Acids, comparative protonation

Maleic and fiimaric acids have physical properties that differ due to the cis and trans configurations about the double bond. Aqueous dissociation constants and solubiUties of the two acids show variations attributable to geometric isomer effects. X-ray diffraction results for maleic acid (16) reveal an intramolecular hydrogen bond that accounts for both the ease of removal of the first carboxyl proton and the smaller dissociation constant for maleic acid compared to fumaric acid. Maleic acid isomerizes to fumaric acid with a derived heat of isomerization of —22.7 kJ/mol (—5.43 kcal/mol) (10). The activation energy for the conversion of maleic to fumaric acid is 66.1 kJ/mol (15.8 kcal/mol) (24). 

The shapes of the titration curves of weak electrolytes are identical, as Figure 2.13 reveals. Note, however, that the midpoints of the different curves vary in a way that characterizes the particular electrolytes. The pV, for acetic acid is 4.76, the pV, for imidazole is 6.99, and that for ammonium is 9.25. These pV, values are directly related to the dissociation constants of these substances, or, viewed the other way, to the relative affinities of the conjugate bases for protons. NH3 has a high affinity for protons compared to Ac NH4 is a poor acid compared to HAc. 

This value is compared with those for other acids and protonic liquids in Table 15.21 " the extent of autoprotolysis in H2SO4 is greater than that in water by a factor of more than lO " and is exceeded only by anhydrous H3PO4 and [HBp3(OH)] (p. 198). In addition to autoprotolysis, H2SO4 undergoes ionic selfdehydration ... 

The Dissociation Constant of Nitric Acid. The largest value of K in Table 9 is that for the (HS04) ion. In Fig. 36 there is a gap of more than 0.2 electron-volt below the level of the (H30)1 ion. As is well known, several acids exist which in aqueous solution fall iu the intermediate region between the very weak acids and the recognized strong acids the proton levels of these acids will fall in this gap. The values of K for these acids obtained by different methods seldom show close agreement. Results obtained by various methods were compared in 1946 by Redlich,1 who discussed the difficulties encountered. 

Everything we have mentioned so far has been the qualitative method for comparing acidity of different protons. In other words, we never said how much more acidic one proton is over another, and we never said exactly how acidic each proton is. We have talked only about relative acidities which proton is more acidic ... 

Compare the product concentrations with the concentration of the acid that is responsible for the various ions in the solution, [H2 CO3] — 0.050 M. The ions generated when this acid undergoes proton transfer with water have concentrations that are two orders of magnitude smaller than the concentration of the parent acid. 

Here, disulfite is functioning as a latent acid, releasing protons and bisulfite upon hydrolysis (Equation 10). At the proper proton concentrations, (x=2, 3), rapid Cr(VI) reduction and fast gelation take place. Therefore at x=2 to 3, the redox reaction should be the same as if acid were added at n=2 to 6 (Equation 9). The gelation reactivity of the two are comparable under these conditions ... 

Despite a wide range of the modifiers used, there is a clear symbasis between the calculated proton affinities and the experimental stability indices Kst (Figure 3). The decrease in concentration of nitric acid in the interlayer space will give rise to a poorer stability of GICs, if the proposed stabilization mechanism is valid. To verify this assumption, the treatment of graphite nitrate with 25-fold excess of acetic acid, compared to that used in the above procedure, was carried out in a glass column. Most of the molecules of nitric acid are probably removed from the interlayer space under such a treatment. It turns out that this procedure reduces the expansion... 

The fact that metal hydrides can be acidic may seem paradoxical in view of the nomenclature that insists that all complexes with a M-H bond be referred to as hydrides regardless of whether their reactivity is hydridic or not. Not only can some metal hydrides donate a proton, but some can be remarkably acidic. Some cationic dihydrogen complexes are sufficiently acidic to protonate Et20 [8], and some dicationic ruthenium complexes have an acidity comparable to or exceeding that of HOTf [9],... 

Treatment of "skipped" enynes H CCH2CH=CHR with alkali amides presumably will lead to extensive isomerization into C>CCH=CHCH2R. With alkyllithium bases inverse addition has to be applied to avoid dimetallation. This isomerization reaction is likely to be even more serious in the case of the diynes HC=CCH2OCR, since the CH2-protons have kinetic acidities comparable with that of the ethynyl proton. 

