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WHAT IS PETR J. BENNETT HILL Sun Oil
Co., Morcus Hook, Po.
There i s an extensive literature of scientific papers on the chemical composition o f petroleum and our knowledge o f the subject is considerable. The purpose o f this paper i s to explore the gaps in our knowledge and to point out what is not yet known. There are some rather fundamental questions in hydrocarbon composition that we cannot
answer: Do the isoparaffins really fade out as molecular weight goes up? What does a typical molecule in lubricating oil look like? Where and how long are the side chains in the big molecules? Ignorance is even greater in the field of the sulfur and nitrogen compounds in petroleum. As to oxygen compounds, knowledge is practically nil.
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is what basic information is available and where extrapolation may be inadequate. One of the first problems is that of branched chain paraffins. It is known that the normal paraffins persist up into the high molecular weight ranges and show up in paraffin wax. The d a t a also show that in most crudes in the Cg to C8range the branched chain paraffins are almost as abundant as the straight chain. But there is considerable evidence that they gradually disappear in the higher molecular weights. Is this in fact the case? D o they disappear completely, or is a compound like 2-methylpentacosane simply lost in the normal paraffins? D o the paraffin wax fractions of lower melting point than the normal paraffins of the same molecular weight owe their lower melting point entirely to naphthene rings, which are known to be present in the fraction, or do they contain branched chain paraffins as well? 4 closer look should be taken a t naphthene rings in paraffin wax. ,4s is the case with all the high molecular weight hydrocarbons, only speculations can be made since no such compounds have been isolated and identified. Hydrocarbon type analysis, however, shows them to be present. If a long chain with a cyclopentyl or cyclohexyl substituent is postulated, it is conceivable t h a t this may occur any place in the chain. I s this what should be expected, or is it rather to be expected a t the end of the chainthat is, in the I-position? I n other words. should such a compound be thought of as a naphthene ring with one long straight chain substituent? Still more complex problems are encountered in the liquid high molecular weight fractions (heavy gas oil and lubricating oil) where rings become even more important. among the ring compounds, consideration might first be given to the aromatic hydrocarbons-those containing one or more aromatic rings. There is more information on these hydrocarbons, particularly in the lower ranges, since they are the easiest to Beparate sharply from the other hydrocarbon series. Bmong the benzene derivatives the data through Cg indicate a greater prevalence of short chains than of longer ones. There is more of the trimethylbenzenes than of the methylethyls. If this concept were extended to the CIOand Cli benzene derivatives, it could be concluded, for example, that in the CIOrange the tetramethylbenzenes would predominate. Williams, Hastings, and Anderson ( I S ) , however, find the following distributions of the CIo benzenes in a virgin naphtha:
I F T Y years ago there was reasonable satisfaction with what was known about the composition of petroleum. College organic chemistry textbooks of t h a t time contain some very illuminating statements, of which the following are typical :
1. American petroleum consists almost exclusively of normal paraffins; yet minute quantities of some of the benzene hydrocarbons (cumene and mesitylene) appear to be preseht. 2. The following paraffins have been obtained from crude petroleum: n- and isopentane, n-hexane and an isomer, and nheptane, all these being present in the so-called gasoline which is obtained by the distillation of petroleum and used for carburetting coal gas. I n those days the constitution of petroleum could not have been part of an “unsolved problems’’ symposium. But as time passed and questions arose as to why different gasolines knocked differently in a n engine or why cracking processes behaved as they did, the realization was reached t h a t our knowledge of petroleum composition contained too many Fide-open gaps. The last 25 years have answered a lot of questions but, as is so characteristic of research, each answered question has posed at least three more, requiring answers.
Hydrocarbons Because petroleum consists mostly of hydrocarbons, any analysis starts here. Over the years much knowledge of the hydrocarbons in petroleum has been accumulated through the work of a great number of individual researchers and through the mass attack of API Project 6 under F. D. Rossini at the S a tional Bureau of Standards and at the Carnegie Institute of Technology, The synthetic work of A P I Project 42 under R. JT. Schiessler a t Pennsylvania State College, of API Project 45 under C. E. Boord at Ohio State University, and of many others has been of tremendous value in permitting identifications, as has also been the critical scrutiny of the physical property data by API Project 44 under Rossini. Despite these advances in knowledge, however, the fact persists t h a t definite isolations of specific hydrocarbons are confined almost entirely to those of low molecular weight. As a matter of fact, except for normal paraffins, no individual hydrocarbon beyond CIZhas as yet been isolated from petroleum and identified. Attempts have been made to extrapolate existing knowledge into the higher molecular weight range, guided by general information from what is called “hydrocarbon-type analysis.” By this method estimates can be made of the average distribution of the carbon atoms between aromatic rings, naphthene rings, and paraffin side chains, and indications can be obtained as to the type of ring structure. These extrapolations are, however, of doubtful validity, and possibly no two interpretations would lead to exactly the same conclusion. The first point to consider 1398
One side chain Two side chains Three side chains Four side chains
284% 5.6% 3.4% 0.6%
Their analysis extended only through 380” F. and the 0.6% figure for the tetramethyls is therefore incomplete, since all three
