Mechanism for proton-transfer reactions

Eigen s mechanism for proton transfer reactions between. acids (AH) and bases (B) proceeds through a neutral hydro-... 

Figure 8.3 Three-step mechanism for proton-transfer reactions of oxygen and nitrogen acids. Line I chemical species and processes, charges are omitted. Line 2 R are the reactants I is the precursor intermediate complex 1 is the successor intermediate complex P are the products t means activated complex. Line 3 a is the association to I pt is the proton-transfer d is the dissociation of I to products, b, c are the bonding and solvent reorganisation. Line 4 free-energy changes.

Three-Step mechanism for proton-transfer reactions in aqueous solution... 

ApA < 1. In Fig. 2 the region of curvature is much broader and extends beyond — 4 < ApA < + 4. One explanation for the poor agreement between the predictions in Fig. 3 and the behaviour observed for ionisation of acetic acid is that in the region around ApA = 0, the proton-transfer step in mechanism (8) is kinetically significant. In order to test this hypothesis and attempt to fit (9) and (10) to experimental data, it is necessary to assume values for the rate coefficients for the formation and breakdown of the hydrogen-bonded complexes in mechanism (8) and to propose a suitable relationship between the rate coefficients of the proton-transfer step and the equilibrium constant for the reaction. There are various ways in which the latter can be achieved. Experimental data for proton-transfer reactions are usually fitted quite well by the Bronsted relation (17). In (17), GB is a... 

Quantum-mechanical tunnelling has been recognized as a possible contributor to the rate of a chemical reaction for many years. For instance, the theory of tunnelling for proton transfer reactions was developed by Bell (1959) in his famous book The Proton in Chemistry. Later, Bell (1980a) published a more thorough treatment of tunnelling in his book The Tunnel Effect in Chemistry. 

In view of the above, several workers, notably Zoltewicz, have stressed the likely importance of the internal return mechanism (Scheme 12) for proton-transfer reactions from heterocyclic compounds. 

DFT calculations were performed for the double proton transfer in bicyclic 2,2 -bis(4,5,6,7-tetrahydro-l,3-diazepine) (Figure 8) <2001CPL591>. Both a concerted and a stepwise mechanism for proton transfer are considered. Though the concerted transition state has two imaginary eigenfrequencies, dynamical calculations have demonstrated that it has to be taken into account in the mechanism of the proton transfer even if it is not a true reaction path. 

Once the gas phase Hamiltonian is parametrized as a function of the inner-sphere reaetion coordinate(s), the free energy is calculated as a function of the proton coordinate(s), the scalar solvent coordinates, and the inner-sphere reaction coordinate(s). Note that this approaeh assumes that the optimized geometries of the VB states are not significantly affected by the solvent. For proton transfer reactions, the proton donor-acceptor distance may be treated as an additional solute reaction coordinate that ean be incorporated into the molecular mechanical terms describing the diagonal matrix elements hf- and, in some cases, the off-diagonal matrix elements (/io)y. If the inner-sphere reaction coordinate represents a slow mode, it is treated in the same way as the solvent coordinates. As discussed throughout the literature, however, often the inner-sphere reaction coordinate must be treated quantum mechanically [27, 28]. In this case, the inner-sphere reaction coordinate is treated in the same way as the proton coordinate(s), and the vibrational wave functions depend explicitly on both the proton coordinate(s) and the inner-sphere reaction coordinate(s). 

The mechanism discussed above for the deprotonation of alkylaromatic radical cations, involving a bimolecular reaction between the radical cation and the base (B), leading to a carbon centered neutral radical and the conjugated acid of the base (BH" ") as described in Scheme 28, has been recently questioned by Parker who provided evidence for an alternative mechanism in proton-transfer reactions between methylanthracene radical cations and pyridine bases [154] this involved reversible covalent adduct formation between the radical cation and the base followed by elimination of BH+ (Scheme 36). 

See footnote 56 in J.M. Sayer and W.P. Jencks, Mechanism and Catalysis of 3-Methyl-3-thiosemicarbazone Formation. A Second Change in Rate-Determining Step and Evidence for a Stepwise Mechanism for Proton Transfer in a Simple Carbonyl Addition Reaction, J. Am. Chem. Soc., 1973,95, 5637. 

ACID-BASE REACTIONS A MECHANISM FOR PROTON TRANSFER... 

A very convincing support for the existence of solvent controlled proton dissociation reactions in aqueous solutions has risen from the theoretical studies of Ando and Hynes [105-108] who have studied the proton dissociation of simple mineral acids HCl and HF in aqueous solutions. The two acids seem to follow a solvent-controlled proton transfer mechanism with a Marcus-like dependence of the activation energy on the acid strength. Recently, a free energy relationship for proton transfer reactions in a polar environment in which the proton is treated quantum mechanically was found by Kiefer and Hynes [109, 110]. Despite the quite different conceptual basis of the treatment the findings bear similarity to those resulting from the Marcus equation Eq. (12.19) which has been used to correlate the proton transfer rates of photoacids with their piG [ 101,102 ]... 

Bronsted and Pedersen [20] indicated that the rate constant for proton transfer from acid to a base cannot continue to increase in accord with a linear Bronsted law but must be limited by an encounter rate. This prediction was confirmed by Eigen s school [21] who showed that changed from 1 to zero as the p/f of the donor acid fell below that of the acceptor base (Fig. 5). Eigen [21] considered the following scheme (sometimes called the Eigen mechanism) for proton transfer from HX to Y where reactions in brackets occur in the encounter complex (Eqn. 28). The overall rate constants are given in Eqns. 29 and 30. 

Table 2 collects some data for acid-catalysed reactions involving pre-equilibrium protonations. The concerted mechanism for proton transfer (S -I- products)... 

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