Halogen-free synthesis of symmetrical 1,3-dialkylimidazolium ionic liquids using non-enolisable starting materials

Abstract

Imidazolium ionic liquids were synthesized from readily available molecules: aldehydes, 1,2-carbonyl components, alkyl amines and acids in a halogen-free procedure. Since the use of enolisable carbonyl functions is not compatible with the modified Debus–Radziszewski reaction, symmetrical 1,3-dialkylimidazolium ionic liquids were made. The use of strong acids, like sulfuric acid, leads to protonation of the amine, low yields and side products that are difficult to remove. Changing the acid in the synthesis to acetic acid greatly improved the isolated yield and produced pure imidazolium acetate ionic liquids. From these imidazolium acetate compounds, many other ionic liquids could be prepared using different metathesis strategies. These strategies were dependent on the acidity of the conjugate acid of the anion, the acid volatility and the hydrophilicity of the used reagents and resulting ionic liquid. The following anions were introduced: bis(trifluoromethylsulfonyl)imide ([Tf₂N]⁻), p-toluenesulfonate (tosylate, [TsO]⁻), bis(2-ethylhexyl)phosphate ([DEHP]⁻) and nitrate ([NO₃]⁻). The impact of the different anions on the properties of the ionic liquids was investigated.