaks, and it indicates
that the primary structure of soybean oil treated with HVACP is
maintained as a triglyceride. A shift to the left was observed in the
spectra (Fig. 3b), mostly in the olefinic carbons. A shift may indicate
changes in the electron density near the carbon that affect the whole
molecule, with a higher shielding of the external magnetic field. These
changes may be related to the formation of the modified double bond of
the dimer.