Nitrophenol, pKa

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... 

Phenol has different chemical properties from those of typical alcohols. Display the electrostatic potential map for phenol. Does this suggest that phenol is likely to be a stronger or weaker acid than any of the compounds discussed above Compare the electrostatic potential map for 4-nitrophenol to that for phenol. What effect does substitution by nitro have on acid strength Explain your result by considering charge delocalization in the conjugate base. Draw all reasonable Lewis structures for phenoxide anion and for 4-nitrophenoxide anion. Which is more delocalized Is this consistent with experimental pKa s ... 

Phenols (ArOH) are relatively acidic, and the presence of a substituent group on the aromatic ring has a large effect. The pKa of unsubstituted phenol, for example, is 9.89, while that of p-nitrophenol is 7.15, Draw resonance structures of the corresponding phenoxide anions and explain the data. 

Among the great variety of organic chromophores such as azo dyes, nitrophenols, phthaleins, sulfophthaleins, aniline-sulfophthaleins, triphenylmethane dyes, polymethines, and others, only a few have so far been considered to be useful and applied to sensor technology (Table l)25 34. Important characterization parameter of pH indicator is its pKa value ... 

A similar treatment can be applied to other dissociating compounds such as the carboxylic acids, nitrophenols. For bases such as amines the pKa is defined as (14 - pKb), and the extent of dissociation is estimated as above. 

Compounds in order of increasing pKa values (in parentheses) Amoxicillin (2.4), Eosin Y (2.7), Lissamine Green ( 3), Erythrosine (3.6), Rose Bengal (3.9), 4-nitrophenol (7.15), 2-nitrophenol (7.22), 3-nitrophenol (8.36), 4-tert-butylphenol (10.16) 4-isoproplphenol (10.19). Erom [47] used with permission... 

The impurities present in aromatic nitro compounds depend on the aromatic portion of the molecule. Thus, benzene, phenols or anilines are probable impurities in nitrobenzene, nitrophenols and nitroanilines, respectively. Purification should be carried out accordingly. Isomeric compounds are likely to remain as impurities after the preliminary purifications to remove basic and acidic contaminants. For example, o-nitrophenol may be found in samples of p-nitrophenol. Usually, the o-nitro compounds are more steam volatile than the p-nitro isomers, and can be separated in this way. Polynitro impurities in mononitro compounds can be readily removed because of their relatively lower solubilities in solvents. With acidic or basic nitro compounds which cannot be separated in the above manner, advantage may be taken of their differences in pKa values. The compounds can thus be purified by preliminary extractions with several sets of aqueous buffers of known pH (see for example Table 19, p. 43) from a solution of the substance in a suitable solvent such as ethyl ether. This method is more satisfactory and less laborious the larger the difference between the pKa value of the impurity and the desired compound. Heterocyclic nitro compounds require similar treatment to the nitroanilines. Neutral nitro compounds can be steam distilled. 

Despite the aforementioned limitation, the use of p-nitrophenyl acyl esters is the most sensitive of the three protocols described, and has an estimated coefficient of variation in the procedure of-5%. It is essential for the user to prepare a standard curve for p-nitrophenol, because the pKa and user-selected pH of the assay... 

Because the ester hydrolysis leads to a change in acidity, as in hydrolytic lipase-or esterase-catalyzed kinetic resolution, an appropriate pH indicator can be used for quantification [14,15]). In an optimized version (Kazlauskas test) [15], a linear correlation between the amount of acid generated and the degree of protonation of the indicator was ensured by using a buffer (e. g., iV,iV-bis(2-hydroxyethyl)-2-(aminoethanesulfonic acid) (= BES), and a pH indicator (e. g., / -nitrophenol) having the same pKa value. The advantage of this system relates to the fact that p-nitrophenol esters are not necessary, i. e., normal substrates such as methyl esters 10 can be used. 

Anti-cancer drugs that are sulfamate esters, ROSO2NH2, appear to act by inhibition of sulfatases. Now, kinetic studies of the aminolysis of p-nitrophenyl sulfamate (109) by secondary alicyclic amines in MeCN at 310 K are reported that model the enzyme reaction. The Brpnsted-type plot was biphasic, the break point at w 18.2 more or less corresponding to the pKa of the ester (109) (17.8). The proposed mechanism (Scheme 30) involves a sequential double deprotonation of (109) leading first, via (110), to the sulfenamine (112) and at higher basic strength to (111) and thence to a novel anionic sulfenamine (113), the products in each case being an V, V-dialkylsulfamide (114) and p-nitrophenol.114... 

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. 

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. 

The pKa of p-nitrophenol is 7.15 it is a stronger acid than phenol by a factor of 700. This is due in part to the electron-withdrawing inductive effect of the nitro... 

Finally, pK determination is also dependent on the ionic strength of the solvent used. In the literature, the pKa of 4-methyl-2-nitrophenol is calculated to be equal to 7.11. Instructors and students can refer to Aptula etal. (2002). 

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... 

With these cyclic peptides it was apparent that no difference of pKa vali s exists between cis and tram isomers and no specific interactions exist between the histidyl and the tyrosyl residues. It was again shown in the hydrolysis of PNPA by these cyclic peptides that the two functional groups do not cooperate for the formation of nitrophenolate ion. On the other hand, for the intramolecular acyl transfer from the acylated imidazole to the phenol group of tyrosine a histidyl side chain in proximity to a tyrosyl side chain facilitated the reaction. This is evident from the fact that the reaction rate was 1,2-cis > 1,2-tram, 1,4-cis > 1,4-trans, and 1,2-cis > 1,4-cis. Thus, a cooperative imidazole function is necessary for the acylation of the tyrosyl phenolic oxygen. Two possibilities illustrated in Fig. 35 a and b are considered... 

The important role of electrostatic interactions is clearly seen in the study of Moreno-Castilla et al. [69] which dealt with the adsorption of substituted phenols (whose pKa ranged from 7.13 for /j-nitrophenol to 10.17 for p-cresol) on a series of well characterized activated carbons, all of which exhibited the expected decrease in uptake at high pH values. Table 27 summarizes the key results. Sample CP-10, with the highest surface area and the most developed porosity, had the highest adsorption capacity for all phenols. However, the pH at which the adsorp-... 

---