Proton donor ability

Hydrogen bonding to the nitrogen lone pair leads to an upheld shift, the extent of which depends on the proton-donor ability of the solvent, and the acceptor ability of the base shifts of some 20 p.p.m. are commonly found. 

The protolytic oxidation of alkanes is also strongly supported by electrochemical studies. In 1973, Fleischmann, Plechter, and co-workers78 showed that the anodic oxidation potential of several alkanes in HSO3F was dependent on the proton donor ability of the medium. This acidity dependence shows that there is a rapid protonation equilibrium before the electron transfer step and it is the protonated alkane that undergoes oxidation (Scheme 5.9). 

Partial silylation of the highly disperse silica surface enhances the adsorption of vitamin E from ethanol solution, and provides the ability to obtain water-soluble nanocomposites containing vitamin E. Immobilization of vitamin C on the silica surface prevents its oxidation. Its interaction with the adsorbent surface leads to a decrease in proton-donor ability of the OH-groups involved in the oxidation of ascorbic acid. Elydrophobized silica nanocomposites are characterized by a prolonged desorption of immobilized vitamins. It has been shown that vitamin C does not lose its antioxidant properties after desorption. 

Miyazawa et al. (92) related rates of decrease of aliphatic hydrogen protons during pyrolysis of ethylene tar pitch to formation of mesophase. Yokono et al, (93) used the model compound anthracene to monitor the availability of transferable hydrogen. Co-carboniza-tions of pitches with anthracene suggested that extents of formation of 9,10-dihydroanthracene could be correlated with size of optical texture. The method was then applied to the carbonization behaviour of hydrogenated ethylene tar pitch (94). This pitch, hydrogenated at 573 K, had a pronounced proton donor ability and produced, on carbonization, a coke of flow-type anisotropy compared with the coarse-grained mosaics (<10 ym dia) of coke from untreated pitch. 

The rate constant of thermal bleaching of derivatives of 1-methylanthraquinone depended on the nature of the solvent. Table 7.14 shows that it decreased by several orders or magnitude as the proton-donor ability increased. 

In dilute aqueous solution, the acidity is measured using pH values. For concentrated acid solutions and non-aqueous acid solutions, pH values are no longer available. Hence, the Hammett acidity function Ho is used as a measure of the acidity of such media [130]. The proton donor ability of an acid in such media is measured by studying the equilibria of a series of indicator bases B (mostly nitroanilines), the UV/Vis absorption spectra of which are markedly different from those of their conjugate acids, so that the indicator concentration ratio I = [B]/[BH+] can be measured spectrophotometri-cally. The acidity function Ho is then given by Hq = p.Kbh+ + Ig f with the subscript zero indicating that the Hq function applies only to neutral bases B [130, 170], For dilute solutions, Ho corresponds exactly to pH in concentrated solutions, the two functions differ appreciably. 

Proton donor ability of surface silanols is believed to be the source of peak tailing for analytes with proton acceptor functionaUty (usually basic analytes). The presence of impurities such as iron, boron, and aluminum [70] in bulk silica decreases the silanol pK and decrease the hydrolytic stability of bonded phases. 

Concluding this section all that one can say is that we found no relationship between anesthetic potency and either the ionization potentials or the frequency of the lowest ultraviolet absorption band. The observation that replacement of a fluorine atom by a hydrogen usually lowers the IP is probably of some value. However, as was pointed out above this could only indicate the possibility of charge transfer interaction if the electron affinities followed the same trend. Unfortunately these have not been determined and the variations in the frequencies of the broad UV bands are too irregular to draw conclusions. It seems that there exists an indirect relationship between the acidity of these molecules and their IPs and what counts is their proton donor ability connected with the acidic hydrogen as has been concluded from the infrared studies described in previous sections. 

The data in Table 6 confirm the presence of an intramolecular hydrogen bond between one hydrogen atom of the NH2 group and the oxygen of the nitro group bonded in position 2 to the amino group132,133. The proton donor ability of anilines resembles qualitatively that of alcohols, and it is lower than that of phenols. 

However, the strong proton donor ability of the sulfonic acids suggests that the acids will loosely associate by hydrogen bonding with weak basic compounds or with proton... 

The greater the value, the greater the proton donor ability of the acid towards water. 

When we dealt with acid-base behaviour in aqueous solution in Chapter 7, we saw that the strength of an acid HX (equation 9.5) depended upon the relative proton donor abilities of HX and [HsO]. ... 

Many other molecules can act either as acids or bases depending on the other molecules they interact with, as illustrated, again, by the reactions of sulfuric acid and ammonia. Sulfuric acid can donate proton to bases such as carbonyl compounds (eq. 9) and can accept proton from stronger acids such as fluorosul-furic acid (eq. 10). The protonation of ammonia with the stronger acid water and its proton donor ability against sodium hydride are given in equations 11 auid 12, respectively. 

In these examples we control the extent of protonation of A by physically adjusting the pH. The important point is that the pH is adjusted relative to the pfCa of the conjugate acid of the base A to change the protonation state. The pH therefore tells us the proton donor ability of the solution toward species A . In other words, it tells us the power of the solution to donate a proton to a particular base. In the next section we introduce a similar concept, acidity functions, which give the proton donor ability of extremely concentrated add solutions. 

Using the insight that the pH gives the proton donor ability of the solution, and the pfC gives the proton donor ability of an acid, we are ready to make predictions as to the protonation states of acids based solely on their pX s and the solution s pH. The following three rules apply ... 

Before launching into an analysis of pXaS of representative acids and a discussion of those factors that influence acidities, we examine gas phase acidities. In the gas phase, the intrinsic proton donor abilities between various acids can be determined without influence from solvents. Hence, an examination of gas phase acidities makes a nice introduction to determining relative solution acidities, where we can take the lessons from the gas phase and perturb them with knowledge of how the solvent influences acidities. For an example of how gas phase acidities have been used to interpret pKa changes in solution, see the Connections highlight given at the end of this discussion. 

The reports by Hoz and Hilmersson underscore the significance of alcohol coordination to Sm(II). Clearly the interplay between coordination and proton donor ability of various alcohols and water will have an impact on the mechanistic pathway of Sml2-based reductions which utilize proton donors. To address this issue, Flowers et al. (Chopade et al., 2004b) stud-... 

It is well-known that the reduction waves of many organic compounds in aprotic solvents differ considerably in form and character from the waves for the same compounds in solutions with fairly high proton-donor ability (e.g., [14-16, 66-68]). Addition of water or other acids to the aprotic solvents changes the mechanism of the electrode process in such a way that the corresponding waves approximate in their character the waves observed in solutions with high proton-donating activity [69, 70]. 

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