Mechanism of acidity generation

A new hypothesis regarding the acidity generation of binary oxides has been proposed by Tanabe el al. the hypothesis predicts what kinds of binary oxides will show acidic properties (Bronsted or Lewis acid) and provides insight regarding the structure of the acid sites. According to the hypothesis, acidity generation is caused by an ex- 

Let us examine another example. In ZnO - Z1O2, there is no excess charge in any part of its composition according to our model structure written by postulates i) and ii), as illustrated in Fig. 3.42. Therefore, the binary oxide is not expected to show any acidic property. This prediction agrees with the experimental result that ZnO — ZrOz does not show acidity larger than the sum of the acidities of the component oxides. 

K valence of positive element, C coordination number of positive element 

Mixed-Oxides a = V/C Acidity increase V alidity of hypotheses  

Thomas hypothesis 15/32=47 % correct. Tanabe et al. s hypothesis 29/32 = 91 % correct. 

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Scheme 7.24 Generalized mechanism of acid generation in onium salts.

The mechanism of acid generation in some PAGs, particularly onium salts, has been studied extensively. Generally, Scheme 7.25 describes the photolytic... 

Figure 8. Proposed mechanism of the generation of peroxyl radicals by reaction of hematin with unsaturated fatty acid hydroperoxides. ...

Guanine is the most easily oxidizable natural nucleic acid base [8] and many oxidants can selectively oxidize guanine in DNA [95]. Here, we focus on the site-selective oxidation of guanine by the carbonate radical anion, COs , one of the important emerging free radicals in biological systems [96]. The mechanism of COs generation in vivo can involve one-electron oxidation of HCOs at the active site of copper-zinc superoxide dismutase [97, 98], and homolysis of the nitrosoperoxycarbonate anion (0N00C02 ) formed by the reaction of peroxynitrite with carbon dioxide [99-102]. 

Carbenes were the earliest proposed Cl intermediates. These were considered to be generated via a-elimination of water from methanol itself [9,10], or from catalyst surface methoxyls [11]. The reaction may be assisted by cooperative action of acid and base sites on the catalyst [10]. Olefins would be formed by polymerization of the (free) CIl2. To overcome the high energetic requirements of carbene generation (vide infra), and the low probability of CH2 self-condensation, Chang and Silvestri [lb] proposed a concerted mechanism of carbene generation with sp3 insertion into the C-H bond of DME to form MeOEt. 

The biosynthesis and breakdown of fatty acids are both essential processes in cellular metabolism. Fatty acids are needed for critical structures such as cell membranes and cell walls, whereas fatty acid catabolism is a mechanism of energy generation in the cell. Although these metabolic pathways are present in all living... 

Thus, to obtain the ANP with the maximum yield, the concentration of sulfuric acid used for treatment of the starting cellulose sample should be in the range of 65—66 wt%. The probable mechanism of the generation of ANP consists of depolymerization, sulfonation, and dissolving of cellulose in a sufficiently high sulfuric acid concentration (65—66 wt%), followed by the regeneration of the amorphous floes and their comminution in the water medium. 

The value of kc for the resists exposed at 248 nm was taken from Reference (75) as the product of the PAG absorption and photoacid quantum yield at 254 nm. Here it is assumed that the quantum yield does not change with minor changes in wavelength and that there are no charge transfer or other indirect mechanisms for acid generation in our resist system. The quantum yield for the resists exposed at 193 nm was determined experimentally. 

Lipoic acid presumably functions only when acyl generation and transfer occur and not in decarboxylation or aceton formation. The mechanism of acyl generation suggested in reaction 2, Fig. 4, is the carbanion cleavage of the disulfide bond to form the free thioester wuth CoA and the free sulfhydryl of lipoic acid. 

Catalytic cracking is essentially carbenium ion chemistry. Thus, the central problem of acid-catalyzed cracking is the mechanism of the generation of carbenium ions. 

Equilibria (7.46) and (7.47) account for tlie fact that complexation is related to acid-base reactions, and that it is involved in the mechanism of charge generation, with proton adsorption/desorption equilibria. The charge o-q. obtained by proton titration, reprc.sents the number of protons released or consumed in all reactions within the Stern layer, and not only the protons involved in the formation of the MO and MOH 2. species. Hence, all ions participating in the creation of charges op and <7h are called potential-determining ions (PDls), although this name is very often reserved for H and HO . 

The chemical pathways leading to acid generation for both direct irradiation and photosensitization (both electron transfer and triplet mechanisms) are complex and at present not fully characterized. Radicals, cations, and radical cations aH have been proposed as reactive intermediates, with the latter two species beHeved to be sources of the photogenerated acid (Fig. 20) (53). In the case of electron-transfer photosensitization, aromatic radical cations (generated from the photosensitizer) are beHeved to be a proton source as weU (54). 

Fig. 20. Proposed photochemical mechanisms for the generation of acid from sulfonium salt photolysis. Shown ate examples illustrating photon absorption by the onium salt (direct irradiation) as well as electron transfer sensitization, initiated by irradiation of an aromatic hydrocarbon.

Fig. 21. Representative nonionic photoacid generators. A variety of photochemical mechanisms for acid production ate represented. In each case a sulfonic acid derivative is produced (25,56,58—60). (a) PAG that generates acid via 0-nitrobenzyl rearrangement (b) PAG that generates acid via electron transfer with phenohc matrix (c) PAG that is active at long wavelengths via electron-transfer sensitization (d) PAG that generates both carboxylic acid and...

In two proposed alternative processes, the chlorine is replaced in the hypochlorination reaction by hypochlorous acid [7790-92-3] HOCl, or tert-huty hypochlorite. In the first, a concentrated (>10% by weight) aqueous solution of hypochlorous acid, substantially free of chloride, chlorate, and alkah metal ions, is contacted with propylene to produce propylene chlorohydrin (113). The likely mechanism of reaction is the same as that for chlorine, as chlorine is generated in situ through the equiUbrium of chlorine and hypochlorous acid (109). 

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