Studies of the acidic components of the Colorado Green River Formation oil shale: Mass spectrometric identification of the methyl esters of extractable acids

Chemical Geology - Elsevier Publishing Company, Amsterdam Printed in The Netherlands

STUDIES OF THE ACIDIC COMPONENTS OF THE COLORADO GREEN RIVER FORMATION OIL S H A L E : M A S S S P E C T R O M E T R I C IDENTIFICATION OF THE METHYL ESTERS OF EXTRACTABLE ACIDS 1

P A T H A U G 2, H.K. S C H N O E S 3 and A.L. B U R L I N G A M E Space Sciences Laboratory, University of California,Berkeley, Calif. (U.S.A.) (Received July 27, 1970)

ABSTRACT Haug, P., Schnoes, H.K. and Burlingame, A.L., 1971. Studies of the acidic components of the Colorado Green River Formation oil shale: mass spectrometric identification of the methyl esters of extractable acids. Chem. Geol., 7: 213-236.

A study of solvent extractable acidic constituents of oil shale from the Colorado Green River Formation (ca. 50.106 years) is presented. Identification of individual components is based upon gas chromatographic and mass spectrometric data obtained for their respective methyl esters. Normal acids (C7-C12), isoprenoidal acids (C9, C10), ~ ,~0-dicarboxylic acids (C12-C18), mono-ot-methyl dicarboxylic acids (C13, C15) and methyl ketoacids (CI], C14) were identified. In addition, the presence of monocycllc, benzoic, phenylaIkanoic and naphthyl-carboxylic acids, as well as cycloaromatic acids, is demonstrated by partial identification. INTRODUCTION The organic m a t t e r of the Colorado G r e e n R i v e r F o r m a t i o n oil s h a l e , which is thought to be derived f r o m a q u a t i c o r g a n i s m s inhabiting a s e r i e s of f r e s h w a t e r lakes during the Eocene Epoch (ca. 50 million y e a r s ) , has been subjected to fairly extensive chemical investigation. Thus, r e c e n t r e p o r t s have established the o c c u r r e n c e of n o r m a l and branched s a t u r a t e d h y d r o c a r b o n s - including isoprenoidal compounds - in that sediment (Cummins and Robinson, 1964; Robinson et al., 1965; Eglinton et at., 1966a) and of polycyclic h y d r o c a r b o n s , such as s t e r a n e s and t r i t e r p a n e s (Burlingame et al., 1965; Hills et at., 1966; Henderson et at., 1968) and p e r h y d r o fl-carotene (Murphy et al., 1967). L e s s detailed, but nonetheless v e r y informative, studies on the porphyrin constituents of the shale have shown

1part XI inHigh-Resolutlon Mass Spectrometry in Molecular Structure Studies series. For part X, see Kupchan, S.M., Anderson, W.K., Bo111nger, P., Doskotch, R.W., Smith, R.M., Saenz Renauld, J.A., Schnoes, H.K., Burlingame, A.L. and SmRh, D.H., 1969. J. Org. Chem., 34: 3858. 2present address: Departments of Chemistry and Geology, Rice University, Houston, Texas (U.S.A.). 3present address: Department of Biochemistry, University of Wisconsin, Madison, Wisc. (U,S.A.).

