Amphiprotic systems

Many molecules of biochemical significance (Including the amino acids) can act as both proton donors and proton acceptors, and we need to be able to treat this dual function quantitatively. 

An amphiprotic species is a molecule or ion that can both accept and donate protons. For instance, HCOj can act as an acid (to form COf ) and as a base (to form H2CO3). Among the most important amphiprotic compounds are the amino acids, which can act as proton donors by virtue of their carboxyl groups and as bases by virtue of their amino groups. 

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Figure 7.1 Nicotinic acid is an amphiprotic system composed of three species in equilibrium. Nicotinic acid at high pH is negatively charged at the carboxylic function, while at low pH it is positively charged at the pyridinyl nitrogen.

One of the likely reasons octanol-water gained such a widespread use is its amphiprotic character, i.e. its abiUty to serve as an H-bond donor and acceptor while, for example, alkane-water systems are inert in that sense, from the point... 

Since 1-octanol has certain limitations (see Section 1.3) many alternative lipophilicity scales have been proposed (see Figure 1.8). A critical quartet of four solvent systems of octanol (amphiprotic), alkane (inert), chloroform (proton donor) and propy-... 

In a very thoughtful investigation of solvent systems to model membrane characteristics, Leahy et al. (1989, 1992) have argued that two receptors sited in different tissues (or membranes) could exist in environments that are very different in hydrogen bonding character one may be surrounded by amphiprotic groups (as in a protein) or by proton donors the other may be surrounded by proton acceptors (as in a phospholipid membrane). 

The impurity gives a signal that disturbs the measuring system. An example is shown in Fig. 10.1 [10], The residual current-potential curves were obtained with a platinum electrode in propylene carbonate (PC) containing various concentrations of water. Because water is amphiprotic, its cathodic reduction and anodic oxidation are easier than those of PC, which is aprotic and protophobic. Thus, the potential window is much narrower in the presence of water than in its absence. Complete removal of water is essential for measuring electrode reactions at very negative or positive potentials. 

Pi is the reference solvent and HBj is an H-bonding parameter. Leahy et al. suggested that a more sophisticated approach incorporating four model systems would be needed to adequately address issues of solute partitioning in membranes (121). Thus, four distinct solvent types were chosen— apolar, amphiprotic, proton... 

Complex systems include solutions containing two acids or bases, which contain or consume two or more protons, and amphiprotic substances that act as both acids and bases. A characteristic of all such systems is that two or more equilibria must be considered in describing their behavior. As a consequence, the techniques for pH data derivation are often more complex than for simple systems. 

Non-aqueous acid-base systems Water plays a crucial role in the acid-base chemistry of aqueous solutions, but in the absence of water it is possible to have other families of acids and bases in which different solvents play a role analogous to that of water. Perhaps the most common of these is the liquid ammonia system . Like water, NH3 is amphiprotic and can engage in autoprotolj is ... 

The pH concept is most commonly used for dilute aqueous media however, a similar formalism can be extended to other systems. The extent of the pH scale, which in aqueous media can be described as 14 units, depends on the autoprotolysis constant of the amphiprotic solvent, so that the equivalent range, e.g., in methanol, equals 16.7 units, in sulfuric acid 2.9 units, and in acetic acid 14.5 units. In such solvents, as in water, the pH of neutrality corresponds to the middle of this range. Such reasoning cannot be extended to protophilic (e.g., pyridine, ethers), and aprotic (e.g., hydrocarbons) solvents, for which the logan+ scale is from one or both sides, respectively, unlimited. 

The range of acidity ApH(HS) = pKap(HS) available in a number of solvents is shown in Figure 1. The various solvents are also compared on an absolute basis. Each solvent is characterized by two pX values, pK and pK", such that if an acid-base system has pX(H20) < pK then on dissolving in this solvent it is essentially completely converted to base + solvent cation, whereas if pK(H20) > pK" the acid-base system is essentially completely converted to acid -t-solvent anion. Evidently pK — pK" = ApH(HS) = pKap(HS). The horizontal scale also represents the Hammett acidity function Hq (see Section 8). In addition to a number of amphiprotic solvents this Figure also illustrates the acidity range available in a solvent such as diethyl ether which has no acid properties, and a solvent such as hexane which has neither acid nor base properties. 

Since acceptor molecules such as B(OH)3 and SbFj can enhance the acidity of water and HF respectively, although they are not simple Bronsted acids, there is considerable merit in defining an acid as any substance which increases the concentration of the characteristic cation of a solvent, and a base as any species which increases the concentration of the characteristic solvent anion. These are the solvent-system definitions of acid and base, and are very useful for amphiprotic and related solvents. 

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