Ionization of the hydroxyl group

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). 

The value of kHO- for the breakdown of 2-hydroxy-1,3-dioxolan is more than 10 times greater than for the breakdown of dimethyl and diethyl hemi-orthoformate (Table 12). This contrasts with what is found with the hydronium-ion catalysed breakdown when acyclic hemiorthoesters react faster than cyclic ones, and may be due either to a greater ease of ionization of the hydroxyl group when attached to the dioxolan ring or to the leaving group in the breakdown of the 2-hydroxy-1,3-dioxolan being p-hydroxy-alkoxide rather than an alkoxide as in the breakdown of dimethyl and diethyl hemiorthoformate. 

Phenylthio-3-trimethylsilylalkanols lose the element of trimethylsilanol on treatment with an acid [208]. Ionization of the hydroxyl group is participated by the sulfur atom and the episulfonium intermediates then undergo desilylation. The Si-C-C-S segment is conjoint and its fragmentation proceeds upon activation by episulfonium ion formation. 

Some of the relevant data are collected in Table XXXII. From these data we note that the apparent nucleophilicity of water is lower than that of OH" by a factor of 55.5 k2/kx = 5.3 x 107. The equilibrium constant for the ionization of the hydroxyl group of adduct 161 is K a. 

Monohydric aliphatic alcohols having no strong electron-withdrawing substituent to facilitate ionization of the hydroxyl group are extremely weak acids in either aqueous or alcoholic media. The dissociation constant in aqueous solution at 25°C for methanol (4) is 2.9 X 10"16, for ethanol (4) and possibly other primary and secondary alkyl alcohols, <—1 X 10"16. In alcoholic media ionization constants may be 1/10-1/100 as large as those in aqueous media (22). 

Comparison of the surface density (pmol/m ) of the functional groups on active carbons oxidized by nitric acid in relation to carbon oxidized in air (see Table 4). shows a significantly higher (about 10 times) concentration of quinoid (carbonyl) structures. The gentler Nernst slope for CWN2—Ox is indicative of reduction with partial ionization of the hydroxyl group, according to the reaction... 

Dehydration of (pentadienol)- or (pentadienylether)Fe(GO)3 complexes with a strong acid is the most common method used for the preparation of acyclic (ry -pentadienyljiron cations. It has been demonstrated that ionization of the hydroxyl group occurs with anchimeric assistance from iron, and that isomerization of the initially generated transotd pentadienyl cation to the more stable cisoid cation occurs with retention of configuration about the C1-G2 bond." ... 

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