Incorporation, carbon chain growth

In Fischer-Tropsch synthesis the readsorption and incorporation of 1-alkenes, alcohols, and aldehydes and their subsequent chain growth play an important role on product distribution. Therefore, it is very useful to study these reactions in the presence of co-fed 13C- or 14 C-labeled compounds in an effort to obtain data helpful to elucidate the reaction mechanism. It has been shown that co-feeding of CF12N2, which dissociates toward CF12 and N2 on the catalyst surface, has led to the sound interpretation that the bimodal carbon number distribution is caused by superposition of two incompatible mechanisms. The distribution characterized by the lower growth probability is assigned to the CH2 insertion mechanism. 

Carbon may exist in different forms, of which carbidic surface carbon (Cs) is the most reactive species. It is readily transformed to carbenic species [Eq. (3.25], which participates in chain growth. This is supported by reevaluation of earlier labeling experiments. New studies showed - that certain surface carbon species may be very reactive and do incorporate into products mainly into methane. Hydrogenation of adsorbed CO without previous dissociation may also lead to surface carbenic species244 [Eq. (3.26)] ... 

DNA chain growth can be blocked by the incorporation of certain nucleoside analogs that have been modified in the sugar portion of the nucleoside (Figure 29.24).7 For example, removal of the hydroxyl group from the 3 -carbon of the deoxyribose ring as in 2, 3 -dldeoxyiriosirie (ddl), or conversion of the deoxyribose to another... 

It has been shown that surface basicity of iron-based catalysts greatly affects probability of chain growth.5 7 Alkali metal incorporation promotes the formation of higher carbon number molecules. As described in Dry s review,7 iron-based catalysts are prepared by precipitation techniques and promoted with Cu, K20, and Si02 (typical composition 5 g of K20, 5 g of Cu, and 25 g of Si02 per lOOg of Fe). This type of catalyst is well suited for heavy liquid fuels produced from a low-temperature... 

The dideoxynucleosides do not have a hydroxyl group on either the 2 - or 3 -car-bon. They can be converted to dideoxynucleoside triphosphates in cells and, like ZDV, terminate chain growth when incorporated into DNA. In the case of the dideoxynucleosides, chain termination results from the absence of a hydroxyl group on the 3 carbon. The HIV virus mutates very rapidly (mostly because reverse transcriptase lacks 3 to 5 exonuclease activity, the proofreading activity) and frequently develops resistance to one or more of these drugs. Therefore, it is recommended that AIDS patients take a number of drugs, including more than one reverse transcriptase inhibitor. 

One of the best of these schemes (10, II), SCG (simple chain growth), involved one-carbon addition to one end of the growing chain at the first carbon with a probability a and to the second carbon with a probability af, if addition had not already occurred on this carbon. The SCG scheme assumed that the growth constants were independent of carbon number and structure of the growing chain, a situation that is fortuitous rather than expected. For a given carbon number, the ratios of branched to normal hydrocarbons are f or 2f for monoethyl isomers and P or 2P for dimethyls the factor one or two depends on whether the species can be produced in one or two ways. This simple mechanism predicted carbon number and isomer distributions moderately well and also seemed consistent with the tagged alcohol incorporation studies of Emmett, Kummer, and Hall (12,13,14). SCG does not produce ethyl-substituted carbon chains, which were subsequently found to be present in about the same concentration as dimethyl species (15,16). 

It is generally accepted that, in the polymerisation of dienes on lanthanide catalysts, the growing chain is attached to the transition metal by an 7t-allyl bond and that the chain growth occurs by incorporation of the monomer via the metal-carbon o-bond. In the case of neodymium catalysts, the delocalised 7t-allyl type structure of the terminal unit has been observed by spectroscopic methods [8, 26, 28, 58-60]. The results reported in these papers show that the relative contents of cis-l,A- and tri2ns-1,4-units in polydienes depend on the type of solvent used, the polymerisation temperature, structure of diene monomer, and the composition of lanthanide-based catalysts. These data can be interpreted in terms of the concept of isomerisation equilibrium between anti- and syn-forms of n-allyl terminal unit. One of the arguments in favour of the existence of this isomerisation... 

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