Affinity for carbon

Quicklime and hydrated lime are reasonably stable compounds but not nearly as stable as their limestone antecedents. Chemically, quicklime is stable at any temperature, but it is extremely vulnerable to moisture. Even moisture in the air produces a destabilizing effect by air-slaking it into a hydrate. As a result, an active high calcium quicklime is a strong desiccant (qv). Probably hydrate is more stable than quicklime. Certainly hydrated lime is less perishable chemically because water does not alter its chemical composition. However, its strong affinity for carbon dioxide causes recarbonation. Dolomitic quicklime is less sensitive to slaking than high calcium quicklime, and dead-burned forms are completely stable under moisture-saturated conditions. 

Specify stabilized grades of stainless steel. An alternative method to prevent chromium carbide formation is to charge the alloy with substances whose affinity for carbon is greater than that of chromium. These substances will react preferentially with the carbon, preventing chromium carbide formation and thereby leaving the chromium uniformly distributed in the metal. The carbon content of the alloy does not have to be reduced if sufficient quantities of these stabilizing elements are present. Titanium is used to produce one stabilized alloy (321) and niobium is used to provide another (347). Note the cautions below. 

The rigorous kinetic modeling with the incorporation of the diffusion step allows explaining the deactivation of the carbon filament growth and the influence of the affinity for carbon formation on the nucleation of the filamentous carbon. 

Wang SQ, Humphreys ES, Chung SY, Delduco DF, Lustig SR, Wang H, Parker KN, Rizzo NW, Subramoney S, Chiang YM, Jagota A (2003) Peptides with selective affinity for carbon nanotubes. Nature Mater. 2 196-200. 

Additionally for rubber compounds, the differences in polarity and unsaturation of the various polymers cause different affinities for fillers and curing additives. In blends of different rubber polymers, the reinforcing filler carbon black for instance locates itself preferentially in the phase with the higher unsaturation and/or polarity, leaving the lower unsaturation or nonpolar phase unreinforced. The affinity for carbon black decreases in the following order of polymers [2] ... 

The area of organogold chemistry has also blossomed in the past ten years. In a 1926 review article (2) it was stated that Gold has no affinity for carbon. Complex organic compounds of gold in which the gold is directly attached to carbon are incapable of existence, or at least cannot be isolated. In fact, only a few workers were active in this field for many years, and the past decade has produced about 75% of the presently existing contributions to the subject. 

AFFINITY. The tendency of an atom or compound to react or combine with atoms or compounds of different chemical constitution. For example, paraffin hydrocarbons were so named because they are quite unreactive, the word paraffin meaning very little affinity." The hemoglobin molecule has a much greater affinity for carbon monoxide Ilian for oxygen. The free energy decrease is a quantitative measure of chemical affinity. 

With 1,3,5-trinitrobenzene no diversity of attack is possible, though for substituted derivatives such as 2,4,6-trinitroanisole or 2,4,6-trinitro-aniline the mode of interaction may vary on changing the nucleophile. Thus structural measurements show that the thermodynamically stable adducts of 2,4,6-trinitroanisole with OMe-, Ns- or NEt2 result from addition at Cl while the apparently stable adducts with S03 or CHg CO CH2 are formed at C3. The failure to detect addition of these latter nucleophiles at Cl may be ascribed to steric strain. This may occur either in the Cl adducts themselves, so that they are no longer thermodynamically preferred to the C3 adducts, or alternatively in the transition states for their formation, so that their formation is very slow. Again the mode of ionization of 2,4,6-trinitroaniline and its N-substituted derivatives depends on the relative affinities for carbon or hydrogen of the particular nucleophile used. Thus sulphur bases such as SEt- and SPh will preferentially add at the 3-position, while with oxygen bases abstraction of an amino proton also occurs. 

