Thermochemolysis using TMAAc and TMAH reagents as means to differentiate between free acids and esters

The reactivity of aromatic and aliphatic esters in on-line thermochemolysis in the presence of methylation reagents was studied using model compounds. Guaiacyl and 2-nonyl palmitates were synthesised, representing aromatic and aliphatic ester bonds, respectively. These model compounds were analysed by on-line thermochemolysis using tetramethylammonium acetate (TMAAc) and tetramethylammonium hydroxide (TMAH) in order to differentiate free acids from esterified acids. The released palmitic acid with TMAH for both model compounds was close to the theoretical palmitic acid content in the model compounds, even if part of the aliphatic ester had not reacted with TMAH. The free palmitic acid content by TMAAc was one third of the theoretical value for aromatic ester and only a few percentages for aliphatic ester. The result indicated that the less basic reagent is able to hydrolyse the aromatic ester linkage to some extent, whereas the aliphatic ester remains intact. Thus, differentiation of free acids by TMAAc from the esterified acids cannot be reliably performed from matrices containing aromatic esters. It was found that transesterification due to the use of methanol as a solvent with TMAAc is an insignificant reaction in on-line thermochemolysis.

► The reactivity of aromatic and aliphatic esters in on-line thermochemolysis was studied.
► Guaiacyl palmitate and 2-nonyl palmitate represented the aromatic and aliphatic ester bonds, respectively.
► Total amount of acids can be determined by TMAH from samples containing aliphatic and aromatic ester.
► The aromatic ester bond is partly hydrolysed in the presence of less basic reagent TMAAc, whereas the aliphatic ester bond is stable against TMAAc.