Hydroxyl group, ionization

Ionization constants of czs-3-substituted acrylic acids have been correlated with the Hammett equation by Hogeveen (58) and by Charton (60). Charton has correlated ionization constants for a number of other c/s-vinylene sets with the Hammett equation (60). Charton and Charton have correlated some cw-vinylene sets with the extended Hammett equation [eq. (2)] (73). Sufficient data are available for twelve sets of cis-vinylene equilibria, of which four sets represent ionization constants of hydroxy compounds (sets 12-1 to 12-4) and eight sets represent ionization constants of carboxylic acids (sets 12-5 to 12-12). All sets have been correlated with eq. (24) and eq. (2). Sets studied are reported in Table XII. Results of the correlations are reported in Table XIII. Sets designated A were correlated with eq. (24), sets designated B were correlated with eq. (2). In the case of the second ionization constant of 2,3,5,6-tetrahydroxy-l,4-benzoquinone (set 12-3), it is uncertain which hydroxyl group ionizes therefore, the value for X = OH was excluded from the correlation. All of the sets 12-1 to 12-4 gave significant correlations with both eq. (24) and eq. (2),... 

A method for the calculation of the surface hydroxyl group ionization constants, based on the zeta potential vs. pH dependence was proposed by Sprycha [115]. This method is based on the following assumptions for the symmetric 1 1 electrolytes pzc agrees with iep, and for lower than... 

Noh and Schwarz proposed a modified Huang s and Stumm method for the calculation of surface hydroxyl group ionization constants, based on Gouy-Chapman model [118]. In this method the reactions of the surface complex formations are neglected ... 

The determination of the ion reaction constants with hydroxyl groups on the surface of metal oxide, may be done in the similar way, as the determination of the surface hydroxyl group ionization constants by the extrapolation or numerical methods. The example of the first one is a method proposed for the oxide surface by Schindler [16]. According to the previous remarks the surface adsorption of the simple cations may take place on two hydroxyl groups at most. Then it may be described as follows ... 

Figure 3. L-Tyrosine (a) groups shown as un-ionized with pK values (b) zwitterionic form, typical at moderate pH levels (c) single anion form, e.g., at pH 10 (idealized case with no hydroxyl group ionization).

The authors did not assume any condensation of SiOH groups as polymerization progrc.s.ses. However, the equation might be rewritten on the assumption that the particles contain anhydrous SiO cores and a surface of Si(OH)i with a certain portion of the hydroxyl groups ionized. Ba.sed on the maximum surface charge on larger... 

The aromatic nature of lignin contrasts with the aliphatic stmcture of the carbohydrates and permits the selective use of electrophilic substitution reactions, eg, chlorination, sulfonation, or nitration. A portion of the phenoUc hydroxyl units, which are estimated to comprise 30 wt % of softwood lignin, are unsubstituted. In alkaline systems the ionized hydroxyl group is highly susceptible to oxidative reactions. 

Like the un-ionized hydroxyl group, an alkoxy group is a weak nucleophile. Nevertheless, it can operate as a neighboring nucleophile. For example, solvolysis of the isomeric p-bromobenzenesulfonate esters 6 and 7 leads to identical prxKluct nuxtures, suggesting the involvement of a common intermediate. This can be explained by involvement of the cyclic oxonium icai which would result from intramolecular participation. ... 

Breakdown of the amide dihydrate occurs by a mechanism similar to its formation. The ionized aspartate carboxyl (Asp in Figure 16.27) acts as a general base to accept a proton from one of the hydroxyl groups of the amide dihydrate, while the protonated carboxyl of the other asparate (Asp in this case) simultaneously acts as a general acid to donate a proton to the nitrogen atom of one of the departing peptide products. 

Figure 1 shows the pH-rate profiles of some active complexes. Both Ni2 + and Zn2 + ion complexes of 8 afford saturation curves with inflection at around pH s 6 and 8, respectively, which represent, most likely, the ionization of the hydroxyl group complexed with a Ni2+ or a Zn2+ ion. The pKa = 8.6 was assigned for the ionization of the hydroxyl group of the latter complex 12). The lower pH for the ionization of the Ni2+ ion complex in respect to that of the Zn2+ ion complex indicates that the ligand 8 coordinates to Ni2+ ion more tightly than to Zn2+ ion, which is in conformity with a larger K value (1120 M) for the Ni2 + ion than for the Zn2 + ion complex (559 M) at pH 7.05 (Table 2). 

