Stearic acid
Auto-oxidation of stearic acid at 210°C generated minor amounts of ester
species giving a 13C signal at 174 ppm, with trace
amounts of C-O carbons (65 to 75 ppm signals) and ketone (210 and 42 ppm
signals) as shown in Figure 2 . Since stearic acid is solid at
25°C, it was examined at 50°C (in CDCl3).1H and 13C NMR revealed an average
of 17 carbons for this commercial stearic acid used (table 1), due to a
smaller (e.g., C16) chain impurity species.
Untreated stearic acid (alone in solution) exhibits one1H DOSY signal at D = (3.2 ± 0.11) x
10-10 m2/s, see Figure S 2
and S3 . After oxidation, the D of unreacted stearic acid
(present in oxidized product mixture), with methylene1H signal at 2.3 ppm (Figure 3 ), is D = 2.4 x
10-10 m2/s, while the minor oxidized
species, giving 1H signals (-CHO-) at 4.0 and 4.9 ppm,
diffuses slower at circa 1.6 x 10-10m2/s (Figure S4 ). Thus, untreated stearic
acid diffuses circa two times faster than the major oxidized product
containing an ester group (Table 2 ). Minor signals from an
ester group are evident at 211 and 42 ppm.