Most of the recent synthetic applications of M-RCM involve one of the above catalysts, particularly G1 or G2, chosen as a function of its own reactivity profile, generally after preliminary reaction assays on the genuine substrate or specific model compounds. The sensitivity of the RCM reaction to steric hindrance is well established. These ruthenium catalysts exhibit high affinity for carbon-carbon double bonds and are compatible with the presence of many functional groups, even the presence of free polar hydroxyl or amino groups. Their use does not require special conditions such as glove boxes, which are required when using Schrock s molybdenum catalyst. 

FIGURE 11.2 Cytochrome P450 has a high affinity for carbon monoxide when drugs are bound to the reduced complex, as observed spectroscopically at 450 nm. 

NiO(250°) contains more metallic nickel than NiO(200°). Magnetic susceptibility measurements have shown that carbon monoxide is adsorbed in part on the metal (33) and infrared absorption spectra have confirmed this result since the intensity of the bands at 2060 cm-i and 1960-1970 cm-1 is greater when carbon monoxide is adsorbed at room temperature on samples of nickel oxide prepared at temperatures higher than 200° and containing therefore more metallic nickel (60). Differences in the adsorption of carbon monoxide on both oxides are not explained entirely, however, by a different metal content in NiO(200°) and NiO(250°). Differences in the surface structures of the oxides are most probably responsible also for the modification of their reactivity toward carbon monoxide. In the surface of NiO(250°), anionic vacancies are formed by the removal of oxygen at 250° and cationic vacancies are created by the migration of nickel atoms to form metal crystallites. Carbon monoxide may be adsorbed in principle on both types of surface vacancies. Adsorption experiments on doped nickel oxides, which are reported in Section VI, B, have shown, however, that anionic vacancies present a very small affinity for carbon monoxide whereas cationic vacancies are very active sites. It appears, therefore, that a modification of the surface defect structure of nickel oxide influences the affinity of the surface for the adsorption of carbon monoxide. The same conclusion has already been proposed in the case of the adsorption of oxygen. 

Such thermodynamic conclusions are only relevant when the system is completely at equilibrium for reactions (4), (5), and, say, (9), but in an open system, such as a catalyst zone in a reformer where the gas is not yet at equilibrium, reaction between the components of that non-equilibrated gas can produce carbon even when the equilibrated gas shows no affinity for carbon formation. This is particularly so when higher hydrocarbons are involved and reaction (7) is possible. Whether carbon is deposited in that zone depends upon the kinetics of the carbon-forming and carbon-removing reactions, which can be influenced... 

Trace Metal Affinites. Trace-element and major-element associations in the oil shales have been a subject of two studies (19-20). In general, the trace elements can be divided into three groups those which have an affinity for carbon (Cu, Cr, Ni, V),... 

Therefore, OH- (or CH30 ) has an abnormally high affinity for the proton, and as a consequence, H02", and other a-nucleophiles, has a greater relative affinity for carbon (and for other electrophilic centers). This finding appears to form the basis of the apparently high reactivity of a-nucleophiles, which is intrinsic to the reagent and should be observed in gas-phase equilibria (see Table III) as well as in solution kinetics. 

Carbon formation is to be expected on a nickel catalyst if the gas shows affinity for carbon after the establishment of the methane reforming and the shift equilibria . 

Wilkinson s catalyst has a strong affinity for carbon monoxide and decarbony-lates aldehydes, therefore alkene compounds containing aldehyde groups cannot normally be hydrogenated with this catalyst under the usual conditions. For example, cinnamaldehyde is converted into styrene in 65% yield, and benzoyl chloride gives chlorobenzene in 90% yield. 

Type II behavior occurs when there is a preference for polymer penetrant pairs to form, or when there are sites in the material that preferentially absorb the penetrant. As an example, consider a black filled rubber and a gas that has an affinity for carbon black. Under these conditions the rate of sorption decreases with increasing pressure. At higher pressures, when all the sites are occupied, small quantities of the gas will then dissolve randomly in the continuous phase and the system behaves ideally. It follows from this that diffusion does not depend on the total concentration but on the much smaller concentration of gas in the continuous phase. 

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