Hydrocarbons precursors

Simple and Complex Organic Molecules. Using modem direct fluorination technology, the synthesis of even the most complex perfluorocarbon stmctures from hydrocarbon precursors is now possible. For example, syntheses of the first perfluoro crown ethers, perfluoro 18-crown-6, perfluoro 15-crown-5, and perfluoro 12-crown-4 (54) have been reported. Perfluoro crown ethers (54,55) are becoming important as the molecules of choice for many F-nmr imaging appHcations (56) in humans and are particularly effective in brain and spinal diagnostics when... 

Direct fluorination involves the treatment of an appropriate hydrocarbon precursor dissolved in an inert Hquid with fluorine gas to yield a perfluorinated precursor to a long-chain carboxyflc acid. Equations 2 and 3 illustrate the process for perfluorooctadecanoic acid (17). 

Most non-graphitic carbons are prepared by pyrolysis of organic polymer or hydrocarbon precursors at temperatures below -1500 °C. Further heat treatment of most non-graphitic carbons at temperatures from -1500 to -3000 °C makes it possible to distinguish between two different types of carbons. 

Fig. 9. Pulse microreactor system for use with 13C-labeled hydrocarbons. D, E, and J are microreactors J contains the catalyst to be used for hydrocarbon skeletal reaction D and E are used, when necessary, to generate the required reactant hydrocarbon from a non-hydrocarbon precursor (e.g., alcohol dehydration in D and olefin hydrogenation in E) reactant injected at C. F is a trap which allows the accumulation of products from several reaction pulses before analysis G is a G.P.C. column, K a katharometer. Traps H collect fractions separated on G for subsequent mass spectrometric study. When generating reactant hydrocarbon in D and E, a two-step process is preferable in which, with J below reaction temperature, the purified reactant hydrocarbon is collected in H, and this is recycled as reactant with D and E below reaction temperature but with J at reaction temperature. After C. Corolleur, S. Corolleur, and F. G. Gault, J. Catal. 24, 385 (1972).

Thus, the 1,2-C pathway dominates when 84 is generated by the thermolysis of a tosylhydrazone salt, but a 1,2-H shift to cyclopropylethene is the major pathway when 84 is generated from either a hydrocarbon precursor or via the atomic carbon abstraction of oxygen from cyclopropylmethylketone at —196°C.U1... 

In the absence of sulfur dioxide, aerosol formation depends strongly on the type of hydrocarbon precursor studied. The following qualitative trends are observed ... 

Grosjean, D. Secondary Organic Aerosob and their Gas Phase Hydrocarbon Precursors. Abstract ENVT 2. In Abstracts of Papers. Centennial ACS Meeting. American Chemical Society, New York, April 4-9, 1976. 

For the measurement of the hydrocarbon precursors of photochemical oxidants, the naturally occurring methane must be separated from the other so-called nonmethane hydrocarbons. Only one procedure, gas chromatography coupled with flame ionization detection, is available for this separation and measurement. Although instrumentation for routinely accomplishing this process is commercially available, its maintenance (continued operation) requires a degree of operational know-how that may be too costly for most public agencies in the United States to support. Consequently, the data currently are insufficient to relate the occurrence of photochemical oxidants and ozone accurately to some of their most important precursors, the nonmethane hydrocarbons. 

Simple HUckel calculations predict the resonance energy of the benzocyclopropenyl cation (273) (3.65 P) to be higher than that of the tropylium ion (274) (2.99 P) and that of the cyclopropenyl cation (275) (2.00 P). Recent ab initio calculations at the 3.2 IG level attribute the same stability to 273 and 275 with respect to their hydrocarbon precursors The enthalpy change in going from 1 to... 

The authors show that hydrocarbon precursors 38-40 of biradical 37 react with O2 at elevated temperature to give the cyclic peroxides 41 (Scheme 5.8). 

All this is not to say that dialkylcarbenes are incapable of the reactions formerly attributed to them exclusively. They often—usually—are able to do the reactions, even in cases in which diazo compound chemistry pre-empts their doing so. For example, homocubylidenes (65) are not the first-formed intermediates from the diazohomocubane precursor (66). The bridgehead alkenes, homocubenes (67) are. However, it was possible to use a complex kinetic analysis involving both the pyridine ylide technique and the alternative hydrocarbon precursor 68 to show that in the parent system the two reactive intermediates 65 and 67 are in... 

Earlier Banks had commented The yield of required fluorocarbon decreases as the molecular complexity of its hydrocarbon precursor increases, and it is difficult to fluorinate hydrocarbons above C1(l without extensive decomposition occurring."... 

In later experiments, Izumi et al. (47, 48) examined aerosol formation during photooxidation of a variety of hydrocarbons in an evacuable smog chamber. No seed particles were used in these experiments, but good estimates of the yield of aerosol from photochemical oxidation of the hydrocarbon precursors were obtained by using EAA data. In some cases, the volumetric yield was found to decrease with decreasing precursor concentration (Figure 4), so the finite vapor pressure of the reaction products limited nucleation, particle growth, or both. 

Another series of compounds, the fluorine-substituted 2,2,4,4-tetra-methylpentanes, have proven to be a good artificial blood candidate (69). The compounds are prepared by the direct fluorination of the hydrocarbon precursor (70a) (see Fig. 15). The overall yield of the reaction can be seen from the figure to be in excess of 99%. The central protons are retained preferentially during the fluorination, a result that is attributed to steric rather than electronic factors (70a). 

The aliphatic alicyclic hydrocarbon precursor is not well recognized as the major aliphatic component in dissolved humic substances, although it was previously postulated to occur (II). This precursor might arise from terpenoid hydrocarbon lipids, but the data presented in this chapter favor polyunsaturated lipid precursors that are oxidatively coupled and cyclized by free-radical mechanisms (20). Degradative studies have not identified this aliphatic component in recognizable fragments. The quantitative, structural-model approach presented here combines the results of 13C NMR, NMR,... 

The continental biosphere is a large source of hydrocarbons. Quantification of these sources in toms of geophysical (e.g. temperature, humidity, light levels) and biogeochemical (soil physical and chemical properties, land use) parameters is much needed for inclusion in atmospheric models. The hydrocarbon oxidation mechanisms in the atmosphere should also be better understood, so that formation of ozone, carbon monoxide, partially oxidized gaseous hydrocarbons, and organic aerosol can be better quantified. The formation of organic aerosol from hydrocarbon precursors and then-capability to serve as cloud condensation nuclei are issues which need to be studied in depth. 

Schleyer, Olah, and co-workers.55 In this method, the sums of all 13C chemical shifts of carbocations with their respective hydrocarbon precursors are compared. A trivalent carbocation has a sum of chemical shifts of at least 350 ppm higher than the sum for the corresponding neutral hydrocarbon. This difference can be rationalized by partly attributing it to the hybridization change to sp2 and to the deshielding influence of an unit positive charge in the trivalent carbocation. Since higher coordinate carbocations (nonclassical ions) have penta- and hexa-coordinate centers, the sum of their chemical shifts relative to their neutral hydrocarbons is much smaller, often by less than 200 ppm. 

Many of these substances have substantial commercial significance as surfactants. The wide range of compounds found In any given surfactant creates a challenging problem If one wishes to analyze It. The nature of the mixtures Is often obscure as the raw materials are Invariably mixtures of fatty acids, alcohols, or hydrocarbon precursors. Lyon et al. (8) have demonstrated that FAB combined with MS/MS can be used to deal with these mixtures. 

Table IV. Kinetic Parameters for the Loss of Thiophene and Hydrocarbon Precursors from Monterey Fm. Kerogens...

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