Chem. Geol., 7 (1971) 213-236

213

the presence of homologous s e r i e s of alkyletioporphyrins, of carboalkoxy porphyrin derivatives, and of cycloalkyl- and alkylbenzoporphyrins (Morandi and Jensen, 1966; Baker et al., 1967). Results of preliminary investigations on other nitrogen containing compounds have also been published (Simoneit et al., 1970; Simoneit et al., 1971). Several repor t s have dealt with the fatty acid content of this sediment. Lawlor and Robinson (1965) described the seri es of homologous saturated acids from C10 to C34, and the occurrence of normal acids of more limited range has been reported by several other groups (Abelson and P a r k e r , 1962; Leo and P a r k e r , 1966). Eglinton and collaborators (Eglinton et al., 1966b; Douglas et al., 1969b)have shown the presence of isoprenoid fatty acids ranging from C14 to C21 (with the exception of the C18 acid, but including phytanic and nor-phytanic acids) and noted that the distribution of these acids paralleled that of the corresponding isoprenoid alkanes. More r e c e n t ly, high-resolution mass spectral studies of the mineral entrapped acids (Burlingame and Simoneit, 1968a; Burlingame et al., 1969a) and acids obtainable by oxidation of the kerogen matrix (Burlingame and Simoneit, 1968b; Burlingame et al., 1969a; Burlingame and Simoneit, 1969) have been reported from this laboratory. These papers report on the o c c u r r e n c e of a large number of different classes of Carboxylic acids, including isoprenoidal, aromatic and polycyclic acids. These studies were motivated p r i m a r i l y by the search for compounds and compound classes which appear distinctly r e lated to biological p r e c u r s o r s , from which some tentative inferences might be drawn on the nature and chemical composition of the ancient flora. Parallel analyses of hydrocarbon and acidic constituents of lower aquatic plants to obtain information on the distribution of these common metabolic products among present-day primitive organisms are essential for such correlative attempts. (For example, r e f e r to P a r k e r and Leo, 1965; P a r k e r et al., 1967; and Hart et al., 1968.) Equally important is the determination of the range of compound classes in a sediment and the recognition of minor, but perhaps unusual, compounds. A study of this kind can provide n e c e s s a r y data for the delineation of possible diagenetic pathways which in turn place the presumed biologically-derived compounds and their abundance in a much c l e a r e r perspective relative to likely p r e c u r s o r s . The work described her e constitutes a survey of the types of carboxylic acids extractable from the Green River Formation oil shale. About a dozen compound classes were recognized, most of which had not been report ed previously from sediments. Some of the results have been the subject of brief preliminary communication (Haug et al., 1967a,b, 1968). It is the purpose of this report to present a documentation of our experimental data and conclusions. EXPERIMENTAL All organic solvents were A.C.S. reagent grade and redistilled before use. Samples of the shale were collected from the side of a cliff at Parachute Creek, 8 miles northwest of Grand Valley, Colorado, latitude 39o37,N, longitude 108°7 ~¢, elevation 7,300 ft. After mechanical removal of the outer surface, 5.3 kg of shale was crushed (3-20 mesh) and the rock chips sonicated in benzene/methanol solvent ( 4/ 1 , v / v ) . Following this p r e 214

