Proton-transfer process, reaction

Notice that specific acid catalysis describes a situation in which the reactant is in equilibrium with regard to proton transfer, and proton transfer is not rate-determining. On the other hand, each case that leads to general acid catalysis involves proton transfer in the rate-determining step. Because of these differences, the study of rates as a function of pH and buffer concentrations can permit conclusions about the nature of proton-transfer processes and their relationship to the rate-determining step in a reaction. 

The details of proton-transfer processes can also be probed by examination of solvent isotope effects, for example, by comparing the rates of a reaction in H2O versus D2O. The solvent isotope effect can be either normal or inverse, depending on the nature of the proton-transfer process in the reaction mechanism. D3O+ is a stronger acid than H3O+. As a result, reactants in D2O solution are somewhat more extensively protonated than in H2O at identical acid concentration. A reaction that involves a rapid equilibrium protonation will proceed faster in D2O than in H2O because of the higher concentration of the protonated reactant. On the other hand, if proton transfer is part of the rate-determining step, the reaction will be faster in H2O than in D2O because of the normal primary kinetic isotope effect of the type considered in Section 4.5. 

The class of proton transfer (PT) reactions plays a major role in many biological processes, including various enzymatic reactions. This class of reactions will be served here as a general example and an introduction for more complicated reactions. As a specific demonstration let s consider a proton transfer between Cys 25 and His 159 in papain. This reaction can be formally described as... 

To draw molecular pictures illustrating a proton transfer process, we must visualize the chemical reactions that occur, see what products result, then draw the resulting solution. When a strong base is added to a weak acid, hydroxide ions remove protons from the molecules of weak acid. When more than one acidic species is present, the stronger acid loses protons preferentially. 

The second group of intermolecular reactions (2) includes [1, 2, 9, 10, 13, 14] electron transfer, exciplex and excimer formations, and proton transfer processes (Table 1). Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. PET is involved in many photochemical reactions and plays... 

The fundamental approach to a proton transfer process, which is crucial to mimic many chemical and biological reactions, has relied deeply on studies of excited-state intramolecular proton transfer (ESIPT) reactions in the condensed phase. 

Concerning the mechanism of O/H insertion, direct carbenoid insertion, oxonium ylide and proton transfer processes have been discussed 7). A recent contribution to this issue is furnished by the Cu(acac)2- or Rh2(OAc)4-catalyzed reaction of benz-hydryl 6-diazopenicillanate 237) with various alcohols, from which 6a-alkoxypenicil-lanates 339 and tetrahydro-l,4-thiazepines 340 resulted324. Formation of 340 is rationalized best by assuming an oxonium ylide intermediate 338 which then rearranges as shown in the formula scheme. Such an assumption is justified by the observation of thiazepine derivatives in reactions which involved deprotonation at C-6 of 6p-aminopenicillanates 325,326). It is possible that the oxonium ylide is the common intermediate for both 339 and 340. 

The assumption that 339 arises from the oxonium ylide by a proton transfer process is supported by the reversed product ratio obtained in the reaction with ethanol in the presence of diazabicyclo[4.3.0]non-5-ene (DBN)-... 

Large numbers of reactions of interest to chemists only take place in strongly acidic or strongly basic media. Many, if not most, of these reactions involve proton transfer processes, and for a complete description of the reaction the acidities or basicities of the proton transfer sites have to be determined or estimated. These quantities are also of interest in their own right, for the information available from the numbers via linear free energy relationships (LFERs), and for other reasons. 

We have already described some proton-transfer processes that occur without disruption of the crystal structure. We now treat two other homogeneous reactions. The first of these is the (2 + 2) photocyclodimerization of a number of benzylidene ketones (185,186). These monomers are based on one of the three frameworks 124 to 126. The three parent molecules and a number of their... 

Both kinetic and thermodynamic data on organometallic hydrides should be very useful. The relative rates of proton transfer processes and other reactions determine a good deal of organometallic chemistry. For example, in our synthesis of cis-0s(C0) (CH )H> reactions 2-4, the comparative rates of... 

We can, however, form alkoxide ions that are monosolvated by a single alcohol group, via the Riveros reaction [Equation (7)]. When the monosolvated methoxide is reacted with acrylonitrile, the addition process reaction (8a), is the major pathway, because there is a molecule of solvent available to carry off the excess energy. The proton transfer pathway, reaction (8b), becomes endothermic, because the methoxide-methanol hydrogen bond, at about 29 kcal/mol, must be broken in order to yield the products. Thus, one can observe either the unique gas phase mechanism in the gas phase, reaction (6b), or the solution phase mechanism in the gas phase, reaction (8a), and the only difference is in the presence of the first molecule of solvent. 

These proton transfer processes increase the driving force of the electron transfer reactions, which can thus be considered in terms of a proton-coupled electron transfer process [57-60]. 

In summary, in this section we have discussed the electronic and steric effects of structural moieties on the pKz value of acid and base functions in organic molecules. We have seen how LFERs can be used to quantitatively describe these electronic effects. At this point, it is important to realize that we have used such LFERs to evaluate the relative stability and, hence, the relative energy status of organic species in aqueous solution (e.g., anionic vs. neutral species). It should come as no surprise then that we will find similar relationships when dealing with chemical reactions other than proton transfer processes in Chapter 13. 

On the other hand, among electrode reactions involving bond-breaking steps, theoretical approaches to the hydrogen electrode reaction have been extensively treated in the electrochemical literature [31, 41, 58— 62]. These reactions can be regarded as heterogeneous proton transfer processes. 

When the alkylation was performed with ethyl allyl carbonate as the precursor of the it-allyl intermediate, only 32% ee was obtained, indicative of a subtle proton-transfer process involved in the catalytic process such as in Scheme 8E.39. The chiral rhodium catalyst was shown to be the primary source of the asymmetric induction because the same reaction in the absence of the rhodium catalyst generated a racemic product in 91% yield. It is interesting that the use of only half an equivalent of the chiral ligand together with half an equivalent of achiral ligand (dppb) with respect to [Pd + Rh] was sufficient to give a high enantioselectivity (93% ee). 

Hydrated electrons react with many Bronsted acids. This reaction is not a proton transfer process but an incorporation of the electron into the acid to form an AH - ion radical, which may subsequently undergo decomposition. This decomposition may occasionally yield a hydrogen atom, but in many cases other pathways of dissociation have been observed. 

These exothermic proton-transfer reactions occur on every collision with well known rate constants, having typical values 1.5 x 10"9cm3s 1 < k < 3 x 10 9 cm s 1. An additional advantage of using primary H30+ ions is that many of their proton-transfer processes are nondissociative, so that only one product ion species occurs for each neutral reactant In order to allow for an accurate quantification of the neutral reactants from measured primary and product ion signals, the reactions of H30+ with the neutrals must occur under well-defined conditions. This is assured in the PTR-MS system by allowing the H30+ reactions to proceed within a DT (Hansel et al., 1998). 

The 1977 review of Martynov et al. [12] discusses existing mechanisms of ESPT, excited-state intramolecular proton transfer (ESIPT) and excited-state double-proton transfer (ESDPT). Various models that have been proposed to account for the kinetics of proton-transfer reactions in general. They include that of association-proton-transfer-dissociation model of Eigen [13], Marcus adaptation of electron-transfer theory [14], and the intersecting state model by Varandas and Formosinho [15,16], Gutman and Nachliel s [17] review in 1990 offers a framework of general conclusions about the mechanism and dynamics of proton-transfer processes. 

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