Hydroaromatic structures aromatization

Dehydrogenation (the conversion of alicycllc or hydroaromatic compounds into their aromatic counterparts by removal of hydrogen and also, in some cases, of other atoms or groups) finds wide application in the determination of structure of natural products of complex hydroaromatic structure. Dehydrogenation is employed also for the s)mthesis of polycyclic hydrocarbons and their derivatives from the readily accessible synthetic hydroaromatic compounds. A very simple example is the formation of p-methylnaphthalene from a-tetra-lone (which is itself prepared from benzene—see Section IV, 143) ... 

I object to the use of the term alicyclicity in this connection. The methods used by Peover, Wender, and Fuks are selective for that group of alicyclic substances capable of yielding aromatic structures on dehydrogenation—i.e., for hydroaromatic rings. If the sulfur method really dehydrogenates any alicyclic structures (e.g. cyclooctane or camphene), then it would yield olefins rather than aromatics and could probably also convert saturated chains to olefins. On the other hand, if it attacks only hydroaromatic structures, then alicyclicity is an incorrect and misleading expression, and hydroaromaticity should be used. 

The process shown is thermal cracking and simultaneous hydrogen disproportionation, leading to aromatization of the hydroaromatic structure. A hydroaromatic unit was used in this example because such units are believed to have a predominant role in the coal structure. 

The formation of the coal extract was interpreted as a free-radical chain reaction leading to the depolymerization of coal and aromatization of some of the hydroaromatic structures. It was suggested that phenanthrene possibly plays the role of a chain carrier in this process. 

Reduction apparently creates fresh hydroaromatic structure (at the expense of the aromatics), and thus the methyl groups attached to the aromatic structures are likely to become amenable to quantitative estimation (by the Kuhn-Roth procedure), provided that the particular aromatic ring is reduced to hydroaromatic. Significantly, the reduced samples of the lower rank coals did not yield much higher values for methyl groups than the original samples. Thus, it would appear that Kuhn-Roth estimation does not completely measure the true C-methyl content in coals, especially in high rank coal samples. 

The apparent decrease in values of C-methyl content with increase in rank observed earlier (14) would now appear to be caused largely by progressive aromatization of methyl-substituted hydroaromatic structures rather than demethylation. Since the Kuhn-Roth method has limitations, an alternative method for assessing the C-methyl content in coals is desirable. 

It is evident, therefore, that the aromatic carbon alone yields coke, and hydroaromatic carbon yields tar. Since neither appears to contribute substantially to the formation of gases (during the low temperature pyrolysis), it seems certain that the gases of low temperature pyrolysis owe their origin largely to the aliphatic structure in coal. At least it is now certain that methane formation is quite independent of the aromatic and hydroaromatic structures in coal. 

Molecular Weight. Aromatization of hydroaromatic structures, rather than removal of aliphatic substituents, was suggested because the weight per molecule remained approximately constant with increasing temperature up to 900°C. The shift to solid products at 1100°C probably represents a significant increase in molecular weight. 

From the table we see that no change in the H/C ratio took place in this time. Work at Mobil (1, 2), Exxon (7, 8), and Oak Ridge National Laboratory (9) indicate that none of the following reactions takes place under the liquefaction conditions described above hydrogenation of aromatic polycyclic hydrocarbons significant aromatization of the hydroaromatic structures or destruction or formation of polycyclic saturated structures. 

Early studies involved pyrolysis of this residue and revealed the presence of aromatic ring systems such as benzene, naphthalene, their alkyl derivatives as well as higher aromatic hydrocarbons. Today, GC-MS analyses of super-critical fluid extracts of hydrous pyrolysates (77), Ha-pyrolysis products (72) as well as solid-state C-NMR spectroscopy (75) of meteorite organic residues are applied to provide insight into the structure of the macromolecular carbon. Most recent, hydrothermal treatment (300 °C at 10 MPa) of demineralized lOM of the Murray meteorite has yielded in the release of a wide variety of carboxylic acids and heteroaromatic compounds including C3-C17 alkyl carboxylic acids and N-, O-and S-containing hydroaromatic and aromatic compounds (74). 

A majority of the proposed models suggest coals to consist of several ring aromatic and hydroaromatic structural units, cross-linked through aliphatic and ether bridges to form the three dimensional structure and in the pores and cavities of this structure reside weakly linked smaller molecules which are easily extracted by solvents. The model proposed by Solomon and Shinn are representative examples of this macromolecular model. 

Hydrogen donors are, however, not the only important components of solvents in short contact time reactions. We have shown (4,7,16) that condensed aromatic hydrocarbons also promote coal conversion. Figure 18 shows the results of a series of conversions of West Kentucky 9,14 coal in a variety of process-derived solvents, all of which contained only small amounts of hydroaromatic hydrocarbons. The concentration of di- and polyaromatic ring structures were obtained by a liquid chromatographic technique (4c). It is interesting to note that a number of these process-derived solvents were as effective or were more effective than a synthetic solvent which contained 40% tetralin. The balance between the concentration of H-donors and condensed aromatic hydrocarbons may be an important criterion in adjusting solvent effectiveness at short times. 

More informative, perhaps, are the marked differences in relative FI signal intensities between the mass spectra shown in Figures 2c and 2d. In agreement with the previously mentioned results reported by Chakravarty et al (15) and Yun et al (16). the mass spectra of the low temperature component (Figures 2b and c) appear to be dominated by homologous series of aromatic and hydroaromatic compounds. Chemical identification of many of the compounds up to MW 350 has been accomplished by high resolution GC/MS (19, 20). although precise identification of the many possible isomeric structures involved will have to await the availability of suitable reference compounds. 

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