Chem. Geol., 7 (1971) 213-236

liminary extraction the r o c k was pulverized (ca. 100 mesh) and twice extracted (2 1 solvent p e r 500 g shale) with b e n z e n e / m e t h a n o l ( 4 / 1 , v / v ) while subjected to sonication and mechanical stirring. The resulting e x t r a c t was taken up in hexane to yield a total of 55 g of hexane-soluble m a t e r i a l . Acidic compounds w e r e obtained f r o m this e x t r a c t by t r e a t i n g 26 and 29 g (each dissolved in 500 ml of hexane) with 1 N sodium h y d r o x i d e (3 × 100 ml), r e s p e c t i v e l y . The aqueous solutions w e r e combined, b a c k extracted with 100 ml of hexane, filtered, acidified to pH 1 and e x t r a c t e d with hexane (3 × 50 ml). The e x t r a c t s w e r e dried_(MgSO4) and after e v a p o r a tion of solvent a total of 0.28 g of acidic m a t e r i a l was obtained. Half of t h i s m a t e r i a l (ca. 140 mg) was utilized in subsequent analysis: a hexane solution (10 ml) was e x t r a c t e d with sodium bicarbonate (3 × 25 ml) and washed with distilled water. The combined aqueous phases w e r e acidified to pH 1 - 2 and extracted with hexane (3 × 25 ml). The residue remaining a f t e r r e m o v a l o f solvent was t r e a t e d with m e t h a n o l / b o r o n trifluoride reagent (Applied S c i e n c e Lab.) and briefly refluxed. The solution was then concentrated, 3 ml of water added and e x t r a c t e d with hexane (3 × 5 ml). Combination of the h e x a n e extracts, washing with dilute base and water, and r e m o v a l of solvent y i e l d e d the e s t e r m i x t u r e utilized in subsequent analyses. Individual e s t e r s and fractions w e r e s e p a r a t e d by gas chromatography and m a s s s p e c t r a obtained using the following p r o c e d u r e : E s t e r s w e r e s e p a r a t e d by chromatography on a 10 ft. × 1 / 4 inch column (5% SE-30 on 80-100 mesh Aeropak, 50 m l / m i n He flow rate), p r o g r a m m e d at 2 ° / r a i n and all fractions collected (ca. 50) w e r e analyzed by low-resolution mass s p e c t r o m e t r y without f u r t h e r p u r i f i c a tion to provide a p r e l i m i n a r y s u r v e y of the types of acids present. F r a c t i o n s for high-resolution m a s s s p e c t r a l analysis w e r e obtained in s i m i l a r fashion. F o r the m o r e detailed investigation, the same column was p r o g r a m m e d a t 4 ° / m i n , and all fractions collected w e r e f u r t h e r purified by c h r o m a t o g r a p h y on a 6 ft. × 1 / 4 inch column (3% H I E P F - 8 - B P on 8 0 / 1 0 0 Gas C h r o m - Q , Applied Science L a b . ) p r o g r a m m e d at 6 ° / r a i n with a helium flow r a t e of 50 m l / m i n . All fractions collected w e r e then analyzed by m a s s s p e c t r o m e t r y . Data and r e s u l t s d i s c u s s e d below a r e based on this two-stage c h r o m a t o graphic separation scheme. ( E s t e r s a r e identified so as to c o r r e s p o n d to the labelling of the g.l.c, peaks of F i g . l ; i.e., e s t e r 13 fraction 2 is a component of g.l.c, peak 13 in F i g . l , "fraction 2" r e f e r r i n g to its elution a s the second peak on the H I E P F column.) The c h r o m a t o g r a m illustrated in Fig.1 was obtained under the following conditions: 10 ft. × 1 / 1 6 inch column, 3% SE-30 on 8 0 - 1 0 0 m e s h Aeropak 30, He flow r a t e of 30 m l / m i n , t e m p e r a t u r e p r o g r a m m e d at 2 ° / r a i n f r o m 50 ° to 280 ° (Varian, model 600 instrument). Except for the c h r o m a t o g r a m of F i g . l , gas chromatography was p e r f o r m e d with a Varian A e r o g r a p h A-90P3. Both low- and h i g h - r e solution m a s s s p e c t r a w e r e obtained with a C.E.C. 21-110B double f o c u s i n g m a s s s p e c t r o m e t e r using the following operating conditions: e l e c t r o n energy: 70 V; ionizing c u r r e n t : 100 gA. F o r low-resolution s p e c t r a m o s t compounds w e r e introduced via the d i r e c t insertion probe (with liquid nitrogen cooling, where n e c e s s a r y ) with the t e m p e r a t u r e of the ion s o u r c e at 80-100°; for high-resolution s p e c t r a , samples w e r e introduced through an a l l - g l a s s s y s t e m (ion s o u r c e t e m p e r a t u r e --200°). Complete h i g h - r e s o l u tion s p e c t r a w e r e r e c o r d e d on photographic plates ( n f o r d Q-2) which w e r e subsequently m e a s u r e d with an automated m i c r o p h o t o m e t e r s y s t e m (Burlingame, 1966) and data r e d u c e d to elemental compositions by c o m p u t e r Chem. Geol., 7 (1971) 213-236

215

50"

4

Fig.1.

•

•

is

Gas chromatogram

100"

I of Colorado

Green

150 °

River

Shale

esters.

