Proton-acceptor basic solvents

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. 

Solvents can be grouped on the basis of their properties as proton donors (acidic), proton acceptors (basic), and dipole interactions. Solvents can be positioned within the Solvent Selectivity Triangle (Fig. 17) on the basis of the relative involvement of each of these three factors as parameters of solubility. Mobile phases consisting of a mixture of three solvents can be optimized by... 

Kinetic studies of the reaction of alcohols with acyl chlorides in polar solvents in the absence of basic catalysts generally reveal terms both first-order and second-order in alcohol. Transition states in which the second alcohol molecule acts as a proton acceptor have been proposed ... 

The possibility of intervention of a solvent bridge between the proton and the basic site, where the solvent molecule acts simultaneously as a proton donor and proton acceptor, should always be considered. Here as elsewhere, the operational recognition of what Hammett (1970) calls the stoichiometric involvement of solvent is not a simple task. 

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

Amino groups may act not only as proton acceptor, but also as proton donor. Acidic N—H protons interact with basic solvents. In these cases an ortho-nitro group in an aniline system competes with the solvent by an internal hydrogen bond66, as depicted in 12. The stretching frequencies (by IR spectra in carbon tetrachloride) of vnh of complexes between A-methylaniline or diphenylamine (and some nitro-anilines66) and solvents depend on the proton accepting ability of the solvent (which is a moderate base)67. The frequency shifts are linearly related to the solvent s donor number (DN)3. 

It should be kept in mind that the terms acidity and basicity of the solvent have to be intended not only according to the Lewis concept (electrophilic vs. nucleophilic properties), but also according to the Bronsted concept (proton donor vs. proton acceptors), or to the hydrogen bonding capacity (hydrogen bond donor vs. hydrogen bond acceptor). 

Organic solvents can also be classified according to their ability to accept or transfer protons (i.e., their acid-base behavior) [20,21]. Amphiprotic solvents possess donor as well as acceptor capabilities and can undergo autoprotolysis. They can be subdivided into neutral solvents that possess approximately equal donor and acceptor capabilities (water and alcohols), acidic solvents with predominantly proton donor properties (acetic acid, formic acid), and basic solvents with primarily proton acceptor characteristics (formamide, N-methylformamide, and N,N-dimethylformamide). Aprotic solvents are not capable of autoprotolysis but may be able to accept protons (ACN, DMSO, propylene carbonate). Inert solvents (hexane) neither accept nor donate protons nor are they capable of autoprotolysis. 

The correlation with basicity will only appear in so far as there is an inverse correlation between proton-acceptor and proton-donor character in the amines concerned. (Zezyulinski [7] has suggested a converse correlation between the shifts in the N—It frequency of pyrrole and the basicity of acceptors, as solvents, from acetone to pyridine.)... 

The mobile phases used in normal-phase chromatography are based on nonpolar hydrocarbons, such as hexane, heptane, or octane, to which is added a small amount of a more polar solvent, such as 2-propanol.5 Solvent selectivity is controlled by the nature of the added solvent. Additives with large dipole moments, such as methylene chloride and 1,2-dichlor-oethane, interact preferentially with solutes that have large dipole moments, such as nitro- compounds, nitriles, amines, and sulfoxides. Good proton donors such as chloroform, m-cresol, and water interact preferentially with basic solutes such as amines and sulfoxides, whereas good proton acceptors such as alcohols, ethers, and amines tend to interact best with hydroxylated molecules such as acids and phenols. A variety of solvents used as mobile phases in normal-phase chromatography are listed in Table 2.2, some of which may need to be stabilized by addition of an antioxidant, such as 3-5% ethanol, because of the propensity for peroxide formation. 

The contribution of solvent-solute hydrogen bonding to selectivity [term (iii) in Eq. (34)] can be generalized for the presence of more than one basic or proton-acceptor solvent in the mobile-phase mixture ... 

The acid and base which differ by a proton according to this relationship are said to be conjugate to one another every acid must, in fact, have its conjugate base, and every base its conjugate acid. It is unlikely that free protons exist to any extent in solution, and so the acidic or basic properties of any species cannot become manifest unless the solvent molecules are themselves able to act as proton acceptors or donors, respectively that is to say, the medium must itself have basic or acidic properties. The interaction between an acid or base and the solvent, and in fact almost all types of acid-base reactions, may be represented as an equilibrium between two acid-base systems, viz.,... 

Ammonia, a strongly basic solvent and therefore a strong proton acceptor, encourages the dissociation of acids ... 

Many solvents are proton donors or proton acceptors and can thus induce basic or acidic behavior in solutes dissolved in them. For example, in an aqueous solution of ammonia, water can donate a proton and thus acts as an acid with respect to the solute ... 

Factors infiuencing the extent of proton transfer are (i) Brpnsted acidity and basicity of the proton donor and acceptor, (ii) solvent, (iii) temperature and (iv) concentration. In the following, we examine these various factors. 

Xanthone, flavone and similar compounds. A difference in chloroform specificity from 2-methoxyethanol is again demonstrated In Figure 11. Xanthydrol is eluted before xanthone and flavone with chloroform as the modifier. Perhaps this shows the useful coupling of a proton donor solvent and proton acceptor solute (a large secondary solvent effect in an adsorption system (43)), the Interaction that was hoped for with the selection of chloroform as one of the selectivity triangle modifiers and apparent here because of a less strong adsorption of the xanthydrol on the fully active silica than some of the other basic solutes used in the preliminary studies. 

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