Nitrophenol, ionization

It has been shown that in some compounds the active species is the non-ionized molecule while the ion is inactive (benzoic acid, phenols, nitrophenols, salicylic acid, acetic acid). Thus, conditions of pH which favour the formation of the ions of these compounds will also reduce their activity. The effect of pH on the ability of acetic acid and phenol to inhibit the growth of a mould is shown in Fig. 11.4. 

Numbers used in this cycle AG° for dissociation of sulfuric acid to sulfur tri-oxide AG° for hydrolysis of bis-p-nitrophenyl sulfate, estimated as described above AG° for hydrolysis of mono-p-nitrophenyl sulfate AG° for esterification to give pNP0S02, estimated as described above AG° for ionization of protonated SO3, estimated as described above AG° for ionization of p-nitrophenol. )... 

In the ionization of the p-nitroanilinium ion, the free base is stabilized by delocalization of electrons involving the canonical structure 13. An analogous structure is not possible for the p-nitroanilinium ion. In the ionization of p-nitrophenol, analogous delocalization is possible in both phenol and phenate species, but is more marked in the ion. Thus, in both the aniline and the phenol system P-NO2 is effectively more electron-attracting than in the ionization of benzoic acid, where the reaction centre is incapable of a —R effect, and indeed shows a small +R effect (14). 

Other evidence for the contribution of electrostatic interactions to retention on SOIL phases may be found in a study by Sun and Stalcup [67]. In this work, it was reported that while the LSFER approach successfully accounted for intermolecular interactions responsible for retention of nonpolar solutes, inclusion of ionizable solutes such as pyridine or nitrophenol isomers seriously degraded the correlation between experimental and predicted retention. Successful global application of an LSFER approach for a training set which includes ionizable analytes required incorporation of an additional descriptor to account for the degree of ionization [68] of the analytes as well as to account for the impact of electrostatic interactions. The additional descriptor incorporated the mobile-phase pH as well as the acid dissociation constant of the analyte. 

Similarly, since p-nitrophenol is ionized above pH 7, its attack on a carbonyl compound cannot be general-base-catalyzed not only is the p-nitrophenolate ion carrying a full negative charge a more powerful nucleophile than p-nitrophenol,... 

Fourage, L., Helbert, M., Nicolet, P., and Colas, B. (1999). Temperature dependence of the ultraviolet-visible spectra of ionized and un-ionized forms of nitrophenol consequence for the determination of enzymatic activities using nitrophenyl derivatives—A warning. Anal. Biochem. 270,184. 

For example, the ratio of the n-octanol/watcr distribution coefficient of the nondissociated species to that of the ionic species is nearly 10,000 for 3-methyl-2-nitrophenol, but only about 1000 for pentachlorophenol because of the greater significance of the hydrophobicity of the ionized form of pentachlorophenol. The logarithm of the -octanol/water distribution coefficient of pentachlorophenol as the phenolate is about 2 (determined at pH 12, and 0.1 M KC1), which indicates significant distribution of the ionized form into the n-octanol phase [8,37], Extraction of such highly hydrophobic ionogenic organic compounds can result from mixed-mode mechanisms that incorporate both the hydrophobic and ionic character of the compound. 

Table 3. 4. The ionization of p-nitrophenol (c = 1.22x1 O 4 M) for different water pool (Wo) parameters of AOT reverse micelles in heptane containing an excess of imidazole...

When an inorganic phosphate buffer or sodium hydroxide is present, without imidazole, only weakly acidic phenol of pKa 11.6 can be detected. Since the phenolic group is adjacent to anionic sulfonates of the AOT, ionization is weakened. / -Nitrophenol in the water pool is influenced primarily by electrostatic interaction between molecular imodazole (Im) and the anionic surfactant group. When excess imidazole (but not methanol or n-butanol) is added to the water pool, a fraction of the interfacial /mitrophenol is displaced into the water pool. 

This compound ionizes to form four ions by cleavage and rearrangement. The loss of a methyl radical leads to the ion at ml z 248 the ion at ml z 154 amu is the p-nitrothio-phenolate ion the ion at mlz 158 is the p-nitrophenolate ion and the ion at mlz 141 is due to loss of the p-nitrophenyl radical. The carrier gas was argon/methane, and the source temperature was 240°C. A PZI79 column was used in the gas chromatograph. This mass spectrum was obtained while the compound was in the source, as indicated in Figure 4. Under these conditions, variations in the field in the source did not alter... 

These reactions are generally fast and formally allow us to use any known fluorogenic, chromogenic, or luminogenic substrate, when the detectable property is based on an ionizable XH group (jfr-nitrophenol, umbelliferone, etc.) (Klein and Reymond, 1998). The antibody catalyzed... 

The modeling data based on nonsteady-state equilibrium predict that volatilization of 4-nitrophenol will be insignificant (Yoshida et al. 1983). The Henry s law constant (H) values for these two compounds (see Table 3-2) and the volatility characteristics associated with various H values (Thomas 1982) can be used to predict that volatilization from water will not be important. The dissociation constant (pKa) values of the two compounds (see Table 3-2) indicate that significant fractions of these nitrophenols will be dissociated at pHs above 6. Since ionic species do not volatilize significantly from water, the ionization may further limit volatilization. 

Methylnitrophenol such as p-nitrophenol is a weak acid similar in its ionization behavior to phenol (Figure 6.1). It has the advantage of being colorless in its conjugate acid form and yellow in its conjugate base form (Figure 6.2). 

Figure 6.1 Ionization of 4-methyl-2-nitrophenol to 4-methyl-2-nitrophenolate.

Another example of nucleophilic hydroxylation of nitro compounds is the formation of nitrophenols by the action on nitrobenzene with aqueous ferrous sulphate in the presence of hydrogen peroxide in a yield of ca. 3% [46a] (Fenton s reagent) [46b] Weiss et al. [46]) and by the action of ionizing radiations on aqueous solution (Weiss and Stein [46c]) ... 

The difference in the pKa values of m- and p-nitrophenol in aqueous medium has aroused interest. In earlier work (Hepler et al., 1965), the enthalpies of ionization were found to be almost identical, and the difference in pATa was attributed to an entropy effect. Since m-nitrophenol had the more negative entropy of ionization, this appeared to be consistent with a greater degree of solvent orientation around its conjugate base as expected since the negative... 

Indicators, if their ionizable groups are present almost exclusively at the surface, and if the colour and optical density of both forms are unaffected by this adsorption, may be used for determining the pH at the surface. Using the triethanolamine salt of cetane sulphonic acid, the pu at the surface of the micelles was found colorimetrically by diphenylazo-o-nitrophenol. p-nitrophenol was employed to find the pu in the solution curiously it was more difficult to find a suitable indicator for the bulk than for the surface pH, since almost all indicators are more or less adsorbed at the surface of these paraffin chain salts. Preliminary experi-... 

Figure 4-38. Elution of a mixture of ionizable components on a XTerra MS CIS (Waters) column with a 60% ACN mobile phase prepared from wpH = 8.0. (A) Phosphate buffer. (B) NH4 /NH3 buffer. Compounds are (1) 2-nitrophenol, (2) 2,4,6-trrmethyIpyridine, (3) 3-bromphenoI, and (4) NN dimethylbenzylamine. (From reference 64, with permission.)...

Figure 1-2. The structural changes associated with the ionization of p-nitrophenol.

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