Proton thermoneutral

For the esterification reaction, there is of course no way of distinguishing between the two proposed mechanisms. The fact that protonated acetic acid reacts with H2lsO to produce CH3C(OH)18OH+ in a thermoneutral exchange with displacement of H20 can be accommodated by either of the mechanisms. Failure to observe the alcoholysis process (76) cannot be rationalized in terms of one mechanism or the other. 

The rate of deprotonation of an acid by a base depends on their structures [41], on the solvent and temperature, and on the difference (ApKa) between the pKa of the acid and that of the base. When acid and base have the same pfCa (ApKa=0) the change of free energy for proton transfer becomes zero and the reaction becomes thermoneutral. Under these conditions the rate of proton transfer is limited only by the so-called intrinsic barrier [34], which is particularly sensitive to structural changes in the reaction partners [39]. When ApKa increases, the rate of proton transfer also increases and approaches a limiting value, which depends on the structures of the acid and base and on the experimental conditions. For normal acids (O-H, N-H) in water the rate of proton transfer becomes diffusion-controlled (ka=10loL mol-1 s"1) when ApKa>2, but in aprotic solvents the limiting proton transfer rate can be substantially lower [42]. 

Scheme 5.1. Approximate relative rates of proton transfer in water at thermoneutrality (ApKa = 0) [35, 39, 51].

It has been suggested that the stabilized CH4T+ ions undergo a thermoneutral proton transfer to CH4, forming tritiated methane, whose yield can therefore be used to evaluate the fraction (ca. 40%) of the excited methanium ions stabilized by collision at the pressure of 760 torr ... 

The reactions proposed for the formation of the labeled products in the triton transfer to ethane, propane, butane and isobutane are illustrated in Fig. 6. The formation of the parent tritiated alkane was ascribed, without invoking the intervention of long-lived protonated ions, to a mechanism involving the protonation reaction (48), followed by the fragmentation process (50b) and by a thermoneutral hydride-ion transfer from the unlabeled alkane to the tritiated alkyl ion ... 

It was proposed that the stabilized protonated cyclopropane can thermoneutrally transfer a proton to c-CsHg, according to reaction (58), which represents the source of the tritiated C3 hydrocarbons, namely propylene and cyclopropane, isolated with a yield of 6-7 and 19-2%, respectively ... 

The stabilized tritiated ions then transfer a proton to CyHg, according to a thermoneutral reaction that yields tritiated toluene ... 

The barrier to thermodynamically unfavorable deprotonation of carbon acids (AGfl, Fig. 1.1) in water is equal to the sum of the thermodynamic barrier to proton transfer (AG°) and the barrier to downhill protonation of the carbanion in the reverse direction (AGr Eq. (1.2)). The observation of significant activation barriers AGr for strongly thermodynamically favorable protonation or resonance stabilized carbanions shows that there is some intrinsic difficulty to proton transfer. The Marcus equation defines this difficulty with greater rigor as the intrinsic barrier A, which is the activation barrier for a related but often hypothetical thermoneutral proton transfer reaction (Fig. 1.2B) [46]. 

There is only a small barrier for thermoneutral proton transfer between electronegative oxygen or nitrogen acids and bases [31]. These reactions proceed by encounter-controlled formation of a hydrogen-bonded complex between the acid and base (ka, Scheme 1.6), proton transfer across this complex (kp, Scheme 1.6), followed by diffusional separation to products (k a, Scheme 1.6) [31]. Much larger Marcus intrinsic barriers are observed for proton transfer to and from carbon [67]. There are at least two causes for this difference in intrinsic barriers for proton transfer between electronegative atoms and proton transfer at carbon. 

Recently, a combination of closed-shell singlet, open-shell broken symmetry singlet and spin triplet DFT calculations was used to probe the reaction coordinate for nitrite reduction catalyzed by mARC. Regarding an oxyl radical transfer mechanism (Figure 2.36), the computed reaction barrier (AH was found to be 14 kcal moF. This reaction is approximately thermoneutral since Mo(v) dioxo species are very unstable toward proton-ation. However, protonation of the equatorial oxo is expected to lead to an overall increase in the exergonic nature of this reaction (Figure 2.36). However, if the 0 oxygen of N02 that is coordinated to the Mo(iv) ion is... 

For the second protonation of the dinitrogen complex a free reaction enthalpy of about -12.5 kcal mol is obtained. From the experimentally determined equilibrium constant of = 1.6 this reaction is found to be thermoneutral (AG = -0.28 kcal mol ). Therefore an error of 12-13 kcal mol between theory and experiment is to be noted. Even larger discrepancies were obtained by Magistrate and Robertazzi and Reiher et al. who... 

The reason for the enhanced reactivity of Oj" probably lies in the free energy of protonation of the peroxide anion (pK of H2O2 = 11-8). On bond strength figures, reaction (40) would be nearly thermoneutral. ... 

It might be argued that the concept of bond orders is more appropriate to reactions involving transfer of hydrogen atoms rather than of protons. It is therefore of interest that a study of 17 reactions between free radicals and thiols does suggest a maximum isotope effect for thermoneutral reactions, though this is less conclusively shown than for proton transfers. 

Note that the addition of a radical to an alkene is often nearly thermoneutral or even strongly exothermic. Therefore, the transition state is early, and has less radical character than the extent of carbenium ion character in the transition state for the endothermic addition of a proton to an alkene. As such, electronic factors are less important in radical addition and steric factors can contribute to regioselectivity. 

With an alkoxide reagent, things are different. Now the product is not a carboxylic acid but another ester. There is no proton that can be removed, and there is a new molecule of alkoxide generated. The reaction is approximately thermoneutral. The reaction is reversible. 

This scheme shows the reactions that can occur in the apparent proton-transfer equilibrium reaction between 2-butyne (PAapp = 778 kJ mol" ) and acetonitrile (PA = 787 kJ mol ). The scheme shows an equilibrium with 1,3-butadiene (PA = 787 kJ mol" ) typical of normal thermoneutral proton-transfer reactions in the gas phase with formation of complexes, 7 and 8, between acetonitrilium ion and 1,3-butadiene, and 1-methylallyl cation and acetonitrile, respectively. Formation of the ion molecule complexes 7 and 8 would be expected to be exothermic by about 60 kJ mol" as a result of charge-induced dipole and charge-dipole interactions. From the... 

---