Metal relative reactivities

Figure Cl. 1.3. Relative reactivity of transition-metal clusters with H2 (full curves, log scale) and tire promotion...

Potassium fluoride [7789-23-3], KF, is the most frequently used of the alkaU metal fluorides, although reactivity of the alkaU fluorides is in the order CsF > RbF > KF > NaF > LiF (6). The preference for KF is based on cost and availabiUty traded off against relative reactivity. In its anhydrous form it can be used to convert alkyl haUdes and sulfonyl haUdes to the fluorides. The versatility makes it suitable for halogen exchange in various functional organic compounds like alcohols, acids and esters (7). For example, 2,2-difluoroethanol [359-13-7] can be made as shown in equation 9 and methyl difluoroacetate [433-53 ] as in equation 10. 

Galvanic corrosion is location specific in the sense that it occurs at a bimetallic couple (Fig. 16.2). It is metal specific in the sense that, typically, corrosion affects the metal that has less resistance in the environment to which the couple is exposed. Hence, in principle, we would anticipate galvanic corrosion of relatively reactive metals wherever they are in physical contact with relatively noble metals in a sufficiently aggressive, common environment. Experience has shown, however, that all such couples do not necessarily result in unsatisfactory service. This is because of the interplay of various critical factors that influence galvanic corrosion. These critical factors are discussed in the next section. 

Magnesium is a relatively reactive metal, and can be chromated in nearly neutral solutions as well as in acid solutions. The range of treatments possible illustrates well the role of pH, activating anion, temperature and duration of treatment in promoting the breakdown of passivity in the chromate solution and the consequent formation of a chromate him. 

Sulfide ores, after preliminary treatment, most often undergo roasting, that is, heating with air or pure oxygen. With a relatively reactive transition metal such as zinc, the product is the oxide... 

Interest in the dehydroxylation of metal hydroxy salts has hitherto largely centred on the hydroxyhalides. Studies of the relative reactivities of comparable salts of this type have included measurements of the influences of the constituent halide and of variations in the ratio of... 

Transition metal complexes that are easy to handle and store are usually used for the reaction. The catalytically active species such as Pd(0) and Ni(0) can be generated in situ to enter the reaction cycle. The oxidative addition of aryl-alkenyl halides can occur to these species to generate Pd(II) or Ni(II) complexes. The relative reactivity for aryl-alkenyl halides is RI > ROTf > RBr > RC1 (R = aryl-alkenyl group). Electron-deficient substrates undergo oxidative addition more readily than those electron-rich ones because this step involves the oxidation of the metal and reduction of the organic aryl-alkenyl halides. Usually... 

Titanium, Ti, a light, strong metal, is used where these properties are critical— in widely diverse applications such as jet engines and dental fixtures such as partial plates. Although titanium is relatively reactive, unlike scandium it is resistant to corrosion because it is passivated by a protective skin of oxide on its surface. The principal sources of the metal are the ores ilmenite, FeTiO , and rutile, Ti02. 

The thiophene ring can be elaborated using standard electrophilic, nucleophilic, and organometallic chemistry. A variety of methods have been developed to exploit the tendency for the thiophene ring (analogous to that of furan and pyrrole) to favor electrophilic substitution and metallation at its a-carbons. Substitution at the p-carbons is more challenging, but this problem can also be solved by utilizing relative reactivity differences. 

The objective of this section is to compare relative reactivities, as well as the activation parameters, for the CO insertion of various types of metal alkyl. Reference will be made again to the classification of these complexes introduced in Section B,l. Some of the data examined are contained in Table III. 

Several examples of intramolecular additions to carbonyl groups by organo-lithium reagents generated by halogen-metal exchange have been reported, such as the two examples shown below. What relative reactivity relationships must hold in order for such procedures to succeed ... 

Bluish, shimmering, brittle, relatively reactive metal. Is guite guickly covered with a protective oxide layer, which is why iron is treated with zinc With copper, forms the popular alloy brass, which was already known in antiquity. Used in batteries and as a stabilizer in plastics. Zinc oxide is used as a white pigment Zinc ions are essential to all life forms, e.g., as a component of alcohol dehydrogenase and many other enzymes. Hence human beings (70 kg) carry about 2.3 g (half as much as iron). 

Table 7-1 Relative Reactivities of Some Metals and Nonmetals...

In fact, the orientation of water at the potential of zero charge is expected to depend approximately linearly on the electronegativity of the metal.9 This orientation (see below) may be deduced from analysis of the variation of the potential drop across the interface with surface charge for different metals and electrolytes. Such analysis leads to the establishment of a hydrophilicity scale of the metals ( solvophilicity for nonaqueous solvents) which expresses the relative strengths of metal-solvent interaction, as well as the relative reactivities of the different metals to oxygen.23... 