~s

200

°

COLORADO

G R E E N RIVER

250

SHALE

°

ESTERS

(Smith, 1967). F o r lack of space, only some c h a r a c t e r i s t i c and r e p r e s e n t a tive examples of the mass spectra of these compounds a r e illustrated h e r e . For complete documentation of these spectra, the r e a d e r is r e f e r r e d to the work of Haug 41967). RESULTS Normal carboxylic acids

Five normal acids were isolated and identified as their methyl e s t e r s . These comprise the homologous s e r i es from C 7 (heptanoic acid) to Cll (undecanoic acid). Identifications ar e based on the typical mass spectral patterns (see Haug, 1967) of their respective methyl e s t e r s (i.e., the peaks of m / e 74, 87, 101, 115, etc., their molecular ions, and M-29, -31 and -45 peaks). In the chromatogram (Fig.1)~ these acids would be represented by peaks labeled 4 (C7-methyl ester), 8 (C8 ), 14 (C9) , 19 (C10)and 22 (Cll). Branched-chain acids

Two isoprenoidal acids and one ~-methyl-substituted saturated acid were isolated. The mass spectra on which these characterizations a r e based are presented in Fig.2A-D. The spectrum of Fig.2A (ester 10p fraction 1) suggests structure of methyl 2-methyloctanonte (MW 172) on the following grounds: The rearrangement ion at m / e 88 demands an ~ - m e t h y l substituent, suggested also by the relatively pronounced peak at m / e 115 (M-59). The compound is eluted after es t e r 8, fraction 1 (the C 9 isoprenoid~ see below) indicating a l e s s e r degree of chain branching. The structure of -methyl caprylic acid would satisfy these data. The spectrum of Fig.2B (ester 8, fraction 1), exhibiting a molecular ion at m / e 172, an intense rearrangement ion at m / e 88 (CH3-CHfC(OH)~), and enhanced peaks at m / e 101, M-15 ( m / e 157)and M-43 ( m / e 129), strongly suggests a C9-isoprenoidal skeleton and is in agreement with structure I - methyl 2,6-dimethylheptanoate. The p r e s e n c e of small amounts of a compound of MW=186 should be noted. A second isoprenoidal acid was identified as methyl 3,7-dimethyloctanoate (II). Its mass spectrum (Fig.2C - es ter 14, fraction 1) is in full accord with this assignment as indicated by direct comparison with the

/~J~.COOCH3 l

/Lv~v~COOCH3 11

authentic C10 isoprenoidal acid e s t e r 1 (Fig.2D). T h e s e t hree branched acids contribute to peaks 8 (C 9 isoprenoidal esterp I), 10 (C 9 ~ - m e t h y l substituted ester) and 14 (Clo isoprenoidal est er, II) in the gas chromatogram of Fig.1. 1We thank Professor James Cason for an authentic sample of the acid. Chem. Geol.. 7 (1971) 213-236

217

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300

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Fig.2. A. Mass spectrum of C9 isoprenoid ester. B. Mass spectrum of C9 branched ester. C. Mass spectrum of C10 isoprenoid ester. D. Mass spectrum of methyl 3,7-dimethyloctanoate.

IllI

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Oxo c a r b o x y l i c

acids

Two gas c h r o m a t o g r a p h i c fractions were obtained which by m a s s s p e c t r o m e t r i c a n a l y s i s could be identified as saturated oxo e s t e r s . One of these ( e s t e r 30, f r a c t i o n 2) exhibited a molecular ion at m / e 214 c o r r e s p o n d ing to a C l l oxo acid. The fragmentation pattern (see Fig.3A) with peaks at m / e 183 (M-31), 157 (M-57), 125 (M-57-32), 97, 87, 74, 69 and 58 is typical for methyl keto e s t e r s (Ryhage and Stenhagen, 1960) and defined its s t r u c t u r e as methyl 10-oxoundecanoate (HI). The second component (ester 40, fraction 5) showed a m o l e c u l a r ion at m / e 256 and the analogous fragmentation p a t t e r n - m / e 225 (M-31), 199 (M-57), 167 (M-57-32), 149 (167-18), 87, 74 and 58 (base peak) - which c h a r a c t e r i z e d this compound as methyl 13-oxotetradecanoate (IV).

