Proton transfer, hydrogen bonds thermodynamics

All these reactions are thermodynamically favourable in the direction of proton transfer to hydroxide ion but the rate coefficients are somewhat below the diffusion-limited values. In broad terms, the typical effect of an intramolecular hydrogen bond on the rate coefficient for proton removal is to reduce the rate coefficient by a factor of up to ca 105 below the diffusion limit. Correspondingly the value of the dissociation constant of the acid is usually decreased by a somewhat smaller factor from that of a non-hydrogen-bonded acid. There are exceptions, however. 

In this region, the equilibrium constant for the proton-transfer step in Scheme 7 has a value K2> 1 and the proton transfer step is strongly favourable thermodynamically in the forward direction. This reaction step is a normal proton transfer between an oxygen acid which does not possess an intramolecular hydrogen bond and a base (B) and will therefore be diffusion-limited with a rate coefficient k2 in the range 1 x 109 to 1 x 1010dm3mol-1 s 1. It follows from (65) that kB will have a value which is... 

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. 

Table 10.2. These results can be interpreted either as the kinetic effects measured for a single-step proton transfer or as the inverse thermodynamic isotope effects in fast preequilibria [5]. Unfortunately, in contrast to the isotope effects measured for classical hydrogen bonds [24], the effects of denterinm on the thermodynamics of dihydrogen bonding are still nnknown.

As we have already noted, classical hydrogen bonds can be thermodynamically very strong but at the same time, easy to transform, due to fast proton transfer along a strong hydrogen bond. Such a dualism can also be seen in the dihydrogen bonding Y-H- H-X,... 

Intermediate 1 could also be stabilised by proton transfer from oxygen to give 3 in Scheme 11.9. The proton acceptor B could be solvent water or a general base catalyst. The reaction will only be catalysed if the rate of breakdown of 1 to regenerate reactants is faster than the rate of proton transfer. In this case, such catalysis would be independent of the base strength of the catalyst B as proton transfer would invariably be thermodynamically favourable and hence occur at the maximum diffusion-controlled rate. If proton transfer to solvent is thermodynamically favourable, such that proton donation to 55.5 M water is faster than to, say, 1 M added base, any observed catalysis by base must represent transition state stabilisation by hydrogen bonding, or a concerted mechanism. 

Proton transfer (PT), i.e., the kinetic aspects of heteroaromatic prototropic tautom-erism, is an important and somewhat neglected topic or, at least, much less studied than the thermodynamic aspects (equilibrium constants, acidity, basicity, pK, etc.). Intermolecular proton transfer between two heterocycles, one protonated and one neutral, occurs along a hydrogen bond (see Sect. 3.5). When the proton transfer occurs in a crystal or in an amorphous solid, we speak of SSPT (vide supra). 

A subsequent study ° from the Arnold group showed an intriguing stereoelectronic effect in oxidative benzylic carbon-hydrogen bond cleavage reactions of substrates 8 and 9 (Scheme 3.7). In this study, electron transfer reactions were conducted in the presence of a nonnucleophilic base. Radical cation formation also weakens benzylic carbon-hydrogen bonds, thereby enhancing their acidity. Deprotonation of benzylic hydrogens yields benzylic radicals that can be reduced by the radical anion of dicyanobenzene to form benzylic anions that will be protonated by solvent. This sequence of oxidation, deprotonation, reduction, and protonation provides a sequence by which epimerization can be effected at the benzylic center. In this study, tram isomer 10 showed no propensity to isomerize to cis isomer 11 (equation 1 in Scheme 3.7), but 11 readily converted to 10 (equation 2 in Scheme 3.7). The reactions were repeated in deuterated solvents to assure that these observations resulted from kinetic rather than thermodynamic factors. Trans isomer 9 showed no incorporation of deuterium (equation 3 in Scheme 3.7) whereas cis isomer 11 showed complete deuterium incorporation. The authors attributed this difference in reactivity to... 

When reaction in the forward direction is thermodynamically favourable (Aptf < 0) the formation of a hydrogen-bonded complex between PhOH and B is rate-determining (step (1)). When reaction in the reverse direction is thermodynamically favourable (Ap/f > 0) step (3) is rate-determining. The proton transfer (step (2)) is not rate-determining except in the region around ApK = 0 and here intermediate values of a and (3 are observed. 

Proper kinetic and thermodynamic meshing of the reactants is necessary for hydride transfer from a donor to an acceptor. There are at least three obviously different mechanisms by which hydride equivalents can be transferred. These are (i) concerted transfer of the proton and two electrons (equation 5) (ii) homolytic cleavage of the carbon-hydrogen bond followed by subsequent transfer of an electron (equation 6) and (iii) initial loss of a proton followed by transfer of two electrons, either together or stepwise (equation 7). Fusion of pathways is imaginable, (tependent on the structures of the participating molecules and possibilities for catalysis. 

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