Proton jumping

The mechanism of enolization involves two separate proton transfer steps rather than a one step process m which a proton jumps from carbon to oxygen It is relatively slow m neutral media The rate of enolization is catalyzed by acids as shown by the mechanism m Figure 18 1 In aqueous acid a hydronium ion transfers a proton to the carbonyl oxygen m step 1 and a water molecule acts as a Brpnsted base to remove a proton from the a car bon atom m step 2 The second step is slower than the first The first step involves proton transfer between oxygens and the second is a proton transfer from carbon to oxygen... 

FIGURE 2.11 Proton jumping via the hydrogen-bonded network of water molecules. 

That is, the H-bonded network provides a natural route for rapid transport. This phenomenon of proton jumping thus occurs with little actual movement of the water molecules themselves. Ice has an electrical conductivity close to that of water because such proton jumps also readily occur even when the water molecules are fixed in a crystal lattice. Such conduction of protons via H-bonded networks has been offered as an explanation for a number of rapid proton transfers of biological significance. 

As mentioned, the frequencies of proton jumps leading to establishment of the equilibrium 14a 14b in solutions of imidazoles and benzimidazoles... 

The Contact between Solvent and Solute Particles Molecules and Molecular Ions in Solution. Incomplete Dissociation into Free Ions. Proton Transfers in Solution. Stokes s Law. The Variation of Electrical Conductivity with Temperature. Correlation between Mobility and Its Temperature Coefficient. Electrical Conductivity in Non-aqueous Solvents. Electrical Conduction by Proton Jumps. Mobility of Ions in D20. 

Electrical Conduction by Proton Jumps. As mentioned in Sec. 24, a hydroxyl ion may be regarded as a doubly charged oxygen ion 0 , containing a proton inside the electronic cloud of the ion, which has the same number of electrons as a fluoride ion. The radius of the hydroxyl ion cannot be very different from that of the fluoride ion. But it will be seen from Table 2 that the mobility of the hydroxyl ion is about four times as great. This arises from the fact that a large part of the mobility is undoubtedly due to proton transfers.1 Consider a water molecule in contact with a hydroxyl ion. If a proton jumps from the molecule to the ion,... 

As will be seen from Table 2, the mobility of the hydrogen ion is even greater than that of (OH)-. This high mobility is ascribed to successive proton jumps of the kind... 

Conductivity due to similar proton jumps is present in other solvents. Consider a bisulfate ion dissolved in sulfuric acid. The relation between... 

When a strong acid is dissolved in ethanol the (C2H50H2)+ ion is likewise formed and it will be seen from Table 5 that the mobility of the hydrogen ion again indicates some contribution from proton jumps, though the effect is smaller than in methanol. 

HC1 in ethyl alcohol causes a marked drop in the electrical conductivity, which is ascribed to the partial suppression of proton jumps resulting from the capture of protons from the (C2H5OH2)+ ions, thus ... 

Since a small trace of water suffices to produce a large effect, the equilibrium of (43) evidently lies far in favor of the right-hand side (see also Sec. 115), indicating that a water molecule dissolved in ethyl alcohol provides a vacant level for an additional proton that lies lower than the level occupied by the protons in the OH2 group of the (C2H3OH2)+ ion. A proton captured in this lower level of (HaO)+ will have to wait until it receives the necessary energy before it can move back to an alcohol molecule. In the meantime the (H30)+ ion can merely contribute to the electrical conductivity by drifting slowly in the field only when the proton has returned to an alcohol molecule can the rapid proton jumps be resumed. 

Turning now to the experimental data, we find that the mobility of the (NH4)+ ion in liquid ammonia does not have any high value that would indicate a contribution from proton jumps.1 Nor docs the (CH,0) ion in methanol or the (C2H5O)- ion in ethanol solution.2 These experimental data do not force us to accept the Hiickel mechanism but if we do not accept the mechanism, we shall have to make some ad hoc assumptions to explain these experimental results. 

Equation (77) shows that if ph lp(R )/4 1 at an optimum distance R between the reactants, proton transfer occurs by means of tunneling between the unexcited states. However, the distance of the proton jump, 2r0(R ), is not equal to the distance between the points of minima of the potential wells of the proton in the equilibrium nuclear configuration. This case is a generalization of the results obtained in an earlier model by Dogonadze, Kuznetsov, and Levich36 (DKL model). 

Jnmps of a proton along the hydrogen bond represent another type of dynamics observed in hydrogen-bonded complexes. Mechanistically, this process is simplest for intramolecular hydrogen bonds. The fast enol-enolic equilibrium shown in Scheme 2.2 illustrates an intramolecular proton-jumping system [27]. Here, substituent X dictates the equilibrium constant as well as the rate of proton transfer. It should be noted that such proton jumps can be stopped on the H NMR time scale only at very low temperatures. 

Scheme 2.2 Schematic representation of a proton-jumping molecular system with fast enol-enolic equilibrium between structures la and lb.

Of special importance are tautomeric equilibria of two forms in which proton jumps lead to a change of the type of conjugation. Katritzky (72KGS1011 91H329) has developed a useful approach to estimating the empirical resonance energies from the constants of tautomeric equilibria which, in their turn, are determined from the pKa values of suitable compounds properly modeling individual tautomers. 

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