Alkanes Summary

A molecular mechanics force field should properly describe molecules in general, with experimental accuracy. It is an excellent tool for showing us what we do not know about chanistry. When the force field does not correctly calculate molecular properties, it indicates either that we simply have used an inadequate force field or the wrong parameters, or that we do not fully understand what is occurring. 

The MM4 force field for alkanes does pretty much what we wanted it to do. It gives us to within chemical accuracy just about everything with which we have tried to deal. Are their further refinements that could be made There are three that we know of, which are (1) the butane barrier, (2) the tilt of methyl groups at the end of alkane chains, and (3) the accuracy of vibrational spectra. 

One item that does seem to be of perhaps more importance concerns the vibrational spectra. MM4 has a root-mean-square (rms) error of about 25 cm in overall vibrational calculations for a test set of alkanes. (This corresponds to only 0.07 kcal/ mol.) Spectroscopists commonly claim that they can measure vibrational frequencies down to 1 cm without difficulty (in fairly small molecules), and numerous spectroscopic force fields in the literature indicate that they can often calculate these frequencies with average errors of 10cm or less. We have not worked on this problem very seriously. The MM4 force field is designed fo calculate a great many different quantities 

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At this writing (2007-2009) we can do calculations on alkanes (and thus on practically everything else) with slightly better accuracy than that given by MM4 because of the continuing availability of more accurate experimental and computational data on molecules (previously discussed under Alkanes Summary in Chapter 4). But the improvements that could be made are not judged as sufficient to warrant the effort required to prepare a new molecular mechanics force field. 

Tabular summaries of the lUPAC rules for alkane and alkyl group nomenclature appear on pages 96-98... 

Summary of lUPAC Nomenclature of Alkanes and Cycloalkanes (Continued)... 

In summary then the chlorination of alkanes is not very selective The various kinds of hydrogens present m a molecule (tertiary secondary and primary) differ by only a factor of 5 m the relative rate at which each reacts with a chlorine atom... 

A brief summary of safety and health hazards follows detailed health hazards, however, should be obtained from producers by requesting Material Safety Data Sheets. Proper protective equipment and exposure hazards should be noted before handling any alkan olamine. Detailed toxicological testing is found in the CTEA Chemical Ingredient Review Board Reports on ethanolamines and isopropanolamines (24). 

In summary a few "generalizations" have been found. First, size selective chemistry is strongly associated with chemisorption that requi res bond-breaking. Second, metal clusters react rapidly with ligands that molecularly chemisorb even when the eventual products involve dissociation of the ligand. Dehydrogenation of Cg-alkanes on small platinum clusters take exception to this. 

Figure 8 Summary of the reactions of osmium atoms with alkanes and alkylbenzenes. Osmium atoms were co-condensed with the indicated substrates at...

In summary, the advances of the past few years have well demonstrated both the challenges and the promise of the Pt(II)/Pt(IV) redox couple for alkane functionalization. It should also be mentioned that the emerging conceptual understanding of the elementary steps involved in this process has also contributed to the development of methods to activate and functionalize alkyl groups in complex organic molecules. 

The choice of appropriate reaction conditions is crucial for optimized performance in alkylation. The most important parameters are the reaction temperature, the feed alkane/alkene ratio, the alkene space velocity, the alkene feed composition, and the reactor design. Changing these parameters will induce similar effects for any alkylation catalyst, but the sensitivity to changes varies from catalyst to catalyst. Table II is a summary of the most important parameters employed in industrial operations for different acids. The values given for zeolites represent best estimates of data available from laboratory and pilot-scale experiments. 

Summary of octane number aromatics, alkenes, and alkynes > cyclic alkanes and branched alkanes > straight-chain alkanes. 

