Protophilic solvents

Proton acceptors (protophilic solvents) amines, ethers. 

Protophilic solvents are substances such as liquid ammonia, amines and ketones which possess a high affinity for protons. The overall reaction taking place can be represented as ... 

The equilibrium in this reversible reaction will be greatly influenced by the nature of the acid and that of the solvent. Weak acids are normally used in the presence of strongly protophilic solvents as their acidic strengths are then enhanced and then become comparable to those of strong acids — this is referred to as the levelling effect . 

Dimethylformamide (DMF) is a protophilic solvent which is frequently employed for titrations between, for instance, benzoic acid and amides, although end points may sometimes be difficult to obtain. 

The pyridinium ion (acid 2) as the analyte can be titrated with quaternary ammonium hydroxide (base 3) as it concerns the determination of H+ of the Brensted acid pyridinium, a potentiometric measurement of the pH titration curve and its inflection point is most obvious. In the aprotic, but protophilic, solvent pyridine no stronger acid can exist (see reactions 4.37 and 4.38) than the pyridinium ion itself hence there is a levelling effect but in theory only on the acid side. 

The above solvents theory (A) and proton theory (B) have shown that in theory the neutrality point (of the pure solvent) lies for the amphiprotic solvents at pH = pKs and for the aprotic protophilic solvents at a pH somewhere between the highest acidity (of the protonated solvent) and an infinitely high pH. However, the true pH of the neutrality point of the solvent can only be obtained from a reliable pH measurement and the problem is whether and how this can be achieved. For water as a solvent, the true pH = - logaH+ = colog aH+ is fixed by the internationally adopted convention E°m ( H2(latm) = 0... 

As would be expected, the larger titration potential ranges offer much more scope for mutually distinguishing between individual acids or bases in amphiprotic solvents, as a consequence of self-dissociation, the potential ranges are rather limited, whereas in the aprotic protophilic solvents and "aprotic inert solvents these ranges are considerably more extensive. 

Such dimerization is impossible in a protophilic solvent such as pyridine. 

Acid-base titrations in protophilic solvents with low e... 

A final subdivision of ionizing protonic solvents can be made in terms of the behaviour of the solvent towards available protons from a solute. A basic or protophilic solvent such as ammonia or an amine will coordinate protons strongly, and in so doing accentuate the acidic properties of the acid. 

Water is the prototype of an amphiprotic solvent and all other solvents with similar acid-base properties are called neutral solvents. Solvents which are much stronger acids and much weaker bases than water are called protogenic solvents, while those which are much stronger bases and much weaker acids than water are designated protophilic solvents. 

Using water as reference, an amphiprotic solvent having an acidity and a basicity comparable to those of water is called a neutral solvent, one with a stronger acidity and a weaker basicity than water is called a protogenic solvent, and one with a weaker acidity and a stronger basicity than water is called a protophilic solvent. The solvent with relatively strong acidity usually has in its molecule a hydrogen atom... 

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

Among amphiprotic solvents of high permittivities, there are water-like neutral solvents (e.g. methanol and ethanol), more acidic protogenic solvents (e.g. formic acid), and more basic protophilic solvents (e.g. 2-aminoethanol). There are also amphiprotic mixed solvents, such as mixtures of water and alcohols and water and 1,4-dioxane. The acid-base equilibria in amphiprotic solvents of high permittivity can be treated by methods similar to those in aqueous solutions. If the solvent is expressed by SH, the acid HA or BH+ will dissociate as follows ... 

When H in AN is titrated with a protophilic solvent as in Table 3.6, a clear pH jump occurs on the titration curve at the equivalence point. 

V. Comparison of Hydrogen Exchange Rates in Liquid Ammonia and in other Protophilic Solvents... 

A. Catalysis of Hydrogen Exchange by Alkali Metal Hydroxides and Alkoxides in Protophilic Solvents... 

VIII. The Kinetic Isotope Effect in the Hydrogen Exchange in Protophilic Solvents and in Metallation... 

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