Protic Impurities

Erotic impurities have to be taken into account for two groups of ionic liquids those that have been produced by an exchange reaction involving a strong acid (often the case, for example, for [BMIM][PEg]), and those that are sensitive to hydrolysis. In the latter case, the protons may originate from the hydrolysis of the anion, forming an acid that may be dissolved in the ionic liquid. 

Eor ionic liquids that do not mix completely with water (and which display sufficient hydrolysis stability), there is an easy test for acidic impurities. The ionic liquid is added to water and a pEf test of the aqueous phase is carried out. If the aqueous phase is acidic, the ionic liquid should be washed with water to the point where the washing water becomes neutral. Eor ionic liquids that mix completely with water we recommend a standardized, highly proton-sensitive test reaction to check for protic impurities. 

Obviously, the check for protic impurities becomes crucial if the ionic liquid is to be used for applications in which protons are known to be active compounds. Eor some organic reactions, one has to be sure that an ionic liquid effect does not turn out to be a protic impurity effect at some later stage of the research  

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Unless working with superdried systems or in the presence of proton traps, adventitious water is always present as a proton source. Polymeriza tion rates, monomer conversions, and to some extent polymer molecular weights are dependent on the amount of protic impurities therefore, weU-estabHshed drying methods should be followed to obtain reproducible results. The importance is not the elimination of the last trace of adventitious water, a heroic task, but to estabhsh a more or less constant level of dryness. 

Apart from halide and protic impurities, ionic liquids can also be contaminated with other ionic impurities from the metathesis reaction. This is especially likely if the alkali salt used in the metathesis reaction shows significant solubility in the... 

Water in an ionic liquid may be a problem for some applications, but not for others. However, one should in all cases know the approximate amount of water present in the ionic liquid used. Moreover, one should be aware of the fact that water in the ionic liquid may not be inert and, furthermore, that the presence of water can have significant influence on the physicochemical properties of the ionic liquid, on its stability (some wet ionic liquids may undergo hydrolysis with formation of protic impurities), and on the reactivity of catalysts dissolved in the ionic liquid. 

Step-growth polymerization processes must be carefully designed in order to avoid reaction conditions that promote deleterious side reactions that may result in the loss of monomer functionality or the volatilization of monomers. For example, initial transesterification between DMT and EG is conducted in the presence of Lewis acid catalysts at temperatures (200°C) that do not result in the premature volatilization of EG (neat EG boiling point 197°C). In addition, polyurethane formation requires the absence of protic impurities such as water to avoid the premature formation of carbamic acids followed by decarboxylation and formation of the reactive amine.50 Thus, reaction conditions must be carefully chosen to avoid undesirable consumption of the functional groups, and 1 1 stoichiometry must be maintained throughout the polymerization process. 

Currently, tin(II) bis-(2-ethyUiexanoate), also referred as tin octoate, is the most widely used catalyst for the ROP of lactones. This popularity stems from its acceptance by the American Food and Drug Administration (FDA) for the formulation of polymer coatings in contact with food. Moreover, tin(II) bis-(2-ethyUiexanoate) is less sensitivity towards water and other protic impurities than aluminum alkoxides, which facilitates its use in the laboratory and in industry. 

The mechanism of the tin(II) bis-(2-ethylhexanoate)-mediated ROP of lactones remained a matter of controversy for many years, and many different mechanisms were proposed. Indeed, tin(II) bis-(2-ethylhexanoate) is not made up of alkoxides but of carboxylates, known as poor initiators for the ROP of lactones. In 1998, Penczek and coworkers made a major contribution in this field. They reported that, if the polymerization is carried out in THF at 80 °C, then tin(II) bis-(2-ethylhexanoate) is converted in situ into a new tin alkoxide by the reaction with either an alcohol, purposely added in the reaction medium, or with any other protic impurity present in the polymerization medium (Fig. 14) [37]. Tin alkoxides formed in situ are the real initiators of the polymerization, which takes place according the usual... 

For 64-70 reversible potentials RED 7SEM can be obtained at -55 °C only in DMF from which protic impurities have been removed One should keep in mind that here the levels RED (and SEM) exist as heterocyclic substituted cumulenes which may rapidly polymerize. 

I11 dipolar aprotic solvents, the second step of the oxygen reduction is 02 -> 02 . However, if H+ is available from the solvent or from protic impurities, 02 reacts with H+ to form ()2I I or 02H2. Sometimes, the ()211 or 02H2 is further reduced at more negative potentials to give H20 as the final product. Detailed information concerning the electrochemistry of molecular oxygen can be found in Ref. [39 b]. 

Because Q2 is a much stronger base than Q , it readily reacts with protons originated from the protic impurity like H20 or from the solvent itself. 

Carbocations formed through protonation of alkenes by proton acids are usually assumed as intermediates in alkylation with alkenes. Metal halides, when free of protic impurities, do not catalyze alkylation with alkenes except when a cocatalyst is present. It was shown that no neat conjugate Friedel-Crafts acids such as HA1C14 or HBF4 are formed from 1 1 molar compositions in the absence of excess HC1 or HF, or another proton acceptor.163-166 In the presence of a proton acceptor (alkene), however, the Lewis acid halides—hydrogen halide systems are readily able to generate carbocations ... 

The enolate that is die most stable usually has die most highly substituted double bond and is called the thermodynamic enolate. If a slight excess of the ketone is used or a trace of protic impurities is present, equilibrium between die enolates is established and isomerization to die more highly substituted enolate occurs. 

Group transfer polymerization meets most of these criteria. However, it is sensitive to protic impurities and the present cost of the initiator is too high. Other living processes for polymerization of (meth)acrylates will be evaluated with respect to these criteria. 

Hexyl trifluoroacetate reacts w ith trimethyl(trifluoromethyl)silane in the presence of a molar equivalent of tetrabutylammonium fluoride to give the silylated hemiketal 29 in. 3.5% yield. Much of the trimethyl(trifluoromcthyl)silanc is converted into the undesired trifluoromethane due to rapid quenching of the incipient trifluoromethide species by the protic impurities in the reaction mixture. "... 

To achieve optimal properties in an AB block copolymer, it is important to control the molecular weight of the blocks, and minimize the amount of A homopolymer produced on addition of the second monomer. Termination reactions do occur in these systems [20], but the rate is fairly slow, particularly at temperatures below about 100 °C. In a practical sense, protic impurities present a much greater challenge. In a two-reactor system it is common practice to prepare the first block in one reactor, titrate out impurities in the B monomer charge in a second reactor by adding small increments of butyllithium to a solution of the B monomer until the first sign of color or exotherm, and then the transfer poly(A)Li solution to the second reactor. 

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