Proton exchange equilibria

It has become a fairly common practice to refer to proton exchange equilibria and derived solvation energies to ammonia. The data from Tables 16 and 17 have been treated in this way to derive the relative solvation energies in HSO3 F for ammonium ions and a... 

The amination of 2-alkenylphenols occurred efficiently compared to 2-allylphenols and -naph-thols69. The mechanism involves a proton exchange equilibrium between the phenolic and amino functions and the photoinduced proton transfer (PPT) from the ammonium ion to the alkenyl group, followed by attack of the amine on the intermediate benzylic carbocation. No photoamination of O-methylated and O-acetylated phenols occurred at all. As a single example of diastereoselective amination, the amine 6 was produced from 5 with good yield and diastereoselectivity, although the configuration was not determined. 

Figure 3.43. Proton exchange equilibrium in meso-tetraphenylporphyrin gives rise to the two contributing structures shown.

Tile tautomeric equilibrium of pyrimido[4,5-h][4, 5 -e]thiazine 157 was studied in DMSO-dg by NMR spectroscopy (92CHE1219). Based on and NMR chemical shifts, fast proton exchange was concluded to occur between 157b and 157c. Monoprotonation of 157 has been assumed to form... 

Tautomeric equilibrium in the symmetrical phenoxy-substituted derivative 136 (R = Ph, r = R = OPh) is fast at ambient temperature on the NMR time scale however, at —84°C the proton exchange becomes frozen and both annular tautomers 136a and 136b can be observed (Scheme 40). The similar exchange was also found for P-aryl-substituted 136 (R = Me, Ft, Ph R = R = Ph). In these cases, the equilibrium is very slow, even at ambient temperature, which was attributed to increased steric demands of four phenyl substituents. Unsymmetrically substituted azaphosphorinanes (R R ) provide even more interesting examples. These compounds (R = Ph R = Me, -Pr R = MeO, -PrO) were found to... 

Eq. (14), which was originally postulated by Zimmerman and Brittin (1957), assumes fast exchange between all hydration states (i) and neglects the complexities of cross-relaxation and proton exchange. Equation (15) is consistent with the Ergodic theorem of statistical thermodynamics, which states that at equilibrium, a time-averaged property of an individual water molecule, as it diffuses between different states in a system, is equal to a... 

More recent work revealed the importance of gas phase proton transfer reactions. [91-94] This implies that multiply charged peptide ions do not exist as preformed ions in solution, but are generated by gas phase ion-ion reactions (Chap. 11.4.4). The proton exchange is driven by the difference in proton affinities (PA, Chap. 2.11) of the species encountered, e.g., a protonated solvent molecule of low PA will protonate a peptide ion with some basic sites left. Under equilibrium conditions, the process would continue until the peptide ion is saturated with protons, a state that also marks its maximum number of charges. 

Measurement of the equilibrium distribution of deuterium relative to hydrogen atoms in a proton-exchanging site compared with the distribution in the solvent or some other standard is a particularly subtle probe that can provide important information about the nature of the environment of the... 

The value of the fractionation factor for any site will be determined by the shape of the potential well. If it is assumed that the potential well for the hydrogen-bonded proton in (2) is broader, with a lower force constant, than that for the proton in the monocarboxylic acid (Fig. 8), the value of the fractionation factor will be lower for the hydrogen-bonded proton than for the proton in the monocarboxylic acid. It follows that the equilibrium isotope effect on (2) will be less than unity. As a consequence, the isotope-exchange equilibrium will lie towards the left, and the heavier isotope (deuterium in this case) will fractionate into the monocarboxylic acid, where the bond has the larger force constant. 

The equilibrium constant for the isotope-exchange equilibrium can be expressed (6) in terms of the solvent isotope effects on the acid-dissociation constants and of the monocarboxylic acid and dicarboxylic acid monoanion, respectively. It follows that a lower value for the fractionation factor of the hydrogen-bonded proton means that the solvent isotope effect on the acid-dissociation constant will be lower for the dicarboxylic acid monoanion than for the monocarboxylic acid. 

It is important to emphasize that the presence of two or three basic centres of comparable basicity in a conjugated molecule may lead to protonation processes at all of them. These processes may occur at various rates, but if all the rates are fast, nmr spectra do not enable us to say what are the relative amounts of variously protonated species present at equilibrium. Increasing the acidity of the medium and lowering the temperature in order to observe the resonances of the captured proton may have the effect of shifting the tautomeric equilibria in favour of one or other of the protonation sites. Information on the position of tautomeric equihbria in protonation processes is thus not obt iinable from nmr spectra under the conditions of rapid proton exchange. 

The light absorption act is fast compared with proton exchange rates and therefore the ultraviolet spectrum of a system in protonation equilibrium (1) is the superposition of the spectra of the base and the conjugate acid, each contributing according to its concentration. So the observed molar extinction coefficient is ven by (4), where a is the fraction of the base converted into the... 

Figure 20.11 Air-water exchange of an organic compound HA undergoing a proton exchange reaction. The conjugate base A cannot leave the water, but it contributes to the diffusive transport across the water-phase boundary layer. 1 = fast acid/base equilibrium (Eq. 8-6), 2 = diffusive transport of HA and A across water-phase boundary layer, 3 = Henry s law equilibrium of HA between water and air, 4 = diffusive transport of HA across air-phase boundary layer.

The proton exchange reaction can be assumed to be at equilibrium everywhere in the water. Thus the ratio of the total and neutral compound concentration is given by ... 

In Fig. 20.13 flux enhancement V / is shown as a function of the reaction/diffusion parameter q for different equilibrium constants Kr. Remember that q2 is basically the ratio of reaction time kr and diffusion time k (Eq. 20-52). Thus, q 1 corresponds to case (1) mentioned at the beginning of this section flux enhancement should not occur (V / = 1). The other extreme (vp 1, that is tT /w) was discussed with the example of proton exchange reactions (Eq. 8-6). We found from Eq. 20-49 that for this case the water-side exchange velocity v/w is enhanced by the factor (1 + Ka /[H+]). By comparing Eqs. 8-6 and 12-17 we see that for the case of proton exchange ATa/[H+] plays the role of the equilibrium constant KT between the two species. Thus, flux enhancement is ... 

From the temperature dependence of the equilibrium constant for proton exchange between some deuterated and undeuterated primary and secondary amines, monitored by high-pressure mass spectrometry, the reaction enthalpy, or difference in proton affinity, could be measured.101 Protonation of the deuterated amine is favored by 0.2kcalmol-1, varying with structure by 0.1 kcal mol-1 but with no obvious pattern. However, the equilibrium, at least for CH3CD2NHCH3, appears to be entropy driven, not enthalpy. 

Indirect Exchange Rates. In this case, the line shape is indirectly related to the acid-base equilibrium. Besides measuring intermolecular processes like the proton exchange rates, DNMR often has been used to measure intramolecular processes like conformational changes that occur on the same time scale. When the activation energy of such a process is very different in the acidic and basic forms for an indicator, DNMR can be used to measure the ionization ratio. 

In the area of carbonation, the pH value decreases to roughly 7, the equilibrium value of saturated calcium carbonate solutions. But if the wall is wet, this results in a proton exchange and therefore no sharp pH border is formed. If a large portion of the air pores (size in the order of a tenth of a millimeter) flooded with water poor in carbon dioxide, the carbonation advances more slowly, because compared to the... 

---