Ionic liquid hydrolytically stable

Ionic liquids are generally regarded as highly stable, and the widely used dial-kylimidazolium ionic liquids are indeed thermostable up to 300 °C [4]. The propensity of the [BF4] and [PF6] anions to hydrolyze with liberation of HF [37], which deactivates many enzymes, has already been mentioned. The [TfO] and [ Tf2N] anions, in contrast, are hydrolytically stable. Dialkylimidazolium cations have a tendency to deprotonate at C-2, with ylide (heterocarbene) formation. Such ylides are strong nucleophiles and have been used as transesterification catalysts, for example [38]. These could cause enzyme deactivation as well as background transesterification when formed in small amounts from anhydrous ionic liquids and basic buffer salts, for example. 

In order to understand these interactions, we consider systems containing second generation ionic liquids (i.e. without functionalisation on the cation substituent, no labile protons on either the cation or the anion, and bearing a hydrolytically stable anion). 

Some ionic liquids have been found to release highly toxic and very corrosive degradation products when undergoing hydrolytic decomposition. This discovery was especially harmfiil for the green image of ionic liquids as it addressed, in particular, the main work horses of ionic liquid research of the time, namely the hexafluorophosphate and tetrafluoroborate systems which indeed release HF on contact with water [47]. Today, we know that many ionic liquids are very stable to hydrolysis. Moreover, we have developed many ionic liquids which are completely halide-free [48] so that the release of HQ or HF can certainly be avoided in both hydrolysis and combustion processes. 

These different ionic liquid acidities were conpared using the results of the alkylation of benzene with 1-dodecene. However, note that ionic liquids with more Lewis acidic anions are less hydrolytically stable. 

Kuhlmaim E, Himmler S, Giebelhaus H, Wassercheid P (2007) Imidazolium dialkylphosphates—a class of versatile, halogen-free and hydrolytically stable ionic liquids. Green Chem 9 233-242... 

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