HI: n = 8 IV: n = l l

0 0 IJ II C H 3 - O - C - ( C H 2) - C - C H 3 n

Confirmation of these assignments was provided by high-resolution m a s s s p e c t r o m e t r y which showed that the peaks at m / e 183, 157, 125 and 58 in the spectrum of HI (Fig.3A) c o r r e s p o n d e d to ions of composition CllH1902, C9I-I1702, C8H13 O, and C3H60, while the peaks at m / e 199, 167, 149 and 58 in the s p e c t r u m of IV calculated for CI2H2302, CllH19Op C l l H 1 7 and C3H60 ~ e r e in a g r e e m e n t with the postulated s t r u c t u r e . The m a s s s p e c t r u m of an impure sample was i n t e r p r e t e d as that of the C13 oxo-acid (MW of e s t e r = 242). The p r e s e n c e of the remaining homologue of this s e r i e s (C12 - M~tV=2$8 of the e s t e r ) was indicated by m a s s s p e c t r a l data on collected chromatographic fractions (Haug, 1967). T h e s e compounds could not be obtained, however, in p u r e f o r m for unambiguous c h a r a c t e r i z a t i o n . Compounds HI and IV a r e eluted as components of peaks 30 and 40 in the gas chromatogram of Fig.1. Dicarboxylic

acids

An interesting s e r i e s of compounds isolated from this shale s p e c i m e n is r e p r e s e n t e d by the g.l.c, peaks 38, 39 and 41 to 54 (see Fig.l). Mass s p e c t r o m e t r i c analysis of these fractions r e v e a l e d two s e r i e s - one r e p r e senting methyl e s t e r s of n o r m a l ~ , w - d i c a r b o x y l i c acids, the other a s e r i e s of dicarboxylic acids p o s s e s s i n g an a - m e t h y l substituent. Complete m a s s s p e c t r a of the n o r m a l dicarboxylic acid e s t e r s isolated from the s e d i m e n t are given by Haug (1967). Two typical examples a r e shown in Fig.3B and C, the f o r m e r (Fig.3B) r e p r e s e n t i n g the s p e c t r u m of authentic dimethyl 1,12dodecanedioate, while the l a t t e r (Fig.3C) shows the s p e c t r u m of a h o m o logue isolated from the shale - the C13 -dicarboxylic acid. The s p e c t r a of these compounds which have been discussed in detail (Ryhage and Stenhagen, 1959, 1964) are c h a r a c t e r i z e d by m o l e c u l a r ions of low intensity ( s o m e t i m e s barely detectable) and intense peaks at M-31 (CH30.)~ M-73 (CH3OCOCH2.), M-73-32 and the base peak at m / e 98, which may be r e p r e s e n t e d by ion V (Ryhage and Stenhagen, 1963, 1964). 220

Chem. Geol.. 7 {1971) 213-236

[~l

+"

OH v Our mass spectral r es ul t s indicate a homologous s e r i e s of normal saturated dicarboxylic acid e s t e r s (of general structure VI) ranging from C12 ( e s t e r 38) to C18 (ester 54).

CH30CO-(CH 2)n-COOCH 3 VI:

n

=

10-16

In the chromatogram of Fig.1 compounds represented by structure VI appear as peaks 38 (VI, n=10), 41, 43, 45, 48, 51 and 54 (VI, n=16), with the C13 and C14 acids (peaks 41, 43) as major components. A second, although very limited, s e r i e s of e s t e r s of dicarboxylic acids (C13 , C15 ) is represented by peaks 39 and 44 in Fig.1. T hei r mass spectra clearly indicate saturated dicarboxylic acids possessing a methyl substituent at one of the ~-positions. Thus, the spectrum of est er 39 shows ~,prominent peaks at m / e 241 (M-31), 199 (M-73), 185 (M-87), 112 (homologue of 98 - V), 98, 88 and 74 in agreement with structure VII for this compound.

c~s°°c~ ~ T CH300C~

A ~ ~ . . . . . C

O

O

COOr..,N3 C

H

VII 3 VIII

An analogous pattern is exhibited by the spectrum of e s t e r 44 (Fig.3D), which thus r ep res e nt s the C15-acid, i.e., dimethyl 2-methyltetradecane-1, 14dioate (VIII). Spectral evidence indicates the presence of the C16 homologue (MW=314) of this s e r i e s , although these data are less clear-cut. Several gas chromatographic fractions containing dicarboxylic acid were analyzed by high-resolution mass spectrometry and the composition of major ions is in agreement with the above assignment.

