Dissociation constant phenolic compounds

In Table 15 are recorded the dissociation constants of certain phenolic compounds. From these data it becomes obvious that the introduction of aldehyde groups, or other substituents, changes the dissociation constant of phenolic hydroxyls by over one-hundred fold. Moreover, oxidation studies carried out in this laboratory have shown that the native lignins from bagasse, white Scots pine and birch contain... 

This preparative scheme leads to only 30% yield due to the side reactions between the meto-astatoaniline diazonium salt and astato-phenol, which cannot be eliminated even by continuous extraction of the product with n-heptane (167). All the astatophenols synthesized to date have been identified by either HPLC (99,104) or TLC (160,166,167). Their dissociation constants (KJ have been established from extraction experiments by measuring the relative distribution of compounds between aqueous borax buffer solutions and n-heptane as a function of acidity. On the basis of these derived values, the Hammett a-constants and hence the field (F) and resonance (R) effects have been estimated for these compounds (167) (see Table VI). The field effect for astatine was found to be considerably weaker than that for other halogens the resonance effect was similar to that for iodine (162). 

The pseudo-first order rate constants (resp. coefficients) for the direct reaction of some compounds may almost be in the order of typical hydroxyl rate constants (kR > 10 M s ), due to high concentrations of the pollutants as well as mass transfer enhancement. For example, Sotelo et al. (1991) measured values of 6.35 106 and 2.88 106 M l s"1 for the dissociating hydroxylated phenols, resorchinol (1,3-dihydroxybenzene) and phlorogluci-nol (1,3,5-tn hydroxybenzene) respectively (pH = 8.5 and T= 20 °C). 

In addition to influencing the rate of a reaction, pH may also control the products where alternate or sequential pH-dependent reactions take place. An example of this type of reaction is the chlorination of phenol. Lee and Morris (37) have shown that the chlorination of phenol proceeds by the stepwise substitution at the 2, 4, and 6 positions of the aromatic ring. The rate of each of these reactions depends on the product of phenate or chlorophenate anion and the hypochlorous acid concentrations. Since each phenolic compound has a slightly different acid dissociation constant, the species of chlorophenols that are formed depend on the pH of the solution. 

Since phenol is benzene with a hydroxyl group, the reactivity of phenol and phenolic compounds is in many ways dictated by the chemical properties of the benzene ring. The first property to consider is acidity. A compound is considered an acid when it can release a proton (H ) while in solution. The acid constant Ka of a compound defines to what extent the proton is released. Strong acids will completely dissociate, whereas weak acids (HA) are at equilibrium with their dissociated state ... 

Tratnyek and Hoigne (1994) investigated 25 substituted phenoxide anions for QSARs that can be used to predict rate constants for the reaction of additional phenolic compounds oxidized by chlorine dioxide (OCIO). Correlating oxidation rates of phenols in aqueous solution is complicated by the dissociation of the phenolic hydroxyl group. The undissociated phenol and the phenoxide anion react as independent species and exhibit very different properties. The correlation analysis should be performed on the two sets of rate constants separately. 

Some representative examples are given of plant sources of the cited compounds (further plant sources can be readily accessed via the Web). ICjo (concentration for 50% inhibition) values are given in round brackets. Ka (dissociation constant) or K (enzyme-inhibitor dissociation constant) values are given in square brackets. For convenience compounds are grouped into alkaloids (also encompassing N-containing aromatic pseudoalkaloids), phenolics, terpenes and other compounds and are listed alphabetically within these four groupings. 

In LC-ESI-MS, the role of the mobile phase pH is complicated. In practice, often a compromise must be strack between analyte retention and ionization. From the perspective of generating preformed ions in solution, the optimum conditions for the ESI analysis of basic compounds, e.g., amines, would be an acidic mobile phase with a pH at 2 units below the dissociation constant pIQ of the analytes, while for acidic compounds, e.g., carboxylic acid or aromatic phenols, a basic mobile phase with a pH two units above the pK, of the analytes is preferred [97]. These conditions are uirfavourable for an analyte retention in RPLC. The analytes elute virtually umetained. In RPLC, it is important to reduce protolysis of basic and acidic analytes, i.e., to assure that the compounds are... 

The antioxidant efficiency of phenolic acids, as determined by the accelerated autooxidation of methyl linoleate and scavenging of the free radical 2,2-diphenyl-1-picrylhydrazyl (141) ° methods, was found to be inversely proportional to the maximal detector response potential in the voltammetric determination of these compounds. No similar correlation was found for the flavonoids . A good correlation was found between the O—H bond dissociation energy of a phenolic compound and its effectiveness as antioxidant, expressed as the rate constant of free radical scavenging . The bond dissociation energy of the phenol O—H bond was estimated by a three-dimensional quantitative structme-activity relationship method incorporating electron densities computed using the Austin Method 1 (AMI) followed by correlation of the... 

