Protic ionic liquid

Wang C, Luo H, Jiang De et al (2010) Carbon dioxide capture by superbase-derived protic ionic liquids. Angew Chem Int Ed 49(34) 5978-5981... 

Given the desire for covalent stability noted above, we must also be wary of protic ionic liquids in general. These species can be written as the product of the reaction of an Arrhenius acid and a base... 

One family of ionic liquids that has to date only been sparsely investigated for use with conducting polymers is the protic ionic liquids, where the cation has one or more mobile hydrogen atoms. A recent manuscript by Bi ak detailed the synthesis of 2-hydroxy ethylammonium formate [109], which melts at — 82 °C and has a room-temperature ionic conductivity of 3.3 mS cm-1, and reported the ability of this protic ionic liquid to dissolve poly(aniline) (17gmL ) and poly(pyrrole) (no concentration specified). The dissolution of conducting polymers into any solvent is of significant interest for a variety of reasons, such as improving their processability and ease of incorporation into different devices. 

Li et al. [93] have used l-ethylimidazolium trifluoroacetate, which is a Bronsted acidic ionic liquid, as a medium for the electropolymerization of aniline. They report that in this ionic liquid the oxidation potential of aniline is lower (0.58 V compared to 0.83 V in 0.5 M H2SO4) and that the growth rate of the polymer is increased. Further, the resultant films are smooth, strongly adhered to the Pt working electrode and are very electrochemically stable. Similar results have been reported by Liu et al. [92], who found that this was the best ionic liquid for the polymerization of aniline, compared to the unsatisfactory results observed in other protic ionic liquids 1-butylimidazolium tetrafluoroborate, 1-butylimidazolium nitrate and 1-butylimidazolium p-toluenesulfonate, as well as the l-butyl-3-methylimidazolium hydrogen sulfate and l-butyl-3-methyimidazolium dihydrogen phosphate. 

Carbene formation can be suppressed by substituting the acidic proton in the 2-position by, for example, a methyl group, although this has the effect of increasing the melting point and the viscosity of the ionic liquid. Alternatively, a protic ionic liquid such as [HC4m][OTf] may be used.1311 Here, the presence of an acidic NH+-functionality prevents formation of a carbene complex and further accelerates the formation of the catalytically active species. 

This relation is shown in Figure 2.1. It has so far been found free of exceptions when base is an amine nitrogen. The protic ionic liquids as a class [16-18] are considerably more fluid than the aprotic ionic liquids [17], for reasons that are not clear at this time (see Section 2.6). 

Surprisingly, it is only a very recent recognition that protic ionic liquids can serve as proton transfer electrolytes in hydrogen-oxygen fuel cells. A current report from the laboratory of Watanabe [40] describes the performance of a hydrogen electrode utilizing, as the electrolyte, the salt formed by proton transfer from the acid form of f>/i-trifluoromethanesulfonyl imide (HTFSI) to the base imidazole. 

Especially to be noted is the high potential of the cell relative to the value obtained when the IL is replaced by 98% H3PO4 with, (0.9 vs. 0.75 V), which is maintained under load when using ethylammonium nitrate as electrolyte. It is evident that with the protic ionic liquid, a much higher exchange current density at the oxygen electrode can be obtained than in other fuel cells. 

Pemak J, Goc I, Mirska I. Anti-microbial activities of protic ionic liquids with lactate anion. Green Chem. 2004. 7, 323-329. 

Greaves T L, Weerawardena A, Fong C, et al. Protic ionic liquids Solvents with tunable phase behavior and physicochemical properties. 

Kreher et al. drew attention to the potential of /V,/V-dialkyl-ammonium N ,N -dialkylcarbamates for use as distillable ionic media [122], However, this class of ILs is generally considered to be a special case since the IL dissociates during distillation to give the respective amine and carbon dioxide. It is also widely recognized that many protic ionic liquids are distillable under easily accessible conditions [123],... 

Two general classes of ionic liquids which are easily distilled are protic ionic liquids (PILs) formed by reaction between a volatile acid and base [86], and dialkylammo-nium carbamate salts such as dimethylammonium dimethylcarbonate (DIMCARB) [72, 73, 206], An example of a distillable PIL is methylpyrrolidinium acetate formed from the exchange of a proton between acetic acid and methylpyrrolidine [86], DIMCARB, a low-cost ionic liquid produced by mixing C02 with dimethylamine, has been used as a reaction medium for the synthesis of monoarylidene and macro-cyclic compounds, where in each case, the product was separated by distillation of the ionic liquid [72, 73, 206],... 

Drummond et al. [54] synthesized protic ionic liquids by combining Bronsted acid/base pairs where the primary amine cations were of the form RNH3 and R(OH) NH3 combined with organic anions of the form RCOO, R(OH)COO or with an inorganic anion. They studied physicochemical properties of synthesized PILs at nominally equimolar ratio of anion and cation (1 1 stoichiometry) and in the presence... 

Scheme 11 Synthesis of a variety of protic ionic liquids...

This class of protic ionic liquids resulted miscible with polar molecular solvents such as acetonitrile, acetone, methylene chloride and ethyl acetate, remaining immiscible with hydrocarbon solvents. Two different applications have been reported by the same authors as solvents in the Heck reaction and as supports for homogeneous supported-phase synthesis. 

Greaves TL, Drummond CJ (2008) Protic ionic liquids properties and applications. Chem Rev 108 206-237... 

Abstract This chapter presents the design and analysis of the microscopic features of binary solvent systems formed by ionic liquids, particularly room temperature ionic liqnids with molecular solvents. Protic ionic liquids, ethylammonium nitrate and l-n-butyl-3-methylmidazohum (bmim)-based ILs, were selected considering the differences in their hydrogen-bond donor acidity. The molecular solvents chosen were aprotic polar (acetonitrile, dimethylsulphoxide and MA(-dimethylformide) and protic (different alcohols). The empirical solvatochromic parameters n, a and P were employed in order to analyse the behaviour of each binary solvent system. The study focuses on the identification of solvent mixtures of relevant solvating properties to propose them as new solvents . Kinetic study of aromatic nucleophilic substitution reactions carried out in this type of solvent systems is also presented. On the other hand, this is considered as a new approach on protic ionic liquids. Ethylammonium nitrate can act as both Bronsted acid and/or nucleophile. Two reactions (aromatic nucleophilic substitution and nncleophilic addition to aromatic aldehydes) were considered as model reactions. 

With respect to ILs as cosolvents, the interest was centred on two types of ILs aUcylammonium and 1,3-dialkylimidazolium-based ionic liquids. For the first type, we selected ethylammonium nitrate, considering that it is a protic ionic liquid (PIL) and that it can act as a potential acid catalyst. With respect to the second type of ILs, we selected those based on l-methyl-3-n-butylimidazolium cation, characterised by a slighter HBD acidity than that of the alkylammonium type (Figs. 13.2, 13.3). These ILs exhibit a wide spectrum of physicochemical properties. It was demonstrated that their water content, density, viscosity, surface tension, melting point and thermal stability are affected by the length of the alkyl chain and the nature of the anion. Several anions were incorporated in this class of ILs. 

Protic Ionic Liquids Nucleophilic and Acid Catalysis Behaviour... 

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