Aromatic acids The shale extract contains a considerable number of acids containing aromatic rings as part of their structure. Very few of these have been characterized completely, but our data pe r m i t the recognition of the st ructural type quite readily, since mass spectral fragmentation of simple a r o matics usually leads to abundant and easily recognizable ions. In addition, the elemental composition of molecular and fragment ions of these compounds was confirmed by high-resolution mass measurement. Among the aromatic acids the classes identified included: alkyl-substituted benzoic, phenylalkanoic and naphthoic acids and condensed cycloalkylaromatic acids, perhaps of the tetraline or indane carboxylic acid type. Chem~ Geol., 7 (1971) 213-236

221

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74

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250

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300

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ESTER

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PERCENT TrITl:lL |0NIZATI0~I

Benzoic acids

Several substituted benzoic acids could be isolated. Two i s o m e r s of molecular weight 150 (as their methyl esters), four of MW=I64 and two of MW=178 appear to be present. The mass spectra of the first two [ester 12, fraction 4 (mass spectrum Fig.4A), and es t e r 13, fraction 3] characterize them as methyl toluates. The patterns [M-31 ( m / e 119)and M-31-28 ( m / e 91)] and the lack of an appreciable M-32 peak ( m / e 118) defines these compounds as methyl p-toluate and methyl rn-toluate. A distinction between them is not easily possible, since the mass spectra of authentic compounds are almost identical (McLafferty and Gohlke, 1959); however, the compound eluted first (ester 12, fraction 4) should be the m - i s o m e r (IX) and the other, therefore, the P - i s o m e r (X). CH 3

IX

COOCH 3

×

Four isomeric members of the next higher homologue (MW=164) appear to be present. They are dimethyl derivatives of methyl benzoate and the relatively intense M-32 peak in all spectra (e.g., Fig.4B) suggest at least one methyl substituent ortho to the carboxyl group. T here a r e four such possible i s o m e r s (i.e., 2,3-; 2,4-; 2,5- and 2,6-dimethyl substitution). Comparison of their spectra (Haug, 1967) with published data (Aczel and Lumpkin, 1962) suggests that one of these compounds should be methyl 2,5dimethylbenzoate. The spectrum of another, est er 18, fraction 3, (Flg.4B), is very similar to that of methyl 2,4-dimethylbenzoate. The remaining isomers might r e p r e s e n t the compounds with 9.,6- and 2,3-methyl substituents, but no comparative data were available for these compounds. Two trimethylbenzoates were obtained. One of these probably r e p r e s e n t s methyl 2,4,5-trimethylbenzoate, since its spectrum compares v e r y well with published data for the authentic compound (Aczel and Lumpkin, 1962). This s er ies of alkyl benzoic acid methyl e s t e r s are components of g.l.c. peaks (Fig.l) 1~ and 13 (methylbenzoates), 16, 17 and 18 ~dimethylbenzoates), and 22 and 23 (trimethylbenzoates).

Substituted phenylalkanoic aczds

The methyl e s t e r s of this group of compounds range from molecular weight 192 to 234. We obtained one compound of molecular weight 192, at least four compounds of molecular weight 206, six cmnpounds of molecular weight 220, and at least two compounds of molecular weight 234. Our data (see Haug, 1967) do not permit definite structural assignments to any of these compounds. They do show, however, that all compounds of this group possess an acid side chain of t hr e e carbons or l arger and (usually) an alkyl substituted phenyl ring and, therefore, all conform to the st ruct ural pattern indicated very schematically by structure XI. 224

