Protogenic solvents

Proton donors (protogenic solvents) H2SO4, carboxylic acids. 

These four solvents can thus be termed protogenic solvents, whilst bromine trifluoride and sulphur dioxide which do not contain hydrogen are non-protonic solvents. Non-ionising solvents include hydrocarbons, ethers, esters and higher alcohols the lower alcohols, especially methanol and ethanol, do show slight ionising properties with appropriate solutes. 

Protogenic solvents are acidic in nature and readily donate protons. Anhydrous acids such as hydrogen fluoride and sulphuric acid fall in this category because of their strength and ability to donate protons they enhance the strength of weak bases. 

In Fig. 4.1a one sees the occurrence of an increasing levelling effect on the acid side when changing from protogenic solvents via neutral to proto-philic solvents (dotted lines below). 

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

HOAc in the equations of the original papers will be replaced in our description by HS as a designation of protogenic solvents in general (see later its use for m-cresol). 

An anion produced as a result of the action of a Brpnsted base on a protogenic solvent or of the action of autoprotolysis (loss of a hydron) of a solvent. Thus, OH is the lyate ion of water. 

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. 

Protic and protogenic solvents (such as ethanol and nitromethane) ... 

In Ref. 26c, solvents are classified as follows protic solvents [amphiprotic hydroxylic solvents (water, methanol, glycols), amphiprotic protogenic solvents (CH3COOH, HF), proto-philic H-bond donor solvents (FA, NMF, NH3) dipolar aprotic solvents [aptotic proto-philic solvents (DMF, DMSO, Py), aprotic... 

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... 

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 ... 

The solvating ability of solvents depends not only on their general polarity, which is a non-specific property, but in a large part to their ability to interact in a specific manner with the solute. This may take place by the donation of a nonbonding pair of electrons from a donor atom of the solvent towards the formation of a coordinate bond with the solute, therefore exhibiting Lewis basicity, or the acceptance of such a pair from a solute, an exhibition of Lewis acidity of a protic or protogenic solvent towards the formation of a hydrogen bond between it and... 

On the other side of the scale, very moderate, but not negligible, Lewis acidities are ascribed, according to their a parameters, to C-H acids, such as chloroform and bromoform, primary and secondary acyclic amines, such as -butylamine and diethylamine, and protogenic solvents, such as methyl-alkyl ketones, acetonitrile, and nitromethane. It can be expected that liquid 1 -alkynes (not on the List), having the grouping H-C=C-R, also have non-negligible a values, being C-H Lewis acids. It can be safely concluded, however, that aprotic solvents other than those of the classes noted above have no Lewis acid character, with a 0 for all intents and purposes. 

Directly related to aldehyde-enol tautomerisation in protogenic solvents are recent results reported by Capon et al. (1979) concerning the decomposition of [61] in D20-[2H3]acetonitrile which generates vinyl alcohol (37), the tautomerisation of which was monitored by H and 13C nmr spectroscopy and compared with ethyl vinyl ether hydrolysis. 

On the other hand an acid or protogenic solvent will be a poor proton acceptor, accentuating basic properties. This elTect is exemplified by the solution or nitric acid in anhydrous hydrofluoric acid, which shows how nitric acid (normally regarded as a strong acid) can behave as a Lowry-Bronsted base. 

Types of Solvent.—In order that a particular solvent may permit a substance dissolved in it to behave as an acid, the solvent itself ifiust be a base, or proton acceptor. A solvent of this kind is said to be proto-philic in character instances of protophilic solvents are water and alcohols, acetone, ether, liquid ammonia, amines and, to some extent, formic and acetic acids. On the other hand, solvents which permit the manifestation of basic properties by a dissolved substance must be proton donors, or acidic such solvents arc protogenic in nature. Water and alcohols arc examples of such solvents, but the most marked protogenic solvents are those of a strongly acidic character, e.g., pure acetic, formic and sulfuric acids, and liquid hydrogen chloride and fluoride. Certain solvents, water and alcohols, in particular, are amphiprotic, for they can act both as proton donors and acceptors these solvents permit substances to show both acidic and basic properties, whereas a purely protophilic solvent, e.g., ether, or a completely protogenic one, e.g., hydrogen fluoride, would permit the manifestation of either acidic or basic functions only. In addition to the types of solvent already considered, there is another class which can neither supply nor take up protons these are called aprotic solvents, and their neutral character makes them especially useful when it is desired to study the interaction of an acidic and a basic substance without interference by the solvent. 

Bases.—The equilibrium between an acidic, i.e., protogenic, solvent and a base may be represented by another form of the general equation (2), viz.,... 

This reaction occurs because the acid possesses some protophilic properties, and these become manifest in the presence of the very strongly protogenic solvent. As may be expected, the stronger the acid is in water, the weaker does it behave as a base in hydrogen fluoride. 

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. 

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