Tom Welton

Ionic Liquids in Synthesis

Preface Contributors INTRODUCTION SYNTHESIS AND PURIFICATION OF IONIC LIQUIDS Synthesis of Ionic Liquids Quality Aspects and Other Questions Related to Commercial Ionic Liquid Production Synthesis of Task-specific Ionic Liquids PHYSICOCHEMICAL PROPERTIES OF IONIC LIQUIDS Melting Points and Phase Diagrams Viscosity and Density of Ionic Liquids Solubility and Solvation in Ionic Liquids Gas Solubilities in Ionic Liquids Polarity Electrochemical Properties of Ionic Liquids MOLECULAR STRUCTURE AND DYNAMICS Order in the Liquid State and Structure Quantum Mechanical Methods for Structure Elucidation Molecular Dynamics Simulation Studies Translational Diffusion Molecular Reorientational Dynamics ORGANIC SYNTHESIS Stoichiometric Organic Reactions and Acid-Catalyzed Reactions in Ionic Liquids Transition Metal Catalysis in Ionic Liquids Ionic Liquids in Multiphasic Reactions Multiphasic Catalysis with Ionic Liquids in Combination with Compressed CO 2 INORGANIC SYNTHESIS Directed Inorganic and Organometallic Synthesis Making of Inorganic Materials by Electrochemical Methods POLYMER SYNTHESIS IN IONIC LIQUIDS Introduction Acid-catalyzed Cationic Polymerization and Oligomerization Free Radical Polymerization Transition Metal-catalyzed Polymerization Preparation of Conductive Polymers Conclusions BIOCATALYTIC REACTIONS IN IONIC LIQUIDS Introduction Biocatalytic Reactions and their Special Needs Examples of Biocatalytic Reactions in Ionic Liquids Conclusions and Outlook OUTLOOK INDEX

Molecular states of water in room temperature ionic liquids

ATR and transmission IR spectroscopy have been used to investigate the state of water in room temperature ionic liquids (RTILs) based on the 1-alkyl-3-methylimidazolium cation with the anions: [PF6]−, [SbF6]−, [BF4]−, [ClO4]−, [CF3SO3]−, [(CF3SO2)2N]−, [NO3]− and [CF3CO2]−. It has been shown that in these RTILs water molecules absorbed from the air are present mostly in the “free” (not self-associated) state, bound via H-bonding with [PF6]−, [BF4]−, [SbF6]−, [ClO4]−, [CF3SO3]−, [(CF3SO2)2N]− with the concentrations of dissolved water in the range 0.2–1.0 mol dm−3. It has been concluded that most of the water molecules at these concentrations exist in symmetric 1 : 2 type H-bonded complexes: anion...HOH...anion. Additional evidence that the preferred sites of interaction with water molecules are the anions has been obtained from the experiments with RTILs of the 1-butyl-2,3-dimethylimidazolium and 1-butyl-2,3,4,5-tetramethylimidazolium cations. Water molecules can also form associated liquid-like formations in RTILs with anions of stronger basicity such as [NO3]− and [CF3CO2]−. When these RTILs are exposed to air the water concentrations exceed 1.0 mol dm−3. The strength of H-bonding between water molecules and anions increases in the order [PF6]− < [SbF6]− < [BF4]− < [(CF3SO2)2N]− < [ClO4]− < [CF3SO3]− < [NO3]− < [CF3CO2]−. The energies of this H-bonding were estimated from spectral shifts, with the resulting enthalpies being in the range 8–13 kJ mol−1. ATR-IR spectroscopy has also been used to study H-bonding between methanol and RTILs.

Diels-Alder reactions in room-temperature ionic liquids

Abstract The Diels-Alder cycloaddition reaction between methyl acrylate and cyclopentadiene has been investigated in a number of air and moisture stable ionic liquids. The endo/exo ratio of the reaction has been used as an initial probe of the nature of the solvents.

Mixtures of ionic liquids

Simple ionic liquids have long been held to be designer solvents, based upon the ability to independently vary their cations and anions. The formation of mixtures of ionic liquids increases this synthetic flexibility. We review the available literature of these ionic liquid mixtures to identify how their properties change and the possibility for their application.

Understanding the polarity of ionic liquids

The polarities of a wide range of ionic liquids have been determined using the Kamlet-Taft empirical polarity scales α, β and π*, with the dye set Reichardts Dye, N,N-diethyl-4-nitroaniline and 4-nitroaniline. These have been compared to measurements of these parameters with different dye sets and to different polarity scales. The results emphasise the importance of recognising the role that the nature of the solute plays in determining these scales. It is particularly noted that polarity scales based upon charged solutes can give very different values for the polarity of ionic liquids compared to those based upon neutral probes. Finally, the effects of commonplace impurities in ionic liquids are reported.

The role of hydrogen bonding in controlling the selectivity of Diels–Alder reactions in room-temperature ionic liquids

The reaction of cyclopentadiene with methyl acrylate has been investigated in a range of ionic liquids. The origin of the endo-selectivity for the reactions and associated rate enhancements has been attributed to a hydrogen bond formed between the cation of the ionic liquid and the methyl acrylate.

Self-assembly in the electrical double layer of ionic liquids.

We have studied the structure of two ionic liquids confined between negatively charged mica sheets. Both liquids exhibit interfacial layering, however the repeat distance is dramatically different for the two liquids. Our results suggest a transition from alternating cation-anion monolayers to tail-to-tail cation bilayers when the length of the cation hydrocarbon chain is increased.

Design of low-cost ionic liquids for lignocellulosic biomass pretreatment

The cost of ionic liquids (ILs) is one of the main impediments to IL utilization in the cellulosic biorefinery, especially in the pretreatment step. In this study, a number of ionic liquids were synthesized with the goal of optimizing solvent cost and stability whilst demonstrating promising processing potential. To achieve this, inexpensive feedstocks such as sulfuric acid and simple amines were combined into a range of protic ionic liquids containing the hydrogen sulfate [HSO4]− anion. The performance of these ionic liquids was compared to a benchmark system containing the IL 1-ethyl-3-methylimidazolium acetate [C2C1im][OAc]. The highest saccharification yields were observed for the triethylammonium hydrogen sulfate IL, which was 75% as effective as the benchmark system. Techno-economic modeling revealed that this promising and yet to be optimized yield was achieved at a fraction of the processing cost. This study demonstrates that some ILs can compete with the cheapest pretreatment chemicals, such as ammonia, in terms of effectiveness and process cost, removing IL cost as a barrier to the economic viability of IL-based biorefineries.

Salts dissolved in salts: ionic liquid mixtures

Solvents and solutions are ubiquitous in chemistry. For instance, in synthesis the solvent allows reagents to mix intimately so that reactions between these may occur. Consequently, understanding how solutes behave in solutions has been one of the major themes of chemistry throughout its history. Ionic liquids (liquid salts) are an exciting recent addition to the range of available solvents. Here we show that these solvents interact with dissolved salts to give solutions that are completely different from those of salts in either traditional organic solvents or water. Observations of these ideal salt solutions will require new models of solvation and polarity and have the potential to lead to new chemical processes.

Solvent?solute interactions in ionic liquidsElectronic supplementary information (ESI) available: synthesis of ionic liquids. See http://www.rsc.org/suppdata/cp/b3/b303095d/

A range of ionic liquids has been investigated using the Kamlet–Taft parameters α, β, and π*. It was found that π* is high for all of the ionic liquids studied and varies with both anion and cation, α is generally moderate and depends mainly on the cation, β is also moderate and depends mainly on the anion. Comparison is made with other polarity measurements in ionic liquids.