Careful kinetic analysis of this thermal reaction shows that the rate of disappearance of the CT band is identical to that of the adduct formation in equation (50). Most importantly, the relative reactivity of the metal hydrides in Table 8 decreases with the increasing ionization potential in the order Bu3SnH < Bu3GeH < Et3SiH. 

As shown by the data for these mixtures also presented in Table II., under the conditions employed here, all of these metals reacted directly with DBDPO. From a mechanistic perspective, that for all three metals the principal DBDPO volatile reaction product was observed to be the octabromodibenzofuran (OBDBF) was considered to be the most important result of these experiments. The observed reactivity of the metal towards the DBDPO decreased in the order Zn° > Sb° > Bi°. Of these data, only the rather large observed difference in the relative reactivities of Sb° (-38% OBDBF) and Bi° (-9% OBDBF) was unexpected based upon the thermodynamics for the formation of their respective halides. 

While cobalt and rhodium have been the focus of most research and are the metals of choice for commercial hydroformylation reactions, numerous other metals have been disclosed as catalysts in the patent literature. However, only some of the carbonyl-forming metals can be seriously considered. Even of these, a comparison of relative reactivity (118) based on cobalt as the standard indicates a decided preference for only two or three metals. This listing may be considered incomplete without the inclusion of platinum and copper, which have recently received significant attention (vide infra). 

In scrutinizing the various proposed reaction sequences in Eq. (26), one may classify the behavior of carbene complexes toward olefins according to four intimately related considerations (a) relative reactivities of various types of olefins (b) the polar nature of the metal-carbene bond (c) the option of prior coordination of olefin to the transition metal, or direct interaction with the carbene carbon and (d) steric factors, including effects arising from ligands on the transition metal as well as substituents on the olefinic and carbene carbons. Information related to these various influences is by no means exhaustive at this point. Consequently, some apparent contradictions exist which seem to cast doubt on the relevance of various model compound studies to conventional catalysis of the metathesis reaction, a process which unfortunately involves species which elude direct structural determination. 

Pampus and co-workers (65) established the relative reactivity of a series of olefins to be 1-butene > 2-butene > isobutylene. This order of reactivity has been confirmed by others, and exactly parallels the reported order of stability of transition metal (Rh) complexes with these olefins (66), thus clearly implicating precomplexation of the olefin with the transition metal prior to metathesis. On a limited scale, Schrock observed a similar order of reactivity for olefins in reactions with (175-C5H5 )TaCl2[=CH(CH3 )3 ], which is known to possess a nucleophilic car-bene carbon (64). This complex also provides the requisite empty coordination site needed for precomplexation. In that study, cyclopropanes or metathesis olefins were not observed as products. 

In addition to the electronic difference between PR3 and PH3, bulkier ligands on the phosphine can change the reaction through their steric effect. Using the R = Bu on the anthraphos system, Haenel et al. calculated the available molecular surface (AMS) around the metal center as a measure of the space available to the alkane (13b). They correlated the AMS to the relative reactivities of the catalysts and the results show that two bulky tert-butyl groups on each P certainly limit the access to the metal center, and thus, may reduce the reactivity. Other theoretical studies on the pincer complexes showed that this steric contribution/ limitation plays a less important role than the activation barriers introduced by the catalyst itself (22), where the increase in energy barrier induced by the bulky 4Bu is smaller than the original barriers calculated... 

Several accounts have described (Z)-dehydroamino acid esters as being less active than the corresponding (F)-isomer [59c, 143-145]. In fact, Bruneau and Demonchaux reported that when reduction of an (E/Z)-mixture of 73 with Rh-Et-DuPhos in THF was not complete, only unreacted (Z)-73 was detected. These findings conflict, however, with results obtained in MeOH [56 d], where the ligand structure was also found to be significant to the relative reactivity of each stereoisomer. As for a-dehydroamino acid derivatives, preformed metal-diphosphine complexes generally perform in superior fashion to those prepared in situ [56d]. 

Some metals are more reactive than others. By comparing how different metals react with the same ions in aqueous solutions, an activity series for the tested metals can be developed. The activity series will reflect the relative reactivity of the tested metals. It can be used to predict whether reactions will occur. 

Many transition-metal complexes have been reported as catalysts of this reaction, including [lr(g-Cl)(coe)2]2 [74] and [lrH2(solv.)(PPh3)][SbF6] [75]. The latter catalyst appeared to be a very active and highly selective. The hydroxyl group can be selectively silylated, even in the presence of other potentially reactive C=C and C=0 groups. The order of relative reactivities of alcohol isomers is secondary alcohol > primary alcohol > tertiary alcohol. 

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