Acidity function explanation

A possible explanation comes from X-ray analyses of the sulfonic acids [45]. All X-rayed crown ether crystals contained water and the sulfonic acid moiety was dissociated. Therefore in crystals of [45], macrocyclic ben-zenesulfonate anions and hydronium ions (sometimes hydrated) are present. The ions are bound to each other by hydrogen bonds. The size of the included water-hydronium ion cluster (varying by the number of solvating water molecules) depends on the ring size. In the 15-membered ring, HsO" was found, whereas in a 21-membered ring HsO and in the 27-membered ring were present. This means the sulfonic acid functions in [45] are... 

Chloro-4,6-dimethoxy-l,3,5-triazine (100) reacts with iV-methylmorpholine at 20 °C to yield an isolable quaternary triazinylammonium salt (101 R = Me, R, R = C4H8O). This salt can then be reacted with a carboxylic acid to yield a 2-acyloxy-4,6-dimethoxy-l,3,5-triazine (102), which, in turn, can be reacted with an amine to yield an amide (103). This sequence of reactions provides an explanation for the activation (formation of reactive ester) of the carboxylic acid function by 2-chloro-4,6-disubstituted-l,3,5-triazines (100) in the presence of hindered amines. Several other hindered amines may replace iV-methylmorpholine in the process, but unhindered amines such as triethylamine and tributylamine were inactive. ... 

An apparently different explanation for the catalysis has been advanced by Kozhevnikov et al. (171, 172). The rate constant, k (min ) for H3PW12O40 and that for H4SiW 2O40 at room temperature show linear relationships with the Hammett acidity function (H0) as shown by Eq. (lib). 

This explanation is supported by the inhibitory activities of the series of adamantanes shown in Figure 4. The simplest compound, adamantane-l-carboxylic acid, is a potent inhibitor, but introduction of a second carboxyl group results in substantially weaker inhibition. Activity is partially restored by the replacement of either or both carboxyl groups with acetic acid functions, in which the additional methylene groups may serve to reduce repulsive interactions. The inhibitory activity of these analogues thus emphasizes the importance of correct substituent orientation in the design of transition state analogues. 

It is obvious that the expression enclosed in the brackets by the author of the present book is nothing but the primary medium effect of O2- expressed via the difference in the values of the equilibrium constants of equation (1.3.6) for the media compared the molten equimolar KCl-NaCl mixture, which was chosen as a reference melt, and for which pKHa/H20 was found to be 14 at 700 °C, and the melt studied. As to the physical sense of the common acidity function Cl, this is equal to the pO of the solution in the molten equimolar KCl-NaCl mixture, whose acidic properties (oxide ion activity) are similar to those of the solution studied. Moreover, from equation (1.3.7) it follows that solutions in different melts possess the same acidic properties (f ) if they are in equilibrium with the atmosphere containing HC1 and H20 and Phc/Ph2o — constant. This explanation confirms that the f function is similar to the Hammett function. Therefore, Cl values measured for standard solutions of strong bases in molten salts allow the prediction of the equilibrium constants on the background of other ionic solvents from the known shift of the acidity scales or the f value for the standard solution of a strong Lux base in the solvent in question. According to the assumption made in Refs. [169, 170] this value may be obtained if we know the equilibrium constant of the acid-base reaction (1.3.6) in the solvent studied. 

Toshiba and Takahashi [214] prepared nanoparticles of colloidal Pt embedded in a micelle (Fig. 18.22). With such a system the initial rate of hydrogenation of 2-undecenoic acid is almost 6 times lower than the initial rate of hydrogenation of 10-undecenoic acid. The explanation is based on the polar effect of the amphiphilic ligand whose polar head stays outside the particle. The polar head fadhtates the coordination of the double bond by preventing the carboxyUc function from approaching the surface of Pt but considering such a micellar system, one can see easily their fragility. 

We may now inquire into the reason for the difference between the behavior of carbonium ions and ammonium or oxonium ions assuming, as is reasonable (87), that the difference in activity coefficient ratios does not reside in the free bases. An attractive explanation may be advanced at once that the carbonium ions derived from aryl carbinols or olefins involve a much greater degree of charge delocalization than that in the onium ions and this should have a profound effect on how tightly the ion is solvated. At least three facts eliminate the de-localization theory. Firstly, the small NO+ ion, in which delocalization is restricted, has activity coefficient behavior like a polyaryl carbonium ion (82). Secondly, aryl and diaryl ketones behave for the most part as proper Hammett bases (324,328) although considerable delocalization occurs in their ions. Thirdly, the same acidity function seems to apply equally well to pyrrole bases whether or not they have much charge delocalization in their ions (357). 

An organic sulfur compound containing an acetal function had been oxidised to the sulfone with 30% hydrogen peroxide in acetic acid. After the liquor had been concentrated by vacuum distillation at 50-60°C, the residue exploded during handling. This was attributed to formation of the peroxide of the acetal (formally a gem-diether) or of the aldehyde formed by hydrolysis, but formation and explosion of peracetic acid seems a more likely explanation. 

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