Diffusion-controlled protonation

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. 

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... 

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. 

The rate constant refers to an essentially diffusion controlled proton transfer it... 

A kinetic study of this process has been reported. The proton exchanges with solvent DaO with a rate constant of 27 s at pH 8.2 and 12 °C using 1-methylxanthine as substrate, and reduction of Mo to Mo v occurs with a rate constant of 1.3 s . A similar process is noted with the cyanide-ion treated desulpho-xanthine oxidase where an exchange rate with solvent DaO of 0.4 s was evaluated. By assuming diffusion-controlled proton transfer, pJCa values of ca. 8 and 10 for the native and desulpho proteins are consistent with experimental data. It is suggested that the protonation process can be represented as... 

Proton transfers between electronegative elements, and especially between oxygen and nitrogen acids and bases, are extremely fast and diffusion controlled (18). In fact, acids with completely diffusion-controlled proton transfer reactions are defined as normal acids. In this case, the proton transfer occurs before the rate-determining step of the reaction. In contrast, when carbon atoms are the basic centers, proton transfers usually take place in a slow, rate-determining step. In such reactions, protonation is considered as the electrophilic attack of the... 

Therefore, it has been postulated that in any reaction the reacting partners form a molecular complex of some kind before they reach the transition state [38, 55]. This must be taken into account when environmental effect on chemical reactivity is interpreted. Many literature data (9, 39, 40, 42] seem to support this postulate. Even diffusion controlled proton transfer is preceded by the formation of a hydrogen-bridged species [50] Let us mention finally that the most fruitful theory of chemical reactivity (intermolecular perturbation) presents charge-transfer between reacting partners as an indispensable and unavoidable step in the overall reaction [51]. 

Diffusion controlled rates are expected for most proton transfers between certain acids and bases in water. As long as the pfC of the conjugate acid of the proton acceptor B" is greater than that of the donor HA by two or more units, the reaction is normally found to be diffusion controlled, and the rate becomes independent of the donor strength. When the pKa of the conjugate acid of the acceptor drops below that of the donor, the forward reaction is endothermic, and hence the reverse reaction becomes diffusion controlled (proton transfer from BH to A ). We examine further the dynamics of proton transfers below. 

The existence of ion pairs of hydroxy aromatic anions with polar groups of cationic micelles was proposed by Zaitsev et al. [66] to explain the effective charge of anions close to zero, observed in acid-base photoreactions of hydroxyaromatics in CTAB solutions. Such a value for the effective charge was found by simulation of the values of the diffusion rate constants of hydrogen ions to excited anions of hydroxyaromatics to make the calculated diffusion-controlled protonation reaction of the excited anions rate constants close to experimentally observed ones. In aqueous solution, the excited anions are protonated with diffusional values of the rate constant with some nonsignificant steric factor [67,68]. The three-phase model can help to interpret the reactivity of polar and charged substances in micellar solutions. 

Fox, J.P., Jencks, W.P. General acid and general base catalysis of the methoxyami-nolysis of 1-acetyl-l,2,4-triazole. J. Am. Chem. Soc. 1974, 96(5), 1436-1449. Bruice, T.C., Donzel, A., Huffman, R.W., Butler, A.R. Aminolysis of phenyl acetates in aqueous solutions. VII. Observations on the influence of salts, amine structure, and base strength. J. Am. Chem. Soc. 1967, 89(9), 2106-2121. Williams, A., Jencks, W.P. Acid and base catalysis of urea synthesis nonlinear Bronsted plots consistent with a diffusion-controlled proton-transfer mechanism and the reactions of imidazole and N-methylimidazole with cyanic acid. J. Chem. Soc. Perkin Trans. 2. 1974, 1760-1768. 

---