Abstract
[AAE]X composed of amino acid ester cations is a sort of typically “bio-based” protic ionic liquids (PILs). They possess potential Brønsted acidity due to the active hydrogens on their cations. The Brønsted acidity of [AAE]X PILs in green solvents (water and ethanol) at room temperature was systematically studied. Various frameworks of amino acid ester cations and four anions were investigated in this work from the viewpoint of structure–property relationship. Four different ways were used to study the acidity. Acid dissociation constants (pKa) of [AAE]X determined by the OIM (overlapping indicator method) were from 7.10 to 7.73 in water and from 8.54 to 9.05 in ethanol. The pKa values determined by the PTM (potential titration method) were from 7.12 to 7.82 in water. Their Hammett acidity function (H0) values (0.05 mol·L−1) were about 4.6 in water. In addition, the pKa values obtained by the DFT (proton-transfer reactions) were from 7.11 to 7.83 in water and from 8.54 to 9.34 in ethanol, respectively. The data revealed that the cationic structures of [AAE]X had little effect and the anions had no effect on the acidity of [AAE]X. At the same time, the OIM, PTM, Hammett method and DFT method were reliable for determining the acidic strength of [AAE]X in this study.
Highlights
Protic ionic liquids (PILs) are an important subset of ionic liquids (ILs)
The Brønsted acidity associated with the Hammett acidity function (H0 ) of [AAE]X was investigated in water to confirm the acidic strength of [AAE]X determined by the Potential Titration Method (PTM) and Overlapping Indicator Method (OIM) [49]
To confirm the acidic strength of [AAE]X determined by the PTM and OIM, the Brønsted acidity associated with the Hammett acidity function (H0 ) of [AAE]X was investigated in water
Summary
Protic ionic liquids (PILs) are an important subset of ionic liquids (ILs). PILs possess strong dissolvabilities, high thermal stabilities, designable structures and broad electrochemical windows [1,2]. Water and ethanol are considered as green media acidic catalytic reactions, which amino acid) analogs and, broader prospects in acid-involving processes [22]. PILs, such as imidazolium salts and quaternarytoammonium have been studied in many acidic catalytic reactions in solvents [29,30]. Determining the acidity of ILs become an intriguing topic, since water ethanolto are promising media acidity for green chemistry. Four acid different were employed to study the Brønsted acidity different amino estermethods cations and anions in water and ethanol carried out. Four different methods were employed to different amino acid ester cations and anions in water and ethanol carried out. Study the Brønsted acidity of [AAE]X PILs with different amino acid ester cations and.
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