Rate of proton exchange

The enol can be observed by NMR spectroscopy and at —20°C has a half-life of several hours. At -1-20°C the half-life is only 10 minutes. The presence of bases causes very r id isomerization to acetaldehyde via the enolate. Solvents have a significant effect on the lifetime of such unstable enols. Solvents such as DMF and DMSO, which are known to slow rates of proton exchange by hydrogen bonding, increase the lifetime of unstable enols. ... 

Comparison of localization energies has frequently been applied to prediction of the relative positional reactivity in polycyclic aromatic hydrocarbons. Simple HMO calculations have only marginal success. CNDO/2 and SCF calculations give results which show good correlation with experimental data on the rate of proton exchange. ... 

Attempts were undertaken to evaluate the rates of proton exchanges 24a 24b 24c and 25a 25b 25c in solutions of 1,2,3-triazole and... 

Physically, dv appears as a broadening of the spectral lines. It has been found that the study of NMR band shapes provides a powerful tool for the measurement of proton-transfer rates through the determination of state lifetimes, related to St. Nuclei such as H and have been studied. This allows the simultaneous determination of different rates of proton exchange. NMR methods are particularly valuable for degenerate equilibria, for which AWJ. and AVj are useless. 

When the proton does not exchange rapidly as on the N-H of the amide group, normal coupling is observed. Since the rates of proton exchange are often critically dependent on the solution conditions, coupling to acidic protons is variable and thus may or may not be observed. 

As we have seen above, a large number of parameters (proton exchange rate, kex = l/rm rotational correlation time,. electronic relaxation times, 1/TI 2(, Gd - proton distance, rG H hydration number, q) influence the inner sphere proton relaxivity. If the proton exchange is very slow (Tlm < rm), it will be the only limiting factor (Eq. (5)). If it is fast (rm Tlm), proton relaxivity will be determined by the relaxation rate of the coordinated protons, Tlm. which also depends on the rate of proton exchange, as well as on rotation and electronic relaxation. The optimal relationship is ... 

Carboxylic Acid Protons Because carboxylic acid protons are bonded to an oxygen next to a carbonyl group, they have considerable positive character. They are strongly deshielded and absorb at chemical shifts greater than 510. Carboxylic acids frequently exist as hydrogen-bonded dimers (shown at left), with moderate rates of proton exchange that broaden the absorption of the acid proton. 

The rate of enolate-carbonyl equilibration " is dependent on the forward and backward rates of proton exchange. Proton exchange from a carbon-based acid is known to be slower than that of a more electronegative atom donor (in particular, O and N atoms) . For a series of closely related molecules usually the more acidic a given molecule is, the faster the rate of proton transfer (high kreu note that thermodynamic and kinetic parameters are not related). For example, benzocyclobutanone (10) is less acidic and the rate of deprotonation is substantially slower (10 times) than the related benzocyclopentanone (12) due to its enolate (11) having unfavourable anti-aromatic character. Deprotonation of the simplest cyclobutanone (13) clearly does not lead to an unfavourable anti-aromatic enolate (14) . By assuming the internal strain of 14 is similar to that of 11, cyclobutanone (13) is evidently 10 " times more acidic than benzocyclopentanone (12). By the same vain, the more acidic propanone (15) has a faster rate of deprotonation (10 times) than the less acidic ethyl acetate (16) . ... 

Rates of proton exchange between benzoic acids and methanol solvent at 25 °C [59]... 

Rate coefficients for exchange of protons between phenols and hydroxylic solvents have been measured by NMR techniques in a similar manner to the corresponding rate coefficients for carboxylic acids. Spin—lattice and spin—spin relaxation times for protons attached to oxygen in acidic aqueous solution of phenol have been measured directly by the spin—echo NMR technique [65], and give a measure of the rate of proton exchange on the phenol. It is found that... 

Rates of proton exchange on phenols in methanol solution... 

The process of acid dissociation can be thought of as comprising two steps, namely ionisation and dissociation of the ion-pair into separate products. The first step may be studied in isolation if a solvent of low dielectric constant is employed, as the second process then occurs to a negligible extent. For example, the rates of proton exchange in acetic acid (dielectric constant, 6.27) are given [73] in Table 15. The measured rate,... 

The rates of proton-exchange involving phosphate ions in aqueous solutions enriched with oxygen-17 have been examined by an nmr line-broadening technique . The process appears to be second order in phosphate species and is attributed to the reaction... 

The spectrum of pure dry ethanol is shown in Fig. 16, in which attention should be focussed on the hydroxyl triplet and the methylene modified-quadruplet. If a little water is added, an extra line due to the HaO-proton appears between the OH and CH2 bands. As the concentration of water increases three things happen—the OH triplet sharpens to a singlet this then broadens and coalesces with the H2O band at the same time the CH2 signal becomes a simple quadruplet. These are all consistent with an increasing rate of proton exchange between the OH group and... 

In this type of rotaxane, the intramolecular hydrogen-bonding interaction between ammonium salt and crown ether is too strong to neutralize the ammonium part with usual bases. In fact, Takata et al. found that the rate of proton-exchange between [2]rotaxane and water was far slower than that between pyrrole and water, suggesting the extremely lowered acidity of the ammonium moiety [74]. However, the ammonium group can be quantitatively acylated by the excess amount of electrophile in the presence of excess triethylamine. Some oligorotaxanes can be prepared by this process (Scheme 12). 

Dryness of solvent. This was confirmed by the addition of small amounts of water to a sample in which slow exchange was observed. This causes an increase in the rate of proton exchange as indicated by the broadening of the... 

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