Chemistry Reference
In-Depth Information
carbonyl carbon peak of the base molecule (173.4 ppm), and an additional
peak appeared at 35.6 ppm due to CH
2
carbons a to the carbonyl of the R3
group. These observations confirm the formation of diester products from
DiOH-SBO and various anhydrides.
Percentage conversions calculated from
1
H-NMR data for signals in the
range 2.95-3.45 ppm (hydroxyl proton of the -CHOH group) yield values
495%. If there is only one substitution on each epoxy carbon (others still
have an -OH group), then the peak position for the remaining hydroxyl
proton may change, and this approach may not give accurate conversion
values. Using specific
1
H-NMR peaks arising from branching groups
(-COCH
3
, R1, R2 and R3), the number of these groups attached to the tri-
acylglycerol structure can be calculated as shown in Table 11.2. In an average
triacylglycerol molecule, there are 8.6 sites available for substitution,
4
sites are substituted by these branching groups, while the others retain hy-
droxyl groups. Steric hindrance may be a possible reason for there being only
one substituent on each epoxy carbon. This explanation seems valid, as the
average number of substituents in diester derivatives decreases with the
bulkiness (chain length) of the branching group.
The presence of hydroxyl and polar ester groups in diester products may
cause enhanced intermolecular interactions
24
resulting in highly viscous
products. GPC was performed on ESBO, dihydroxy and diester products to
check the extent of internal polymerization. It was found that oligomeriza-
tion occurred at some stage during the reaction. Apart from the main peak of
the expected products, there were approximately 20% oligomerized products
(with higher molecular weights) in the DiOH-SBO and diester products. The
molecular weights (M
w
) of the main product peaks for the starting and final
products are: ESBO 901 Da; DiOH-SBO 1107 Da; Ace-SBO 1193 Da; But-SBO
1286 Da; Isobut-SBO 1277 Da; and Hex-SBO 1653 Da as estimated by using a
calibration mixture of polystyrene, fatty acid methyl ester, and mono-, di-,
and triolein.
B
11.3.2.3 Anhydride Determination
FT-IR and TGA were used to check for unreacted anhydride in the final
diester products. It was found that in Hex-SBO, and But-SBO unreacted
anhydride remained in the product even after vacuum distillation. The
amount of unreacted anhydride was quantified using TGA. For the analysis,
the sample was heated in the presence of nitrogen at 20 1Cmin
1
to 500 1C
in high-resolution mode. Hexanoic anhydride (HA) has the highest boiling
of all the anhydrides used, and was found to evaporate before 200 1C. No
evaporation loss below 200 1C was detected in ESBO. On using a solution of
20% HA in ESBO, at 200 1C,
20% weight loss was detected, which can be
assigned to anhydride present in that mixture. Using TGA, the unreacted
anhydride in the crude Hex-SBO obtained after vacuum distillation, was
found to be 28% (Figure 11.4). Kugelrohr's distillation was then used at
100 1C under 0.2 Torr vacuum to remove the unreacted anhydride.
Figure 11.4 shows the TGA thermograms of ESBO, the 20% HA in ESBO,
B