Other complexes of 4c, 10, and 11 show linear plots with slopes close to unity in the a pH range 6.5-8.5. These linear plots also seem to represent the ionization of hydroxyl groups, but their pKa values must be higher than 8.5. Unfortunately, it is difficult to examine higher pH s due to precipitation of the metal hydroxide. 

In aromatic diazonium compounds containing an ionized hydroxyl group ( —O-) in the 2- or 4-position, it is necessary to consider delocalization of electrons and, therefore, two mesomeric structures (1.7a-1.7b) (see Sec. 4.2). This fact has implications for nomenclature compounds of this type are considered as quinone derivatives following IUPAC Rule C-815.3 (Exception) compounds of this class are called quinone diazides. As a specific compound 1.7a-1.7b is indexed in Chemical Abstracts as 4-diazo-2,5-cyclohexadien-l-one. If reference is made specifically to mesomeric structure 1.7b, however, it is called 4-diazoniophenolate. 

A similar effect can be observed in anthaquinones, mainly for the presence of an hydroxyl group. The ionization of hydroxyl groups under basic conditions also undergoes a bathochromic shift. Alizarin has two absorption bands in the vis region, simated at 567 and 609 nm carminic acid has a visible absorption maximum at around 500 nm and kermesic acid at 498 mn. 

The acidic form of the indicator (HIn) retains the hydrogen on each hydroxyl group the conjugate base form of the indicator (In-) contains one ionized hydroxyl group (—O-). The pAa value for the acid ionization is 9.9 thus, Ka = 10-9-9 = 1.3 x 10-I0.n] A discernible color change is noted when the pH of an aqueous solution of the indicator is in the range of 9.4 to 10.6. 

The pH dependence of the reaction of m-tolyl acetate with cyclohexa-amylose implies a pAa of 12.1 for the catalytically active secondary hydroxyl group (Van Etten et al., 1967b). Although this pK at first appears low for the ionization of an aliphatic alcohol, it is consistent with the value of 12.35 determined thermodynamically for the ionization of the secondary hydroxyl groups of the ribose moiety of adenosine (Izatt et al., 1966 Christensen et al., 1966), and with the value of 12.2 reported by Lach for the... 

The conclusions derived from the preceding experiments may be summarized with the aid of the reaction mechanism illustrated in Scheme II. The ester undergoes a rapid, reversible association with the cycloamylose, C—OH. An alkoxide ion derived from a secondary hydroxyl group of the cycloamylose may then react with an included ester molecule to liberate a phenolate ion and produce an acylated cycloamylose. This reaction is characterized by a rate constant, jfc2(lim), the maximal rate constant for the appearance of the phenolate ion from the fully complexed ester in the pH range where the cycloamylose is completely ionized. Limiting rates are seldom achieved, however, because of the high pK of cycloamylose. 

The one exception to this observation is the hydrolysis of bis( p-nitrophenyl) methylphosphonate which, in the presence of cycloheptaamylose, produces only 1.7 moles of phenol. Probably two competitive pathways are available for the hydrolysis of the included substrate (1) nucleophilic attack by an ionized cycloheptaamylose hydroxyl group, and (2) nucleophilic attack by a water molecule or a hydroxide ion from the bulk solution. Whereas the former process produces two moles of phenol and yields a phos-phonylated cycloheptaamylose, the latter process produces only one mole of phenol and a relatively stable p-nitrophenyl methylphosphonate anion. The appearance of less than two moles of phenol may be explained by a combination of these two pathways. Since the amount of p-nitrophenyl methylphosphonate produced in this reaction is considerably larger than expected from an uncatalyzed pathway, attack of water may be catalyzed by the cycloheptaamylose alkoxide ions, acting as general bases (Brass and Bender, 1972). 

In an attempt to prepare a catalytically active cycloamylose derivative which would retain the binding properties of an unmodified cycloamylose,7 Gruhn and Bender (1971) attached a relatively small hydroxamate function to a secondary hydroxyl group of cyclohexaamylose. The initial and most important step in the synthetic sequence is the reaction of ionized cyclo-... 

The hydroxyl groups of the cellulose appear to be somewhat acidic. While studies of the composition of alkali cellulose and adsorption of sodium hydroxide have not clearly proved the presence of any sodium compound in alkali cellulose, the reactions of alkali cellulose with carbon disulfide and with etherifying agents would seem to justify the assumption that such an intermediate exists or that the hydroxyl hydrogen at least ionizes. This view is strengthened by the fact that the rate of etherification is proportional to a high power of the concentration of alkali.19... 

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