Acids, acid weak, deprotonation

The conjugate base of an acid is the base formed when the acid has donated a proton. The conjugate acid of a base is the acid that forms when the base has accepted a proton. A strong acid is fully deprotonated in solution a weak acid is only partially deprotonated in solution. A strong base is completely protonated in solution a weak base is only partially protonated in solution. 

An alternative approach to this problem is to assume that deprotonation of a weak acid in the micellar pseudophase will be related to the concentration of bound OH-, which should follow the ion-exchange model (Section 5). As a result much of the work has been based on the use of very weak acids which are deprotonated only at high pH. 

Tetraborane(lO) is a very weak acid. Therefore, deprotonation requires a very... 

Because under basic conditions carboxylic acids are deprotonated to the carboxylate ions, which are no longer electrophilic enough that a weak nucleophile like MeO- can attack them. Upon workup the carboxylate is neutralized to give back the carboxylic acid. 

FIGURE 2-17 The titration curve of acetic acid. After addition of each increment of NaOH to the acetic acid solution, the pH of the mixture is measured. This value is plotted against the amount of NaOH expressed as a fraction of the total NaOH required to convert all the acetic acid to its deprotonated form, acetate. The points so obtained yield the titration curve. Shown in the boxes are the predominant ionic forms at the points designated. At the midpoint of the titration, the concentrations of the proton donor and proton acceptor are equal, and the pH is numerically equal to the pAfa. The shaded zone is the useful region of buffering power, generally between 10% and 90% titration of the weak acid. 

Except for B1QH14, a more detailed consideration of the interaction of donor ligands with boranes is beyond the scope of this review. Decaborane is a multifunctional species that simultaneously acts as a Bmnsted acid and a Lewis acid. Weak bases fail to direcdy deprotonate decaborane but do react resulting in the evolution of H2 and the formation of species that contain ligands coordinated at the six- and nine-positions of the decaborane skeleton (see Fig. 11). 

Now suppose instead that the base A" is a stronger proton acceptor than H20. The tug-of-war for the proton now favors the protonated base—its conjugate acid HA. For instance, if the base is CN , then the acid is HCN, and a high proportion of HCN molecules survive in solution. Such an acid is weak, for it is only slightly deprotonated in aqueous solution. 

These principles apply to any solvent (Fig. 10.16) an acid is strong if its conjugate base is a weaker proton acceptor than the solvent. In such a case, the tug-of-war for the proton is resolved in favor of the solvent, so the acid is completely deprotonated. The acetate ion, CH3C02, is a weaker proton acceptor than an NH3 molecule, so acetic acid is a strong acid in liquid ammonia. A base is strong if it has a greater proton-pulling power than the solvent if the opposite is true, the base is weak. 

When we turn to polyprotic acids other than sulfuric, we find that the acidity constants of successive deprotonation steps are normally widely different. As a result, we can treat a polyprotic acid or the salt of any anion derived from it as the only significant species in solution. This approximation leads to a major simplification to calculate the pH of a polyprotic acid, we just use JCal and take only the first deprotonation into account that is, we treat the acid as a monoprotic weak acid. Subsequent deprotonations do take place, but they do not affect the pH significantly and can be ignored. 

Figure 5.32 shows the pH-pseudo-ftrst-order rate constant profile of the degradation of aspartame (an ampholyte, pKal = 3.19 and pKa2 = 8.14). In the hydrolysis of aspartame, the protonated/undissociated form predominates at low pH (<3), while the deprotonated/dissociated form exists at high pH (>8). As demonstrated for monoprotic weak acids and weak bases, certain terms in the numerator of Equation (5.173) become negligible. For example, the hydrolysis of the protonated/undissociated form by OH-, of the protonated/dissociated form by H+ and OH-, and the deprotonated/dissociated form by H+ are not likely to occur. Then, Equation (5.173) for the ampholytic drug is written as ... 

The simple amine, piperidine, will have a normal amine pKaH of about 11 so it will be easy to protonate with even very weak acids. Any mineral acid like HCl will do as would weaker acids like RCO2H. Deprotonation will remove the NH proton as nitrogen is more electronegative than carbon but a very strong base such as BuLi will be needed as the pK will be about 30-35. 

Carbohydrates are the third most important class of biopolymers, next to proteins and DNA. However, their analyses by CZE are still in infancy because of the complexity and diversity of their structures. In terms of ionization ability, carbohydrates may be divided into acidic and weakly ionizable classes. Acidic carbohydrates are negatively charged at neutral pH and can be conveniently separated. Weakly ionizable carbohydrates are neutral at mild pH, but deprotonate at extremely basic condition (e.g., pH > 12). 

The strongest acids appear on the left side of the figure and the strongest bases on the right side of the figure. Any base can deprotonate any acid on the left side of it, a weaker base. Acetic acid, a weak acid will ionize (or get deprotonated)... 

Preparation.—A historical account of the development of the synthesis of methylenephosphoranes has been given. Details have appeared - of the use of epoxides in the generation of ylides from phosphonium salts. Weakly acidic salts are deprotonated by the anions such as XCHaCH20 formed by attack of the phosphonium covmter-anion on epoxide, but strongly acidic phosphonium salts are deprotonated more rapidly than these species are produced. The base in these cases must be either epoxide or original anion. Side-reactions in olefin syntheses using epoxides as base include acetal formation from aldehyde and epoxide, cyclopropane formation from ylide and epoxide, and decomposition of quinquecovalent phosphoranes, e.g. (1), formed from phosphonium salt and... 

The observed first-order rate constant for carbanion formation may be controlled through the choice of the basic proton acceptor. Relatively strong carbon acids undergo detectable deprotonation by the weak base water in a pseudo-first-order reaction (Scheme I.IA), but stronger general bases (Scheme I.IB) or hydroxide ion (Scheme I.IC) are required to give detectable deprotonation of weaker carbon acids in bimolecular reactions. 

Many drug molecules are weak acids or weak bases. The pH of the reaction medium may in that case strongly influence the results obtained. The photosensitized oxidation of mefloquine and other antimalarials shows a clear pH dependency (Tpnnesen and Moore, 1991). Valuable information can be lost if experiments are carried out only in one medium, i.e., organic solvent that cannot differentiate between protonated and deprotonated forms of the molecules, or at nonphysiological pH. This is clearly demonstrated for the antimalarials of 4-aminoquinoline structure... 

Needless to say, complexes formed by protonation, especially where HA is a strong acid, are readily deprotonated, even by bases as weak as diethyl ether, and are highly sensitive to solvent media and trace water. These properties relate in large measure to the high acidity of certain H2 complexes, which can have pK as low as -6, e.g., when generated from triflic acid (see Chapter 9). 

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