Proton transfer diffusion controlled

In some situations, the oxygen or proton reduction reaction during metal corrosion will be mass transfer (diffusion) controlled, due to poor fluid agitation and/or a low concentration of H or dissolved oxygen in solution (this is especially trae for O2, which has a low solubility in water at... 

Table 4-1 lists some rate constants for acid-base reactions. A very simple yet powerful generalization can be made For normal acids, proton transfer in the thermodynamically favored direction is diffusion controlled. Normal acids are predominantly oxygen and nitrogen acids carbon acids do not fit this pattern. The thermodynamicEilly favored direction is that in which the conventionally written equilibrium constant is greater than unity this is readily established from the pK of the conjugate acid. Approximate values of rate constants in both directions can thus be estimated by assuming a typical diffusion-limited value in the favored direction (most reasonably by inspection of experimental results for closely related... 

Diffusion-limited rate control at high basicity may set in. This is more eommonly seen in a true Br nsted plot. If the rate-determining step is a proton transfer, and if this is diffusion controlled, then variation in base strength will not affect the rate of reaction. Thus, 3 may be zero at high basicity, whereas at low basicity a dependence on pK may be seen. ° Yang and Jencks ° show an example in the nucleophilic attack of aniline on methyl formate catalyzed by oxygen bases. 

Up to the present the principal interest in heteroaromatic tautomeric systems has been the determination of the position of equilibrium, although methods for studying fast proton-transfer reactions (e.g., fluorescence spectroscopy and proton resonance ) are now becoming available, and more interest is being shown in reactions of this type (see, e.g., references 21 and 22 and the references therein). Thus, the reactions of the imidazolium cation and imidazole with hydroxyl and hydrogen ions, respectively, have recently been demonstrated to be diffusion controlled. ... 

The result of the fast reactions in the ion source is the production of two abundant reagent ions (CH5+ and C2H5+) that are stable in the methane plasma (do not react further with neutral methane). These so-called reagent ions are strong Brpnsted acids and will ionize most compounds by transferring a proton (eq. 7). For exothermic reactions, the proton is transferred from the reagent ion to the neutral sample molecule at the diffusion controlled rate (at every collision, or ca. 10 9 s 1). 

Proton transfers between oxygen and nitrogen acids and bases are usually extremely fast. In the thermodynamically favored direction, they are generally diffusion controlled. In fact, a normal acid is defined as one whose proton-transfer reactions are completely diffusion controlled, except when the conjugate acid of the base to which the proton is transferred has a pA value very close (differs by g2 pA units) to that of the acid. The normal acid-base reaction mechanism consists of three steps ... 

However, not all such proton transfers are diffusion controlled. For example, if an internal hydrogen bond exists in a molecule, reaction with an external acid or base is often much slower. In the following case ... 

Mass transfer for this technique was examined by studying the protonation and aggregation kinetics of 5,10,15,20-tetraphenylporphyrin (H2TTP) at the dodecane-aqu-eous interface [61]. The rate law for the diffusion-controlled protonation of H2TTP at the interface was derived. 

A normal proton transfer was defined by Eigen as one whose rate in the thermodynamically favourable direction was diffusion-controlled (Eigen, 1964). By use of relaxation techniques Eigen was able to show that many proton transfers involving oxygen and nitrogen acids and bases were in this category. If the reactions (5) of an acid (HA) with a series of bases (B-) shows normal proton-transfer behaviour, the rate coefficients in the forward... 

In the two examples of buffer catalysis of proton transfer from an intramolecularly hydrogen-bonded acid which have been discussed, it seems reasonably certain that the mechanism in Scheme 7 applies. The reactions are of the first order with respect to the catalyst B and it therefore follows that proton removal from the non-hydrogen-bonded species is rate-limiting k j > 2[B]- If this step consists of diffusion-controlled proton removal from a low concentration intermediate, the value k2 lx 109dm3 moP s-1 will apply for proton transfer to an amine. In the case of proton removal by hydroxide ion from 4-(3-nitrophenylazo)salicylate ion, the reaction was found to be of the first order in hydroxide ion up to the highest concentrations which could be studied (0.003 mol dm-3) with a rate... 

For this mechanism, values of kr =k 2 = 1.3 x 10lodm3mol-1s-1 are calculated from the experimental value for k in (94), and this means that the proton-transfer steps in (96) are diffusion-controlled in the thermodynamically favourable directions. The hydroxide ion catalysed tautomerisation... 

In another study (40) we found that protonation of pyridine is diffusion-controlled with a one-to-one solute-methanol complex as the reactive species. Thus, while methanol plays essentially no role in the proton transfer to dipicrylamine in the first study, it is indeed intimately involved in the proton transfer to pyridine. 

Whereas in acetonitrile the rate limiting step is an opening of the solvent shell of a reactant, in benzonitrile the back reaction of (5) between the protonated acridine orange cation (BH ) and the 3-methyl-4-nitrophenolate ion (A ) to form the ion pair is diffusion controlled (although the overall reaction to the neutral molecules is an endothermic process). Because of its lower dielectric constant than acetonitrile, the electrostatic interactions between reactants in benzonitrile outweigh specific solvent effects. In other words, in benzonitrile a rate limiting coupling of proton transfer to the reorientation of solvent dipoles does not occur and the measured rates are very fast. The ion recombination (I) + (II) in benzonitrile has a diffusion controlled specific rate (theoretical) k = 9 -1 -1... 

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 radical anion pathway (e-c-P-d-p Scheme 2) requires a rate-determining protonation after cyclization, i.e., a slow protonation of a hard oxyanion. However, such proton transfer rates are usually diffusion controlled, so this seems unlikely [32,33], On the other hand, the carbanion closure (e-P-d-c-p) portrayed in Scheme 4 requires a very reasonable suggestion that the ratedetermining step corresponds to protonation of the soft, weakly basic radical anion 42, prior to cyclization [32-35] this is the preferred mechanism. One must use caution, however, realizing that these conclusions are drawn for the particular set of substrates which were examined. In some cases, radical anion cyclization remains a viable option. 

From the thermodynamics of such dynamical hydrogen bonds , one may actually expect an activation enthalpy of long-range proton diffusion of not more than 0.15 eV, provided that the configuration O—H "0 is linear, for which the proton-transfer barrier vanishes at 0/0 distances of less than 250 pm. However, the mobility of protonic defects in cubic perovskite-type oxides has activation enthalpies on the order of 0.4—0.6 eV. This raises the question as to which interactions control the activation enthalpy of proton transfer. 

The mechanism of reaction between barbiturate and 1,3-dimethylbarbiturate ions with o-nitro-, j -nitro-, and 2,4-dinitrobenzaldehyde has been explored rate dependence on solvent viscosity is indicative of involvement of a diffusion-controlled proton transfer in the rate-determining step at pH 2-4. Unexpected values of Brpnsted a for the acid-catalysed process have been explained. 

General base catalysis of the cyclization of ethyl 2-hydroxymethyl 4 nitrobenzoate [35], where the acyl group is further activated by electron withdrawal by the nitro-substituent, is characterized by a Bronsted coefficient of 0 97, i.e. unity within the limits of error, suggesting that proton transfer is a diffusion-controlled process (Fife... 

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