Solvents protophobic

Difficult in protophobic solvents, but fairly easy in protophilic solvents. 

With aromatic hydrocarbons, the potential of this step is usually 0.5 V more negative than the first step. The dianions Q2 are more protophilic (basic) than Q and are easily converted to QH2 (or QH ), withdrawing protons from the solvent or solvent impurities (possibly water), although Q2 with delocalized charges can remain somewhat stable. With compounds like 9,10-diphenylanthracene and in protophobic solvents like AN, the formation of dications has been confirmed in the second oxidation step (Section 8.3.2) ... 

Aprotic protophobic solvents like the above, but do not act as proton or Bronsted bases. This group includes ketones, nitriles and esters. 

Aprotic protophobic solvents Acetic anhydride Acetone... 

Water has high permittivity and moderate acidity and basicity. Thus, in water, many cations and anions are easily solvated (hydrated) and many electrolytes are highly soluble and dissociate into ions. Water has fairly wide pH and potential ranges and a convenient liquid temperature range. Of course, water is an excellent solvent. However, as in Table 1.7, the reaction environment can be expanded much wider than in water by use of a solvent of weak acidity and/or basicity. This is the reason why dipolar aprotic solvents, which are either protophilic or protophobic, are used in a variety of ways in modern chemistry. 

The solvation of H+ is stronger in protophilic DMSO but much weaker in protophobic AN than in amphiprotic water. The solvation of CH3COCT is much weaker in aprotic AN and DMSO than in water (for the strong solvation of CH3COO- in water, see Section 2.2.1). As for the solvation of CH3COOH, there is not much difference among the three solvents. From Eq. (2.7) and pKa = 4.76 in water, we get pKa = 11.0 and 23.2 for DMSO and AN, respectively, in fair agreement with the experimental values of 12.6 and 22.3 [7]. 

The hydrogen ion in protophobic aprotic solvents is very reactive. For example, judging from the values of transfer activity coefficient, H+ in AN is 10s times more reactive than in water. Thus, if basic substances are added to the solution in AN, they easily combine with H+. Table 3.6 shows the complex formation con-... 

As mentioned in Section 2.6, triple ion formation is not limited to low permittivity solvents. It also occurs in high permittivity solvents, if they are of very weak acidity and basicity for example, Kt for the formation of Li2Cl and LiCl2 in AN has been determined by polarography to be 105 M 2 [19]. Li+ and Cl- in AN are only weakly solvated and tend to be stabilized by forming triple ions. For conductimetric studies of triple ion formation in dipolar protophobic aprotic solvents, see Ref. [20]. 

The following criteria should be used to select an appropriate solvent (i) the electroactive species under study and the reaction products must dissolve and remain sufficiently stable in the solvent (ii) a polar solvent of weak acidity is suitable for an electrode reaction that occurs at negative potentials or whose measurement is affected by acidic solvents (iii) a polar solvent of weak basicity is suitable for an electrode reaction that occurs at positive potentials or whose measurement is affected by basic solvents (iv) the solvent should be easy to purify, low in toxicity, benign to the environment, reasonable in price, and should dissolve enough supporting electrolyte. Generally, DMF and DMSO (protophilic aprotic) and AN (protophobic aprotic) are used in case (ii), while AN is used most frequently in case (iii). 

The solubilities of alkali metal halides in various solvents are shown in Table 11.2 [2], In the table, water can dissolve all of the halides listed. Because water has a high permittivity and moderate acidic and basic properties, the hydration energies of the halides are large enough. Polar protic solvents like MeOH, HCOOH, FA and NMF can also dissolve many of the halides to considerable extents. However, in polar protophobic aprotic solvents like AN and Ac, halides... 

If the supporting electrolyte and the electrode material are chosen appropriately, the potential window in such protophobic aprotic solvents as AN, NM, PC and TMS easily exceeds 6 V (Table 8.1, see also 15) in Chapter 8). In aqueous solutions, the potential window never exceeds 4.5 V, even when a mercury electrode is used on the negative side and a diamond electrode on the positive side. This difference is important not only for electrochemical measurements but also for electrochemical technologies of, for example, rechargeable batteries and supercapacitors. For more information on the potential windows in non-aqueous solutions, see Ref. [10]. 

In this section we shall consider the state of protonic acids in the pure state and in solutions of three classes of solvents (i) amphiprotic-protogenic (mineral and carboxylic acids), (ii) aproticdipolar-protophobic (e.g., acetonitrile and nitromethane), and (iii) aprotic-inert (aliphatic and aromatic hydrocarbons and their haloderivatives). While classes (ii) and (iii) represent the only two families of solvents relevant to cationic polymerisation (with the possible exception of the polymerisation of N-vinylcarbazole, wdiich can be carried out in certain dipolar-protophilic solvents), class (i) is interesting because it represents the interaction between two Br nsted acids, the initiator and the solvent, as a direct source of protonating species. Althou the latter combination has not been used in cationic polymerisation, we will discuss its potentials and possible drawbacks. 

Amphiprotic protogenic solvents have higher acidic properties, but lower basic ones (always in comparison to water). Examples are formic and acetic acid. Amphiprotic protophilic solvents have lower acidity and higher basicity than water, with formamide or ethanolamine as examples. Aprotic dipolar solvents have low acidity and (occasionally) basicity as well, with A,A-dimethylformamide and dimethylsulfoxide as examples for protophilic dipolar solvents and acetonitrile for a protophobic dipolar solvent. 

The donor number is frequently used in various fields of polymer chemistry (see Chapter 10). Another classification based on acidity/basicity of solvents allows the division of solvents into six groups containing protic-neutral protogenic protophilic aprotic-protophilic aprotic-protophobic and aprotic-inert. ... 

Hojo M, Miyauchi Y, Nakatani I, Mizobuchi T, Imai Y (1990) Conductometric studies on the triple ion and quadrupole formations from lithium and tributylammonium trifluoroacetates in protophobic aprotic solvents. Chem Lett 6 1035-1038. doi 10.1246/cl.l990.1035... 

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