Phenol ionization

It is possible to measure equilibrium constants and heats of reaction in the gas phase by using mass spectrometers of special configuration. With proton-transfer reactions, for example, the equilibrium constant can be determined by measuring the ratio of two reactant species competing for protons. Table 4.13 compares of phenol ionizations. 

Table 4.13. Comparison of Substituent Contributions to Phenol Ionization in the Gas Phase and Solution" ...

Monobromination is achieved with nonpolar solvents such as CS to decrease the electrophilici-ty of Brj and also to minimize phenol ionization. 

By using the fraction of phenol ionized (a) with pH and pKa, together with rate constants for phenol and phenolate oxidations, it becomes possible to estimate the observed rate constant (kobs) for oxidation of phenols as a function of pH ... 

Methods for evaluating the microconstants are discussed in the paper by Martin et al. (1958), and by Edsall et al. (1958). Martin et al. (1958) found pfci2 = 9.63 0.02 (phenolic ionization constant for tyrosine carrying ammonium and carboxylate ions) and pkm = 10.04 0.03 (phenolic ionization constant for tyrosine carrying uncharged amino group and carboxylate ion), at 25°C and ionic strength 0.16. 

Gas-phase properties of a molecule have, by definition, an intrinsic character and they could be modified by the environment. Although the formation and reactions of gaseous ionized phenol 21 (cf. Chart 5) and its cyclohexa-2,4-dienone isomer 22 have been studied in numerous ionization and mass spectrometric studies , thermochemical parameters of these isomers as well as information on other non-conventional isomers, such as the distonic ion 23, were rather scarce. Conventional cations of analogous aromatic systems (X—CeH5) + and their distonic isomers generated by simple 1,2-hydrogen shifts within the ring were demonstrated to be observable gas-phase species . In addition, the mechanism of the CO-loss upon phenol ionization has only recently been unraveled . ... 

Transport of solutes through the LM occurs by either passive transport or by carrier-facilitated transport. Phenol, for example, is soluble in both phases, and treatment of an aqueous phenol solution with an emulsion results in a lowering of the external concentration of phenol as it passively diffuses through the hydrocarbon (HC) layer and into the internal aqueous phase. Equilibrium is reached when the concentrations of phenol in both aqueous solutions are equal (assuming no other conditions are present which would alter the distribution between the aqueous and HC phases). One way to alter this equilibrium is to trap phenol inside with a sodium hydroxide solution. Phenol ionizes at high pH, and the phenolate ion cannot permeate a HC layer trace amounts of phenol have been completely removed from wastewaters by this system (10, 11). This exclusion of charged molecules by the aliphatic hydrocarbon LM layer is desirable in some applications, but to employ LM enzyme reactors and/or separation systems with amino acids, it is necessary to incorporate carriers into the HC phase. 

Figure 3. Dependence on basicity of the rate constants for the reactions of phenoxide, alkoxide, and hydroxide ions with p-nitrophenyl thioacetate. The solid line for phenoxide ions (circles) has a least-squares slope of Pnuc = 0.68, and a dotted line of slope 0.17 has been drawn through the points for ethoxide and trifluoroethoxide anions (triangles). The dashed line shows the expected rate constants if the full solvation energy change for phenolate ionization is added to AG (see the text). (Reproduced from reference 25.

In phenol ionization, pKa values (water, 25 °C) for some p-X, o-X pairs are as follows CN, 7.95,6.90 F, 9.91,8.71 Cl 9.42,8.53 Br, 9.36,8.44 NO2,7.15,7.23. (Various different values may be found in the literature for some of these substituted phenols. As far as possible the individual values quoted for the members of each pair have been determined by the same authors.) Here the pattern for CN is similar to those for F, Cl and Br, but different from that shown by NO2, for which the isomeric phenols differ little in acidity. The increase in acidity when an electron-attracting group is moved from para- to nr /zn-position in phenol is doubtless due to an increased transmission of the inductive effect to the reaction centre, which more than outweighs any unfavourable effect of change in orientation. The anomalously low acidity of n-nitrophenol is usually attributed to stabilization of the undissociated form by hydrogen-bonding between OH and ONO. 

For correlations in which cr values are considered relevant, the ordinary Gm and (revalues of CN should be used (Section III. C). Similarly, for correlations of electron-demanding processes in which the use of g values is appropriate for-i substituents, the ordinary Gm and Gp values of CN should be used (Sections VI. C and VI. D). When the use of Gp values is appropriate for +R substituents, the value 0.88 for CN may be used in association with other values based on phenol ionization, while a value of 1.00 may be used in association with other values based on anilinium ionization (Section III. D). 

Benzoic acids ionization (aq), equilibrium Phenols ionization (aq), equilibrium Benzoylation of aromatic amines in benezene, rate... 

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