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isomers boil above this point. It is still interesting, however, t h a t the greatest prevalence is of two side chains-that is, the methylpropyls and the diethyls-for which there are most isomers. What will be the optimum number of side chains in the benzene hydrocarbons in the even higher molecular weight ranges? What is the optimum number of substituents on the naphthalene structure or on the phenanthrene structure? Where more than one aromatic ring is present they might be separated as in diphenylmethane, directly linked as in biphenyl, o r condensed as in naphthalene. Although actual isolation has not gone high enough in these molecular weight ranges to establish the trend, the evidence from other sources indicates t h a t condensed rings are the prevalent structures. There is some evidence, however, that diphenyl derivatives, or even totally discrete rings, may be present. This point needs to be established if, for example, something is t o be known about the structure of our lubricating oil molecules. What condensed structures obtain beyond two rings? Do the petroleum hydrocarbons follow the coal t a r pattern of phenanthrene and chrysene rather than anthracene and naphthacene? The evidence to date is t h a t they do. Are there in virgin petroleum highly condensed structures like pyrene, which are indicated to be present in cracked products? The naphthene hydrocarbons in petroleum comprise what is probably the greatest field of ignorance. They are rather well known through CS,but only a few Cg’s have been isolated and identified 80 far and nothing beyond that. Until recently it was believed t h a t they consisted of the cyclopentane and cyclohexane derivatives. Now Project 6 (IO) has found cycloheptane. True, the quantity is small, but so too is the quantity of cyclopentane compared with its homologs. May not the higher homologs of cycloheptane be similarly important? Also, if cycloheptane, why not cyclobutane and cyclo-octane? Why not even spiropentane? Returning to the five- and six-carbon ring naphthenes, Project 6 has shown that in the CSto C, range the relative proportions of the two series vary widely from crude to crude (5). B u t in any one crude i t is still impossible as yet to establish any trend as t o the general prevalence of one series over the other in the bigger molecules. It is interesting to note from the Project 6 data on their exhaustively analyzed crude, that, whereas in the C, range there is more cyclopentanes than cyclohexanes, in the Cs range there is nearly twice as much cyclohexanes as cyclopentanes. Granting that there are not enough data to establish a trend, the question is raised whether the five-carbon ring is actually as important in the larger molecules as has been generally assumed. Perhaps the five-carbon ring may fade out, just as it is suspected the branched chain paraffins do. The only condensed ring naphthene that has been isolated and identified is trans-decalin. Others are assumed to be present, including such compounds as hydrindane derivatives, but about the only basis so far for these assumptions is t h a t the corresponding condensed aromatic and naphthene ring compounds, derivatives of indan (hydrindene) and tetralin, have been isolated. How far is it possible t o reason by analogy from the aromatic hydrocarbons into the naphthenes? Project 6 (8) from their investigations of the lubricating oil range ( C Z ~ drew ~ O the ) conclusion that all molecules contained at least one naphthene ring, whether or not aromatic rings were present. If this conclusion is correct, another interesting problem arises. I n the lower ranges, up to Clz, simple aromatics are present-that is, hydrocarbons containing the benzeneor naphthalene ring structure with paraffin side chains. If among the larger molecules there are no such compounds, where do they cease to be present? As questioned earlier, how important are cyclopentane rings in these molecules? As t o the side chains, i t is known that they constitute a considerable portion of the molecule, but there is no knowledge whatever as t o their length or number, much less as t o their structure.
In a Tetralin structure, for example, are the side chains more a p t to be attached t o the naphthene ring or t o the aromatic ring? A lubricating oil molecule having thirty carbons and two aromatic and two naphthene rings might look like any one of the pictures in Figure 1 or any one of a vast variety of other combinations, and so far there is no assurance as t o which predominates. The answering of this question will require long and tedious work. Among the means available are the following: 1. Actual isolation and identification of hydrocarbons in this range. (This appears t o be a long way off.) 2. The isolation and identification of compounds in a lower range but enough higher than those now isolated to make extrapolations more plausible. 3. Correlations between physical properties and constitution. 4. Spectroscopic data. 5. Synthesis of possible structures and comparison of properties with isolated narrow fractions.
These are the methods which are being used all the way through the range and b y which gradual closing in on the problem is being accomplished. The next ten years should reveal much. CHI
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Figure 1.
Possible Types of Structure of Lubricating Oil Molecules
Before leaving hydrocarbons, olefins should be mentioned. It used to be stated categorically that crude petroleum contained no olefins. But the recent evidence ( 4 , 5 ) t h a t certain crude petroleums do contain molecules with olefinic linkages is too strong to be ignored. How far this will further complicate the picture is unknown. Asphalt
There is available a great deal of empirical information on asphalt and its rather arbitrary separation into asphsltmes, petrolenes, etc. Basic knowledge of composition, however, is such t h a t this topic cannot with any degree of certainty be put under hydrocarbons, sulfur compounds, or oxygen compounds. It is known (6) t h a t the asphaltenes are very high molecular weight (2500-5300) substances with a high carbon t o hydrogen ratio, but t h a t is about as far as it is possible t o go on asphalt composition. It is still a n unsolved problem. Sulfur Compounds
Sulfur occurs in petroleum in amounts varying from 0.1 t o 7.5%. The average sulfur content of U. S. petroleum is about 0.65%. Work on the sulfur compounds has not been popular, probably largely because most laboratories, both industrial and academic, do not wish t o put up with their foul odors. Then, too, the instability of the sulfur compounds, especially in the presence of air, makes research difficult. Despite these troubles a great
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deal of scattered work has been done on the subject, but still very little is known about what the sulfur compounds are. A new concentrated attack on the problem is being made by API Project 48A under the leadership of H. M. Smith of the Bureau of Mines, and excellent work is being done. So far, however, the surface has really just been scratched. The presence of elemental sulfur in certain crudes is pretty well established (8). I n the naphtha fraction, where knowledge is greatest, various thiols (mercaptans) and sulfides, both open and closed chain,
Metallic Constituents
&
CHOLESTEROL
HO
&CH