Along with the electrostatic effect, a statistical or entropy effect is also present and accounts for the increased dependence of the equilibrium constant on the degree of neutralization, f, with decreased acid strength of the polyelectrolyte. Thus, cations from the base compete with protons from the acid for positions on the PAA chain. There are more possibilities or opportunities for a cation with respect to replacing a proton when f is below 0.5 (that is when neutralization is less than one half). The counter is true when f is above 0.5. Thus, PAA is a stronger acid compared with monoacids such as propenoic acid at low fractions of neutralization, whereas PAA is a weaker acid at high fractions of neutralizations. However, this entropy effect is overwhelmed for PAA by the electrostatic effect with the end result that PAA is a weaker acid than propenoic acid throughout its neutralization. 

Our experience to this point has been that C—H bonds are not very acidic. Compared with most hydrocarbons, however, aldehydes and ketones have relatively acidic protons on their a-carbon atoms. pATa s for enolate formation from simple aldehydes and ketones are in the 16 to 20 range. 

Quantifying precisely the acidity of zeolites or other acidic solids is a goal, which up to now has not been satisfactorily achieved. The main problem is the lack of an acceptable scale of solid acidity comparable to pK scale for aqueous solutions or proton affinities for gas-phase reactions. For this reason all available physical and theoretical methods of investigation have been applied over the years on this subject and a large number of papers have been published. Several reviews are available. 

Phenolic compounds are, in general, weak acids. Compared to the hydroxyl group of unsubstituted aliphatic alcohols, however, the phenolic OH-group is more acidic. The reason for this is that the anion formed after abstracting the proton from the hydroxyl group is relatively stable because of the existence of several mesomeric structures. The anion is referred to as the phenolate anion. Hence, phenol (2.5) is a weak acid, with a pKa value of 10. This places phenol in between carboxylic acids (pKa = 4-5) and aliphatic alcohols (pKa = 16-19). 

The smaller the pA a, the more acidic the proton is. This probably seems strange, but that s the way it is. A compound with a pA a of 4 is more acidic than a compound with a pKa of 7. Next, we need to know what the difference is between 4 and 7. These numbers measure orders of magnitude. So the compound with a pA a of 4 is 103 times more acidic (1000 times more acidic) than a compound with a pA a of 7. If we compare a compound with a pA a of 10 to a compound with a pA a of 25, we find that the first compound is 1015 times more acidic than the second compound (1,000,000,000.000,000 times more acidic). 

In addition, sulfonamides bear more acidic, compared to carboxamides, protons that can be selectively A-alkylatcd or V-acylatcd. Thus, alkylation of sulfonamide catenane and rotaxane with Frechet-dendrons of 2nd generation led to the first representatives of the dendrocatenane 15 and dendrorotaxane 16 [46], respectively. These compounds were of especial interest, since they allowed for the first time to study chiral induction of topologically chiral cores on appended dendrons and to compare to the analogous centrochiral dendrimers for which phenomenon of the crypto-optical activity was postulated [47],... 

Nucleic Acid Base Resonances The chemical shifts of the nonexchangeable protons in poly(dA-dT), the Nuc/D = 24 complex and the Nuc/D = 8 complex in 1 M NaCl solution are plotted as a function of temperature in Figure 19. The nucleic acid nonexchangeable proton chemical shifts in the duplex state are either unperturbed (adenosine H-8, H-2, and thymidine CH3-5) or shift slightly upfield (thymidine H-6) on complex formation (Figure 19). By contrast, the thymidine H-3 exchangeable proton located in the center of the duplex resonates 0.35 ppm to higher field in the Nuc/D = 8 proflavine complex compared to its position in the... 

Enolate ions formed from, ketones or aldehydes are extremely important in the synthesis of more complex organic molecules. The ease with which an enolate ion is formed is related to the acidity of the a proton. The pKa of propane (acetone) is 19.3 that means that it is a stronger acid compared to ethane (pKa 60) and a much weaker acid than acetic acid (pKa 4.7), i.e. strong bases like sodium hydride, sodium amide, and lithium diisopropylamide LiN(i-C3H7)2 are needed to form an enolate ion. 

An alkyl halide can undergo an elimination reaction if it has a susceptible proton situated on a (1-carbon, i.e. the carbon next to the C-X group. This proton is lost during the elimination reaction along with the halide ion. In some respects, there is similarity here between alkyl halides and carbonyl compounds (Following fig.). Alkyl halides can have susceptible protons at the (1-position whilst carbonyl compounds can have acidic protons at their a-position. By comparing both structures, it can be seen that the acidic/ susceptible proton is attached to a carbon neighbouring an electrophilic carbon. 

Having explored the relationships between solution pH and pKa values, we can now explore the relative acidities of various hydrogen atoms and how these values are influenced by neighboring functional groups and heteroatoms. In this arena, it is important to remember that how a reaction proceeds is largely dependent upon the relative acidities of protons (hydrogen atoms) compared to one another and not on the absolute acidity of a given proton. 

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