Solvents, acceptor properties acidity/basicity

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

Protophillic H-bond donor solvents solvents such as amides, amines or and other compounds with at least one N—II bond, which may be shared or donated. These solvents also have a highly basic character in the Bronsted sense i.e., they have a likelihood of accepting a free proton or a proton from a proton donor molecule (protophillic). These solvents also show high electron donor and acceptor properties (basic and acidic in the Lewis sense). 

Physical organic chemists have tended to examine parameters based on shifts in the absorption peaks in the spectra of various dyes or indicator molecules. The a and P scales of Taft and Kamlet, the ET(30) scale of Dimroth and Reichardt, the 7t scale of Taft and co-workers and the Z value of Kosower are all examples of this type of parameter. The definitions and measurement means for these parameters, as well as important references, are shown in Table 5. An alternative definition of the Dimroth-Reichardt parameter is the dimensionless, ETN, which is now preferred by some organic chemists (for a discussion see Ref. 15). The Z value is important in that it led to the scale of Dimroth and Reichardt, which overcomes many of the limitations of the earlier scale. Several workers have shown that relationships exist, with good correlation coefficients, between similar parameters. Thus, DN is linearly related to p, both parameters being designed to measure the donor properties (or Lewis basicity) of solvent molecules. Also, Lr(30) is related to a as well as to AN all three parameters purport to measure the electron acceptor properties (or Lewis acidity) of solvent molecules. It has been found that different solvent types have different coefficients in linear relationships between n and the dipole moment. The Taft and Dimroth-Reichardt parameters, in particular, have been found to correlate with free energies and... 

We have tried to cover important aspects of the physical chemistry of the ionic liquids currently under study, and to relate them to what is known about other types of low-melting ionic media. In concluding, we must emphasize that much of the success in their application, particularly in the Green Chemistry area where there is hope they will replace volatile solvents of environmentally hostile character, will depend on the important chemical properties of these media. These we have not addressed at all in this chapter. Properties such as donor and acceptor character, acidity and basicity, are in fact aU within the capacity of physics to describe, though they are most commonly invoked in a more empirical manner based on experience, as described in [1—4]. An excellent treatment of acid base character of ionic liquids has recently been given by MacFarlane and Forsyth [45]. 

Solvents may be classifiedf broadly according to their proton donor-acceptor properties as either amphiprotic, that is, both acidic and basic, or aprotic, neither acidic nor basic. 

Acid-base properties of oxide surfaces are employed in many fields and their relationship with PZC has been often invoked. Adsorption and displacement of different organic molecules from gas phase was proposed as a tool to characterize acid-base properties of dry ZnO and MgO [341]. Hammet acidity functions were used as a measure of acid-base strength of oxides and some salts [342]. Acidity and basicity were determined by titration with 1-butylamine and trichloroacetic acid in benzene using indicators of different pAg. There is no simple correlation between these results and the PZC. Acid-base properties of surfaces have been derived from IR spectra of vapors of probe acids or bases, e.g. pyridine [343] adsorbed on these surfaces. The correlation between Gibbs energy of adsorption of organic solvents on oxides calculated from results obtained by means of inverse gas chromatography and the acceptor and donor ability of these solvents was too poor to use this method to characterize the donor-acceptor properties of the solids [344],... 

At present, various kinds of donor-acceptor interactions are considered to be acid-base ones, in the framework of an appropriate acid-base concept [10-12]. The last century witnessed the development of a few such definitions, which can be translated conditionally to definitions of carriers of acidic (basic) properties, those for a solvent system, and principles for the prediction of the behaviour of acid-base reactions in various media. 

Trichloroacetic acid is stable in toluene, carbon disulfide, or 6N-sulfuric acid, i.e., in solvents having little or no proton-acceptor properties. The situation is different in basic solvents, e.g., in water in aqueous solution trichloroacetic acid decomposes to chloroform and carbon dioxide when warmed to 70°. Thus the undissociated acid is moderately stable, but the anion is not. Cor-... 

One such methodology is the Kamlet-Taft Solvatochromic parameter approach. In this methodology, a solvent can be characterized by three parameters, tt, a measure of the polarity and polarizability of the fluid, a, the acidity or hydrogen bond donor capability and P, the hydrogen bond acceptor capability or basicity. Each of these parameters is determined from the shift in UV-visible absorbance of a series of select indicator species dissolved in the solvent. Rather than depending on the bulk properties of the fluid, as is the case with the cohesive energy approaches, the solvatochromic parameters are derived from the interactions between the indicator solute and the immediate solvent shell, in effect they are a measure of how a solute sees the solvent. In each case, the scale of values has been normalized to between 0.0 for cyclohexane... 

However, donor-acceptor interactions are affected not only by the Lewis acid and base strengths, but also by other, steric and electron structural, factors. Thus, even in systems where either solely the donor or the acceptor property of the solvent is manifested, solvents with different space requirements may interact to different extents because of the steric properties of the reference solute and a reference acceptor with a tendency for dative 7c-bonding (back-coordination) will interact more strongly with jr-acceptor solvent molecules (e.g., acetonitrile) than would be expected from their basicity. The solvent donicity investigations by Burger et al [Bu 71, 74] with transition metal complex reference acceptor model systems have clearly shown the great extent to which such secondary effects may distort the solvent scale. 

The clear evaluation of the experimental results is also hindered by the difficulties encountered in the perfect purification of non-aqueous solvents. Several of them are hygroscopic, and even an extremely low water content may cause fundamental changes in the chemical properties of numerous solvents. As an electron-pair donor, the water molecule may behave as a ligand, and as a consequence of the ability of its hydrogen atoms to form hydrogen bonds, it may also act as an acceptor. This may lead to the occurrence of unexpected side reactions. In acidic solvents water behaves as a base, and in basic solvents as an acid, thereby disturbing the courses of the reactions to be investigated. The removal of trace amounts of water and the performance of work under anhydrous conditions is a difficult task. 

Acidic/Basic Lewis acidity asicity determines the solvent s ability to donate or accept a pair of electrons to form a coordinate bond with solute and/or between solvent molecules. A scale for this aeid ase property was proposed by Gutman (DN and AN -donor and acceptor number, respeetively) based on ealorimetrie determination. The complete proton transfer reaetion with formation of protonated ions is determined by proton affinity, gas phase aeidity, aeid or base dissoeiation constants. Both concepts differ in terms of net ehemieal reaetion. 

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. 

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). 

It is hoped that the terms donor and acceptor strengths will be reserved for inferences made about Lewis acid-base properties from data in the gas phase or poorly solvating solvents. This is to be contrasted with the more complex phenomena contributing to acidity and basicity. 

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. 

Compounds that are more polar, and which can better hydrogen-bond with water, require less drastic alterations to the solvent environment to cause dissolution to occur. On the right side of Figure 3, we associate solute polarity with each formulation concept. Drugs that are good hydrogen donors, in the extreme sense, have acidic properties. Likewise, those that are very good acceptors have basic properties. For these compounds, formation of a salt by protonation or deprotonation is a feasible route. 

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