Acidic protogenic hydrogen

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. 

Most protic solvents have both protogenic and protophilic character, i.e. they can split off as well as bind protons. They are called, therefore, amphiprotic. These include water, alcohols, acids (especially carboxylic), ammonia, dimethylsulphoxide and acetonitrile. Solvents that are protogenic and have weak or practically negligible protophilic character include acid solvents, such as sulphuric acid, hydrogen fluoride, hydrogen cyanide, and formic acid. 

Protolytic reactions can also occur in fused salts. The solvent participates in these reactions provided that at least one of its ions has protogenic and/or protophilic character. An example of a solvent in which the cation is aprotic and the anion protophilic is ethylpyridinium bromide (m.p. 114°C). The acid HA is protolysed in this solvent (HA -I- Br HBr + A"). Hydrogen bromide acts as a solvated proton and the acidity is expressed as... 

Class 2 Other liqnids that have both active hydrogen atoms and donor atoms (O, N, F see Chapter. 3), but do not form three-dimensional networks (rather forming chainlike oligomers), e.g., primary alcohols, carboxylic acids, primary and secondary amines, nitro compounds with a-positioned hydrogen atoms, liqnified ammonia, etc. They are generally called protic or protogenic snbstances. 

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... 

ACIDIC SOLVENT, A solvent which is strongly protogenic, i.e., which has a strong tendency to donate protons and little tendency to accept them. Liquid hydrogen chloride and hydrogen fluoride are acidic solvents, and in them even such normally strong acids as nitric acid do not exhibit acidic properties, since there are no molecules that can accept protons but, on the contrary, behave to some extent as bases by accepting protons yielded by the dissociation of the HQ or the HF. See Acids and Bases. 

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... 

Initiation by a Lewis acid requires the presence of a proton donor (protogen) such as water, alcohol, or hydrogen halide. Generation of the carbocation is preceded by link-up of the initiator with the protogen, as shown here for polymerization of isoprene induced by BF3 [83] ... 

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. 

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. 

Lewis acids are seldom effective sdone rather they require the presence of trace amounts of water or some other proton donor (protogen) such as hydrogen halide, alcohol, and carboxylic acid, or a carbocation donor (cationogen) such as t-butyl chloride or triphenylmethyl chloride, which, on reaction with the Lewis acid, forms the electrophilic species that initiates polymerization. Thus dry isobutylene is unaffected by dry boron trifluoride but polymerization occurs immediately when trace amounts of water are added. The initiation process for boron trifluoride and water is... 

The pK of proteinous acids is pK > 4. Thus the fraction of active site retaining a free proton will be 10-4 or less. Consequently, the true presentation of a proton, even in a protogenic site, is not as a free hydrated proton but as a covalently linked hydrogen atom. 

Protogenic Capable of acting as a proton (hydron) donor strongly or weakly acidic (as a Bronsted acid). The term is preferred to the synonym protic or the more ambiguous expression acidic by itself. Also called HBD (hydrogen-bond donor) solvent. ... 

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