To evaluate the DES as a reaction solvent, 6.0 mL of 35% ChCl DES
was combined with 2.57 g (10.0 mmol) palmitic acid, 2.57 g
(80.2 mmol) methanol, and 0.6 mL (7.84 mmol) trifluoroacetic acid.
This mixture was heated with stirring at 60 °C in a closed container
for 4 h. The reaction mixture was transferred to a separatory funnel,
where the methyl palmitate product formed a solid upon cooling
(methyl palmitate has a melting point of 29–31 °C). The bottom DES
layer remained liquid; thus the reaction products are simple to separate
for analysis. 1H NMR analysis in CDCl3 of the top layer indicated complete
conversion to methyl palmitate. 1H NMR analysis of the bottom
layer in D2O showed the characteristic peaks for the DES plus excess
methanol.
A recycling experiment was carried out to determine if the DES was a
suitable solvent for re-use. For this experiment, an esterification of
palmitic acid was performed as described above. After the two layers
had separated, the DES was removed and placed in a clean 25 mL
round bottom flask. Methanol, trifluoroacetic acid, and palmitic acid
were added as above, and another esterification experiment was performed.
After 1H NMR analysis of the resulting methyl ester layer, the
DES was again removed and used for another esterification reaction.
This procedure was repeated for a total of five esterification reactions
using the same DES. 1H NMR analysis in D2O of the DES after five
recycling experiments showed the characteristic peaks for the DES
plus residual methanol.