Hydroaromatic structures

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. 

In another set of experiments the effect of prior dehydrogenation on the yields of gases (especially that of methane) was studied. As shown earlier, dehydrogenation with either sulfur (17, 18) or iodine (22) leads to the complete inhibition of tar formation and fixation of the corresponding carbon (alicyclic) in char. It is thus possible to study the contribution, if any, of the hydroaromatic structure towards gas formation by partial or complete fixation of the hydroaromatic carbon in char. 

However, the considerable decrease in methane yield in the above case does not indicate the contribution of the hydroaromatic structure towards methane formation. It has been established (19, 23) that complete or even a partial dehydrogenation inhibits tar formation completely (though a minimum dehydrogenation is necessary for this effect) and retains all the hydroaromatic carbon in the char. The decrease in methane yield on pyrolysis of... 

All these observations indicate that hydroaromatic structures do not as such contribute significantly to methane formation. Evidently this is also true for the amount of carbon dioxide and carbon monoxide found in low temperature pyrolysis gases. 

The fact that the freshly created hydroaromatic structure in reduced coals contributes little to the formation of gases (except for some unsaturates, Table V) is perhaps also evident from the correlation shown earlier between the yield of extra tar and the extra hydroaromatic carbon created by reduction (12). These results are recast in Figure 3 to show more effectively the pyrolysis characteristics of reduced coals and hence throw light on the formation of gases. 

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. 

The presence of a considerable proportion of methylated bodies in low temperature tar and its origin must be explained. The fact that the yield of methane remains largely the same even when tar formation is completely inhibited would indicate that the methyl groups of coal possibly do not participate in forming the methylated bodies in tar. It is not unlikely, therefore, that such methylated bodies in tar are synthesized during pyrolytic reaction of the hydroaromatic structure (via methylenes). 

Coals are macromolecular, —i.e., low rank coals, at least, appear to be able to absorb certain molecules such as methanol and hydrocarbons. In low and medium rank coals the ultimate units are linked by chemical and physical forces, and in high rank coals physical forces predominate. The presence of hydroaromatic structures in low rank coals must lead to rather distorted frameworks. Although it is not difficult to visualize that spaces exist in which foreign... 

Accurate prediction of "ring strain" in hydroaromatic structures is not possible at present (28a), so available hydrogenation or other direct determinations should generally be used in preference to estimated values. This can be a very troublesome source of uncertainty. 

This reaction could cause homogeneous equilibration of certain hydroaromatic structures. 

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. 

A brief comparison of this data set with the results obtained for Illinois No. 6 is informative (2). For O-methyl Illinois No. 6, a high-volatile bituminous coal, values of 0.63 0.03, 1.7 0.1, and 2.6 0.1 methyl groups per 100 coal carbons were measured after three serial alkylations using 9-phenylfluorenyllithium, fluorenyllithium, and trityllithium, respectively. Although the two data sets for these bituminous coals demonstrate a similar preference for fluorenyllithium as base, and thus a significant number of C-H sites with 19 < pKa 22, they differ in that Illinois No. 6 contains important quantities of acidic C-H groups with pKa < 19 and 22 < pKa 31. Because the class of structures with 22 < pKa < 31 includes 9,10-dihydroanthracene and its analogues, we conclude that this type of six-membered hydroaromatic structure is absent, or at least below the detection limit, in PSOC 1197. 

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. 

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