The first chapter concerns the chemistry of the oxidation catalysts, some 250 of these, arranged in decreasing order of the metal oxidation state (VIII) to (0). Preparations, structural and spectroscopic characteristics are briefly described, followed by a summary of their catalytic oxidation properties for organic substrates, with a brief appendix on practical matters with four important oxidants. The subsequent four chapters concentrate on oxidations of specific organic groups, first for alcohols, then alkenes, arenes, alkynes, alkanes, amines and other substrates with hetero atoms. Frequent cross-references between the five chapters are provided. 

This paper is a summary of our current understanding of this system. In particular, we will be discussing the observations in terms of selectivity with respect to the availability of reactive lattice oxygen. The organization of the paper is as follows. First, the general features of the reaction scheme for alkane oxidation on vanadate catalysts will be presented. This is followed by a discussion of results on the effect of ease of removal of oxygen from the lattice on the selectivity, and then a discussion on the importance of the atomic arrangement of the active sites. 

In summary, we can conclude that at moderate salt concentrations typical for seawater ( 0.5 M), salinity will affect aqueous solubility (or the aqueous activity coefficient) by a factor of between less than 1.5 (small and/or polar compounds) and about 3 (large, nonpolar compounds, n-alkanals). Hence, in marine environments for many compounds, salting-out will not be a major factor in determining their partitioning behavior. Note, however, that in environments exhibiting much higher salt concentrations [e.g., in the Dead Sea (5 M) or in subsurface brines near oil fields], because of the exponential relationship (Eq. 5-28), salting-out will be substantial (see also Illustrative Example 5.4). 

A more detailed interpretation of the chemistry of catalytic cracking was based on studies with pure hydrocarbons.121-123 A simplified summary put forward by Heinemann and coworkers123 (Fig. 2.1) shows how Cg open-chain and cyclic alkanes are transformed to benzene by the action of both the hydrogenating (metal) and acidic (halogenated alumina) functions of the catalyst. 

The initial coordination of reactants has indeed been proposed to explain the selective oxidation of alkenes in the presence of saturated hydrocarbons. It was argued that, owing to the hydrophobic nature of titanium silicates, the concentration of both hydrocarbons inside the catalyst pores is relatively high and hence the alkenes must coordinate to TiIv. Consequently, the titanium peroxo complex will be formed almost exclusively on Tilv centers that already have an alkene in their coordination sphere, and will therefore oxidize this alkene rather than an alkane which may be present in the catalyst (Huybrechts et al., 1992). Objections to this proposal are based on the fact that the intrinsically higher reactivity of alkenes with respect to saturated hydrocarbons is sufficient to account for the selectivity observed (Clerici et al., 1992). But coordination around the titanium center of an alcohol molecule, particularly methanol, is nevertheless proposed to explain the formation of acidic species, as was previously discussed. In summary, coordination around Tiiv could play a more important role than it does in solution chemistry as a consequence of the hydrophobicity of the environment where the reactions take place. 

In summary, the oxidation of alkanes with iron-based catalysts remains a challenging task. Much progress has been made but the field is still far from mature. Higher efficiencies in the transformation ofhydrocarbons to alcohols and ketones are desirable. 

In summary, Raney Ni desulfurisation of the polar fraction of the Northern Apennines Marl further supports the presence of (poly)sulfide-linked phytanyl, docosanyl and cholestanyl moieties (some of them with additional intramolecular sulfur linkages) in the resin fraction as proposed from the pyrolysis experiments. In addition, a number of other structural units are revealed e.g. pentakishomohopane, carotenoids, n-alkanes and isoprenoid alkanes. The reason why these structural units are not revealed by the pyrolysis experiments may be (i) their much lower relative abundance (e.g.9 other n-alkanes and isoprenoid alkanes), (ii) their attachment in the macromolecules by more than one (poly)sulfide linkage, which make their release from the macromolecule by flash pyrolysis unlikely ([Pg.522]

In summary, all the BCF oils analyzed showed very similar patterns for minor components and aromatics, and appeared to be related. Major compositional trends reflect variations in the amounts of normal alkanes. 

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