Reprotonation rate

The pK of tyrosine explains the absence of measurable excited-state proton transfer in water. The pK is the negative logarithm of the ratio of the deprotonation and the bimolecular reprotonation rates. Since reprotonation is diffusion-controlled, this rate will be the same for tyrosine and 2-naphthol. The difference of nearly two in their respective pK values means that the excited-state deprotonation rate of tyrosine is nearly two orders of magnitude slower than that of 2-naphthol.(26) This means that the rate of excited-state proton transfer by tyrosine to water is on the order of 105s 1. With a fluorescence lifetime near 3 ns for tyrosine, the combined rates for radiative and nonradiative processes approach 109s-1. Thus, the proton transfer reaction is too slow to compete effectively with the other deactivation pathways. 

The binding of pyranine to phosphatidylcholine (lecithin) vesicles as a function of the probe and electrolyte concentration has been investigated [103], The binding of the probe to the internal leaflet of lecithin small unilamellar vesicles (SUVs) was found to be larger than that to the external leaflet. The addition of salt up to 2 M did not prevent binding, even at low probe concentrations. The ground-state reprotonation rate constant was found to depend on the probe content per vesicle. 

At low pH any excited naphtholate ion formed from excited 2-naphthol is quickly reprotonated but as the pH increases the reprotonation rate constant k2 becomes smaller (and increases from zero) until beyond pH3 reprotonation becomes negligible and 0/0o settles down to the plateau value 1/(1 +A,r0) and 0 /0o to... 

Recent preliminary evidence indicates that even for alkyl substituents attached to the enamine, the reprotonation rate constant may be very much below diffusion-controlled. This conclusion is based on the ability to observe the enamine 2 (in equation 3) derived from compounds with R1 = OMe, R2 = -pentyl, in partially aqueous solutions and only in the presence of cyclodextrins of appropriate cavity to accommodate the alkyl chain33. 

The relatively low pX values seen for the benzoyl acetanilides, especiaHy as two-equivalent couplers, minimize concerns over slow ionization rates and contribute to the couplers overaH reactivity. But this same property often results in slow reprotonation in the acidic bleach, where developer carried over from the previous step can be oxidized and react with the stiH ionized coupler to produce unwanted dye in a nonimage related fashion. This problem can be eliminated by an acidic stop bath between the developer and the bleach steps or minimized by careful choice of coupling-off group, coupler solvent, or dispersion additives. 

The catalyst reported by Grotjahn and Lev (11-13) for alkyne hydration (2) is capable of isomerizing alkenes, but veiy slowly. Because we knew that the rate of alkyne hydration was unchanged in the presence of excess phosphine ligand, we thought that like alkyne hydration, alkene isomerization would require loss of acetonitrile ligand (14) and alkene binding. Subsequent deprotonation at an allylic position would make an q -allyl intermediate which when reprotonated at the other... 

This equilibrium value is the same as that found for the chelated acid (Kk = 1.0) (17) so it is likely that the dipeptide carbanion, like 15 (Scheme 15), shows less kinetic discrimination for reprotonation (k-i/ k-2) than does the ester carbanion 18. In D20 H-exchange and epi-merization occur at the same rate, kohs = 6.4 X 10 4 s 1 (pD = 8.4, 34°C), to give the same equilibrium distribution of A-S-S and A-R-S products (24). 

Let us consider the possible events following excitation of an acid AH that is stronger in the excited state than in the ground state (pK < pK). In the simplest case, where there is no geminate proton recombination, the processes are presented in Scheme 4.6, where t0 and Tq are the excited-state lifetimes of the acidic (AH ) and basic (A- ) forms, respectively, and ki and k i are the rate constants for deprotonation and reprotonation, respectively, kj is a pseudo-first order rate constant, whereas k i is a second-order rate constant. The excited-state equilibrium constant is K = k /k 7 ... 

The acido-basic properties of water molecules are greatly affected in restricted media such as the active sites of enzymes, reverse micelles, etc. The ability of water to accept or yield a proton is indeed related to its H-bonded structure which is, in a confined environment, different from that of bulk water. Water acidity is then best described by the concept of proton-transfer efficiency -characterized by the rate constants of deprotonation and reprotonation of solutes - instead of the classical concept of pH. Such rate constants can be determined by means of fluorescent acidic or basic probes. 