The simpler equation (4) embodies the Ostwald conception with the difference that Km is not the true dissociation constant but the apparerd constant of the indicator since it represents the product of the true dissociation constant and the equilibrium constant for the normal and aci-forms. The latter equilibrium favors the normal compound in the case of p-nitrophenol so that this substance appears to be a very weak acid. With o-nitro-phenol, however, the existence of the aci-form is favored so that this compound behaves as a stronger acid. The ratio of aci to normal is so large in the case of picric acid that relatively much of the aci- or ionogen form, as compared with the pseudo-compound, is present in aqueous solution. Consequently this substance is a rather strong acid. As the apparent dissociation constant increases, the intensity of the yellow color of aqueous solutions must likewise grow because more of the aci-form will be found in solution. This statement can be confirmed easily. Picric acid in water solutions is yellow, but colorless in organic solvents due to the predominance of the pseudo-form. 

The pK a and therefore the physicochemical properties of a series of aminosulfonate-based compounds of phenol is correlated with the irreversible inhibition of the enzyme estrone sulfatase (ES). A strong correlation exists between the observed pK a and inhibitory activity. The stability of the phenoxide ion, as indicated by the acid dissociation constant, is an important factor in the irreversible inhibition of this enzyme. 

Because of the formation of emulsions at phase boundaries for APE surfactants, EEE is limited to the degradation products APs, alkylphenol monoethoxylate to triethoxylate (APE(l-3)) and alkylphenol ethoxy carboxylate (APEC). Dichloromethane and hexane are the solvents commonly used in the extraction of APs and APE(l-3) from liquid samples.For phenolic compounds including BPA, OP, and NP, water samples are often acidified to pH < 4 with hydrochloric acid. Acidification of water samples suppresses the dissociation of phenols and prevents the ionization of the analytes, which increased the efficiency of the extraction. Del Olmo et al. " studied the effect of pH on extraction of BPA using sodium hydroxide and hydrochloric acid for adjustment. The result obtained showed that the extraction efficiency remains constant for pH values lower than 6.5, decreasing sharply for higher values. This behavior agrees with the weak acid nature of BPA. 

For deductions to be made as to the state of the aromatic amino acid residues in proteins, the conditions under which the amino acids and the proteins are examined must be similar. Thin-film data on crystalline material are scarcely comparable with those obtained from the study of the same compounds when combined in macromolecules in solution, although they may have intrinsic interest of their own. Similarly, caution must be observed in comparing data obtained at low temperatures with those at room temperature where there is a possibility that ionic dissociation is involved. The meaning of pH at low temperatures in solid media and the values of the relevent dissociation constants are scarcely possible of definition. Keilin and Hartree (1949) have demonstrated the decolorization of certain dyes and indicators on cooling, which they ascribe to decrease of ionization. For instance, a red phenol-phthalein solution at pH 10.0 becomes colorless on cooling to — 15°C. 

Table 4 Atomic partial charge (ape) values calculated by the AMI program, and reference and predicted dissociation constants pK values) of some phenolic compounds.

The predictive ability of the MLC QRAR model is compared in Fig. 9.12 with a three-variable QSAR model. The predicted value of log 1/C was calculated from the fitted log 1/C vs. k and log 1/C vs. (log Pow, acid dissociation constant pK and resonance parameter R) plots, by using the leave-one-out technique (the compound predicted was left out in the derivation of the model). As observed, the QSAR model had difficulties in predicting the toxicity of highly lipophilic phenols, as indicated by the curvature in this region. The results show that a single MLC retention... 

The fluorescence of certain compounds as a function of pH has been used for the detection of end points in acid-base titrations. For example, fluorescence of the phenolic form of l-naphthol-4-.sulfonic acid is not detectable by the eye because it occurs in the ultraviolet region. When the compound is converted to the pheno-late ion by the addition of base, however, the emission band shifts to visible wavelengths, where it can readily be seen. It is significant that this change occurs at a different pH than would be predicted from the acid dissociation constant for the compound. The explanation of this discrepancy is that the acid dissociation constant for the excited molecule differs from that for the same species in its ground stale. Changes in acid or base dis-... 

An example in which rate constants are related to equilibrium constants involves the base-catalyzed hydrolysis ofN-phenyl carbamates (Fig. 13.16). As discussed above, these compounds hydrolyze with the dissociation of the alcohol moiety being the rate-determining step. Hence, by using the pA a s of the leaving groups (phenols and aliphatic alcohols), we find a nice correlation to the rates of these reactions. 

The oxidation of substituted phenols illustrates the importance of including speciation. Dissociation of the phenolic hydroxyl group results in an equilibrium mixture of the parent compound and its dissociated form, the phenoxide (or phenolate) anion. The undissociated phenol and the phenoxide anion react as independent species with very different rate constants, designated kArOU and kAr0. For the oxidation of 4-nitrophenol (pKa = 7.2) by C102, 1(1,0,11 = 1.4 x 10 1 M 1 s, andkArCr = 4.0 x 103 M 1 s 1 (42). Estimates of the pH-corrected second-order rate constant, kM, can be made using... 

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