Type B oxides

FIGURE 5.41 Structural possibilities for binary oxides, (a) Type A oxides metal excess/anion vacancies, (i) This shows the two electrons that maintain charge neutrality, localized at the vacancy, (ii) The electrons are associated with the normal cations making them into M. (b) Type B oxides metal excess/interstitials. (i) This shows an interstitial atom, whereas in (ii) the atom has ionized to and the two liberated electrons are now associated with two normal cations, reducing them to M. (c) Type C oxides metal deficiency/interstitial anions. The charge compensation for an interstitial anion is by way of two ions, (d) Type D oxides metal deficiency/cation vacancies. The cation vacancy is compensated by two cations. 

Type B oxides have a metal excess which is incorporated into the lattice in interstitial positions. This is shown in (b)(1) as an interstitial atom, but it is more likely that the situation in (b)(ii) will hold, where the interstitial atom has ionised and the two electrons so released are now associated with two neighbouring ions, reducing them from to M. Cadmium oxide, CdO, has this type of structure. Oxygen is lost when zinc(II) oxide is heated, forming Zn +JD, oxide vacancies form and to compensate, Zn ions migrate to interstitial positions and are reduced to Zn ions or Zn atoms. Electron transfer can take place between the Zn and ZnTZn resulting in the yellow coloration seen when ZnO is heated. 

The possibility of introducing new d-bands for Type "b" oxides (with filled d-bands) by introducing dopants into the host lattice was also discussed in Ref. 65 with examples. Other authors have advocated this approach as well (Ref. 26, for exam pie). The review in Ref. 32 contains further examples of this approach for effectively shrinking the original Eg and sensitizing the oxide to visible portions of the solar spectrum. We shall return to this aspect for the specific case of TiCte later in this Chapter. 

Type B Oxidizing or inert atmospheres. Beware of contamination. For high temperatures. 

Classically, type B pyrazine syntheses involve self-condensation of an a-aminoacyl compound to yield a 3,6-dihydropyrazine which is subsequently oxidized to the pyrazine (Scheme 54) (70CC25). The aromatization usually proceeds under very mild conditions. 

The creation of the N—N bond as the last step of the ring synthesis is common in indazoles and very rare in pyrazoles. In indazoles this method is well known (type B synthesis (67HC(22)l), for example, the dehydration of oximes (570) with acetic anhydride yields 1-acetylindazoles (571), and in basic medium the indazole 1-oxides (573) are formed from the nitro derivatives (572). 

Soluble sulfides (i.e., H S, HS" and S ", with sulfur at minus two oxidation state) are chemically very reactive. The two general types of soluble-sulfide reactions may be identified as precipitation reaction (type A) and redox reaction (type B). 

Type B (redox) reactions are more complex. Sulfide in this reaction is converted into some other oxidation state of sulfur. For example, sulfides can be converted to a zero oxidation state of elemental sulfur by oxygen ... 

The majority of potentiometric titrations involve chemical reactions which can be classified as (a) neutralisation reactions, (b) oxidation-reduction reactions, (c) precipitation reactions or (d) complexation reactions, and for each of these different types of reaction, certain general principles can be enunciated. 

Ubiquitous mitochondrial monoamine oxidase [monoamine oxygen oxidoreductase (deaminating) (flavin-containing) EC 1.4.3.4 MAO] exists in two forms, namely type A and type B [ monoamine oxidase (MAO) A and B]. They are responsible for oxidative deamination of primary, secondary, and tertiary amines, including neurotransmitters, adrenaline, noradrenaline, dopamine (DA), and serotonin and vasoactive amines, such as tyramine and phenylethylamine. Their nonselec-tive and selective inhibitors ( selective MAO-A and -B inhibitors) are employed for the treatment of depressive illness and Parkinson s disease (PD). 

The beneficial effect of deprenyl in Parkinson s disease was su ested to be in part due to its effect on increasing the levels of SOD activity in several brain regions (Carrillo et al., 1993). Deprenyl is known to inhibit monoamine oxidase type B, which results in a reduction in hydrogen peroxide formation by blockade of the oxidative deamination of dopamine. That is believed to be the major mechanism of action of this drug in inhibiting the progression of Parkinson s disease. 

It was postulated (169) that these amides are 8,8a-secobenzophenanthridine alkaloids produced by oxidative cleavage of ring B of the corresponding benzophenanthridines. The success of Baeyer-Villiger-type oxidations of the immonium bond of benzophenanthridine skeletons (168,171,172,175) indicates that this type of oxidation could be a real biological pathway. 

Hu, B., Chen, C., Frueh, S.J., Jin, L., Joesten, R. and Suib, S.L. (2010) Removal of aqueous phenol by adsorption and oxidation with doped hydrophobic cryptomelane-type manganese oxide (K-OMS-2) nanofibers. Journal of Physical Chemistry C, 114, 9835-9844. 

Increasing temperature shortens the induction time and increases the maximum chemiluminescence intensity in the case of chemiluminescence of PP powder (type (a), see Figure 15), whereas it increases the initial chemiluminescence intensity in the case of poly(2,6-dimethyl-l,4-phenylene oxide) (type (b), see Figure 5). This is perhaps not surprising as the rate of oxidation reaction increases with temperature as well. 

In type A reactions one electron is removed from one of the two double bonds to form a cation radical, and allylic substitution and oxidative addition take place as the following reactions. On the other hand, in type B reactions the initial electron transfer from the double bond is accompanied by a transannular reaction between the two double bonds. 

The difference between dienes reacting according to type A and those according to type B is clearly reflected in their oxidation potentials (Table 5). 

This result suggests that in the anodic oxidation of type B, the cation radical formed from one of the two double bonds is stabilized through transannular interaction with another double bond. 

Harry B. Gray and Walther Ellis,13 writing in Chapter 6 of reference 13, describe three types of oxidation-reduction centers found in biological systems. The first of these, protein side chains, may undergo oxidation-reduction reactions such as the transformation of two cysteine residues to form the cystine dimer as shown in equation 1.28 ... 

In the case of Type B linear correlations of two presumably related processes, the main problem is to find a suitable partner to a heterogeneous catalytic reaction the requirements include a good knowledge of its mechanism, easy measurement of structure effects, and the possibility of using the same reactants in both series. It already has been mentioned that this task may be more easily fulfilled with heterogeneous acid-base reactions but may be impossible with reactions on metals or some oxides. 

The oxidation of alkenes by nitrous oxide on silver at 350°C has been studied from the viewpoint of structure effects on rate by Belousov, Mulik, and Rubanik (J40), and very good correlations of Type B have been found with ionization potentials and with the rate of oxidation by atomic oxygen (series 110 and 111). 

Structure effects on the rate of selective or total oxidation of saturated and unsaturated hydrocarbons and their correlations have been used successfully in the exploration of the reaction mechanisms. Adams 150) has shown that the oxidation of alkenes to aldehydes or alkadienes on a BijOj-MoOj catalyst exhibits the same influence of alkene structure on rate as the attack by methyl radicals an excellent Type B correlation has been gained between the rate of these two processes for various alkenes (series 135, five reactants, positive slope). It was concluded on this basis that the rate-determining step of the oxidation is the abstraction of the allylic hydrogen. Similarly, Uchi-jima, Ishida, Uemitsu, and Yoneda 151) correlated the rate of the total oxidation of alkenes on NiO with the quantum-chemical index of delo-calizability of allylic hydrogens (series 136, five reactants). 

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