The kinetics and thermodynamics of the act-nitro equilibrium of picrylacetone (105) in 50 50 and 30 70 (v/v) H20-MC2S0 mixtures have been reported. Rate of general base-catalysed deprotonation of (105) and general acid-catalysed reprotonation of the resulting anion (106) have been monitored at low pH a fast equilibrium protonation of (106) to give a directly observable short-lived nitronic acid species (107) has been found to precede conversion to (105). The constants pAf and pATj,... 

The Brpnsted coefficient /3b = 0.52 for deprotonation of 3-phenylcoumaran-2-one (108) by a series of bases in 50% (v/v) water-dioxane, and q bh = 0.48 for reprotonation by the conjugate acid of the buffer, are indicative of a fairly symmetrical transition state for proton transfer, although the primary KE, ku/ku = 3.81, found for proton abstraction by HO is lower than expected. " The moderate intrinsic rate constant for deprotonation of (108) suggests that generation of the charge in the transition state is accompanied by only a small amount of molecular and solvent reorganization. In acidic solution, below pH 5, O-protonation of (110) occurs initially to form (109)... 

It should be noted that the rate of racemization (or the rate of hydrogen exchange in Section 10.1.1) is exactly the same as the rate of enolization, since the reprotonation reaction is fast. Hence, the rate is typical of a bimolecular process and depends upon two variables, the concentration of carbonyl compound and the concentration of acid (or base). 

The following data would appear to substantiate this premise. At high nitroaromatic concentrations Reaction 12 should be able to compete with the reprotonation of the carbanion and the rate of ionization should become equal to the rate of oxygen absorption. Since the stoichiometry of the oxidation did not change on adding the nitroaromatic catalysts, the assumption that the absorption of only one molecule of oxygen occurred for each electron transfer step is legitimate. 

Unlike with the breakdown of the more reactive hemiorthoesters a mechanism which involves a rapid and reversible ionization followed by a unimolecular breakdown of the monoanion appears to be a valid one for the hydroxide-ion catalysed breakdown of these nitrogen containing tetrahedral intermediates. Not only have the ionized forms of [124] and [126] been detected but the values of k l, the rate constant for reprotonation of the ionized form calculated as above, is always much greater than k2. 

It was originally thought that the high basicity of the amido complex would make the reprotonation a diffusion controlled process so that k, was always much greater than k2. This is indeed true for the majority of Co1" systems examined and for all studied cases of Cr111, RuUI, Rh111 and Ir111 complexes. Under these conditions, the expression for the rate constant reduced to ks = 2 nlk, k2 //c i or 2 K k2 where Kl is the equilibrium constant for the proton transfer process (1). With proton transfer as a rapid preequilibrium, such systems exhibit specific base catalysis. 

The a-secondary IE of two deuteriums on the rate of base-catalyzed CD exchange of toluene, 3A ( PhC112D)/k(PhCD is 1.31, and the [3-secondary D IE on the rate of base-catalyzed a-C-D exchange of ethylbenzene, k(PhCHDCH3)//t(PhCHDCD3), is 1.11 0.03.58 Similarly, from the rates of base-catalyzed a-C-D exchange of tolucne-a,4-r/2, -a,2,4,6-c/4, and -a,2,3,4,5,6-d6 and with an assumption of linearity of IEs, the contributions of ortho, meta, and para deuteration lead to rate retardations of 2.4, 0.4, and 1.8%, respectively.59 These are all kinetic IEs, but to the extent that the transition state resembles closely the carbanion, or to the extent that the reverse reprotonation is encounter-controlled and independent of isotopic substitution, these kinetic IEs represent equilibrium IEs on acidity. The IEs were interpreted in terms of an electron-donating inductive effect of D relative to H. 

Linear alkanes, which are known to be less reactive, also undergo H-D exchange by the same mechanistic scheme at slower rates at and above 150°C.54,55 This exchange reaction occurs in a very clean way because no side products from cracking and isomerization are observed. The cations that are adsorbed on the surface are prone to deprotonation, but the alkenes that are formed are rapidly reprotonated before substantial oligomerization can take place. 

At higher concentrations of acetaldehyde, bimolecular trapping of the enolate in Scheme 4.7 will become faster, so, at some stage, this will compete effectively with the reprotonation of the enolate. When the bimolecular capture of enolate by another acetaldehyde molecule becomes much faster than the reprotonation of the enolate, i.e. when /c4[CH3CHO] k, 1 + k-2[H+] + /c 3[BH+], another limiting approximation to the complex rate equation predicted from the mechanism (Equation 4.17) is obtained, Equation 4.19 ... 

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