Deprotonation reaction, equilibrium

The rates of protonation and deprotonation reactions are sufficiently large in comparison with corresponding diffusion rates, so that it can be assumed that these reactions are at equilibrium even when current is flowing, that is, the following equations are valid everywhere at t > 0, t being the time ... 

TLM Activity Coefficients. In the version of the TLM as discussed by Davis et al. (11), mass action equations representing surface complexation reactions were written to include "chemical" and "coulombic" contributions to the overall free energy of reaction, e.g., the equilibrium constant for the deprotonation reaction represented by Equation 2 has been given as... 

A relatively stable aqua complex or protonated hypoastatous acid [H20At] has been assumed similarly, a protonated hypoiodous acid has been reported to exist in aqueous solutions (15). The equilibrium constant for the deprotonation reaction [Eq. (9)] has been estimated by extrapolation of data accrued from the lighter halogens to be < 10" (80), indicating that [H20At]" is a fairly weak acid. Another structure, the symmetric diaqua cationic complex [H20-At-0H2]", has also been proposed (79, 80). 

Redus, M., Baker, D.C., and Dougall, D.K., Rate and equilibrium constants for the dehydration and deprotonation reactions of some monoacylated and glycosylated cyanidin derivatives, J. Agric. Food Chem., 47, 3449, 1999. 

The solvent affects the chemical equilibria of reactions. Second-order rate constants and equilibrium constants have been determined for the benzoate ion promoted deprotonation reactions of (m-nitrophenyl)nitromethane, (p-nitrophenyl)nitromethane, and (3,5-dinitrophenyl)nitromethane in methanol solution. The pKa values for the arylnitromethanes in methanol are the following pKa = 10.9, 10.5, and 9.86 for m-nitrophenyl)nitromethane, (p-nitrophenyl)nitromethane, and (3,5-dinitrophenyl)nitro-methane, respectively, relative to benzoic acid (pKa = 9.4). A Bronsted B value of... 

The time required to reach equilibrium very much depends on the pKd value of the acid. An acid with a pKa value of 4, for example, deprotonates with a rate of 106 s Thus, the equilibrium is established within a few microseconds. On the other hand, an acid with a pKa value of 7 dissociates with a rate of ca. 103 s"1, and the equilibrium becomes established only on the millisecond time range. In a pulse radiolytic experiment, a large part of the radicals will thus have disappeared in bimolecular termination reactions, before an equilibrium is reached. Buffers speed-up the protonation/deprotonation reactions, and their addition can overcome this problem. Yet, they deprotonate acids and protonate their corresponding anions typically two to three orders of magnitude more slowly than OH and H+ (for a DNA-related example, see Chap. 10.4 for potential artifacts in the determination of pKa values using too low buffer concentrations, see, e.g., von Sonntag et al. 2002). 

It has been mentioned above that the pyrimidine radical cations are reasonably strong acids and rapidly deprotonate at a heteroatom. As all protonation/ deprotonation reactions at heteroatoms are reversible [e.g., equilibrium (22)], the radical cations are regenerated upon reprotonation. Deprotonation at carbon or reaction with water yield the final free-radical products. For the l,3Me2Thy system, where the deprotonation/reprotonation equilibria such as reaction (22) fall away, reactions (25)-(28) have been postulated to account for the fact that in the presence of 02 l,3Me25HOMeUra and l,3Me25(CHO)Ura [reaction (29)] are formed in a combined yield of 80% of primary S04 radicals (Rashid et al. 1991). The formation of these products has been taken as evidence that a free radical cation must be an intermediate. It is, however, also possible that the allylic radical is formed in a concerted reaction HS04 elimination. For such a process, a six-membered transition state can be written. 

The pAT-behaviour of 1-, 2- and 9-anthroic acids in the excited state was studied by Vander Donckt and Porter (1968a). Directly determined p/ (T )-values were found to lie nearer to the p/ (S1)-values calculated using the Forster cycle than to pA (S0). In a study of the fluorescence of 1- and 2-anthroic acids over a wide acidity range (Schulman et al., 1973a), it appeared that the deprotonation reactions did not come to equilibrium in the excited state. For protonation of the carboxyl group only 1-anthroic acid showed an excited state reaction and, as expected, it became more basic in the Si state. Fluorescence lifetime measurements on the prototropic species derived from 1- and 2-anthroic acids help in understanding the failure to reach equilibrium ... 

Reaction of acetylene with ethoxide forms acetylide and ethanol. Because the stronger acid is the product of the reaction, equilibrium favors the starting materials. The base ethoxide is not strong enough to deprotonate acetylene. 

The second group of values came from studies where it was assumed that polymerization reactions occurred, such as the formation of H5As206 (aq>, in addition to the deprotonation reaction. For chemical and mathematical reasons, the dissociation constant calculated from a set of measurements becomes smaller as one introduces polymeric anions into the model. The differences of the models chosen, at first appearance, could serve to explain the differences of the equilibrium constants given in the previous table. Unfortunately, the situation, from the perspective of data evaluation, is more complex. In principle, there should be a sufficient dilution of arsenious acid for which one would not expect the formation of a significant proportion of species like HsAsaOe caq) upon addition of base. For such a condition, the equilibrium constant determined assuming that only the monomer exists, should approach that determined for the multi-species model. Britton and Jackson (1934) performed potentiometric titration at two concentrations of arsenious acid (0.0170 and 0.0914 molar) and obtained essentially the same... 

The Br0nsted acidity of a molecule is its capacity to give up a proton. It can be expressed either by the equilibrium constant, the pfsTa value, the change of standard free energy (AGj) or simply the energy of the deprotonation reaction AH A - - H+. The acidities of phenols were measured experimentally " , including a series of 38 meta-... 

Zinc ester enolates may also be obtained by the addition of ZnX2 to lithium or sodium enolates as first described by Hauser and Puterbaugh (equation 6)P This approach has so far received little attention but similar reactions have been used to obtain zinc ketone enolates. In this regard, it should be noted that Heathcock and coworkers have shown that deprotonation reactions of ketones with zinc dialkylamide bases reach equilibrium at only about 50% conversion (equation 7). This result implies that attempts to prepare zinc enolates from solutions of amide-generated lithium enolates will be successful only when the lithium enolate is made amine-free. 

Obviously, protons near the surface are affected by its electrostatic field, and their activities must include the Boltzmann factor. Because of this at equilibrium protonation and deprotonation reactions must be subjects to equalities... 

When a semiconductor is immersed in an aqueous solution, and OH ions in the solution will continuously adsorb and desorb from the surface. A dynamic equilibrium will be established, which can be described by the following protonation and deprotonation reactions ... 

The conjugate acid of butyllithium is butane. Butane has a pK > 40, so it is a much weaker acid than diisopropyl amine with a pK of about 25. If butane is a much weaker acid than diisopropyl amine, for the reaction will be large (equilibrium is pushed to the right), facilitating the deprotonation reaction of the amine to give the conjugate base, lithium diisopropylamide. 

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