Chem. Geol., 7 (1971) 213-236

I

R = alkyl substituent. (s)

~

n > 2 (may be bPonched )

.~c ~):-coocH3 "l n

R XI

Since definitive structures cannot be assigned for this series solely on the basis of t h e i r mass spectra, only two typical mass spectra are presented. Fig.4C represents the mass spectrum of an ester of molecular weight of 206, the mass spectral fragmented pattern of which permits suggestion of the tentative structure XH. Authentic phenylvaleric acid methyl ester shows a s i m i l a r fragmentation pattern. The tentative s t r u c t u r e X]H is suggested for the ester spectrum in Fig.4D. 119

XII

119

(cH3) 2 XIII

F o r the other compounds of this s e r i es, structures similar to these, bearing alkyl substituents on the phenyl ring a n d / o r on the side chain, may be proposed. Specific structural proposals, however, appear too speculative to warrant extensive discussion at this time. All compounds of this class elute between peaks 9.5 and 34 of the gas chromatogram (Fig.l). The ester of molecular weight 192 contributes to peak 26; is o me r s of molecular weight 206 are components of peaks 25, 27 and 29; the group of molecular weight 220 appears as peaks 26-31, and the i s o mer s of molecular weight 234 contribute to peaks 32 and 34. Naphthoic acids and condensed cycloaromatic acids

Peak 33 of the gas chromatogram contains two i s o m e r s of methyl methylnaphthoate (general structure XIV). No definite substitution pattern can be assigned to them, but the presence of a considerable M-32 peak in the spectrum of Fig.SA suggests an ortho relationship of the methyl and carbomethoxy grouping, whereas the absence of this peak in the spectrum of the second component indicates one of the other possible substitution patterns. High-resolution mass spectra which showed that the peaks at m / e 900, 169 and 141 corresponded to ions of composition C13 H120 , C12H90 and C l l H 7, respectively, confirmed the assignment of general structure. High- and low-resolution spectra of crude fractions indicate that three homologues of~his s e r i e s - the compounds of MW=214, 228 and 242 ar e present in small quantities. I ~

t - COOCH3 CH3

xIV Chem. Geol.. 7 (1971) 213-236

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The last group of a r o m a t i c acids includes compounds of the m o l e c u l a r weight s e r i e s 204, 218, 232 and composition CnH2n_100 2. The m a s s and composition of the m o l e c u l a r ion and the pronounced loss of 15 m.u. suggests condensed c y c l o a l k y l a r o m a t i c s y s t e m s , such as alkyl (methyl ?)substituted indane or t e t r a l i n e - c a r b o x y l i c acid e s t e r s (see Haug, 1967). At least t h r e e compounds of this general class a r e p r e s e n t , including components of MW=204, 218 (Fig.SB) and 232. T h e s e compounds o c c u r in g.l.c, peaks 31, 33 and 34 (Fig.l). Saturated monocyclic carboxylw acids The m a s s s p e c t r a of cyclic acid e s t e r s a r e even m o r e difficult to i n t e r p r e t in p r e c i s e s t r u c t u r a l t e r m s than those of the a r o m a t i c e s t e r d i s cussed above. Substitution patterns and s t e r e o c h e m i s t r y can have d r a m a t i c effects on the fragmentation of the molecule (Khodair, 1965; Cason and Khodair, 1967a) and, thus, even the determination of such basic f e a t u r e s a s ring size or the n a t u r e of the side chain or alkyl substituents, is r a r e l y possible. The lack of detailed and extensive information on the mode of fragmentation of this compound class, the lack of authentic m a t e r i a l suitable for comparison, the lack of other physical data on these compounds, and in p a r t i c u l a r , the probable, or (in some c a s e s ) quite apparent, inhomogeneity of isolated fractions prohibit extensive s t r u c t u r a l speculations and allow only an indication of the range of compounds p r e s e n t in the extract. The shale contains a homologous s e r i e s comprising e s t e r s of m o l e c u l a r weight 156 (methyl e s t e r of C s - c y c l o a l k y l acid), 170 (C9), 184 (C10), 198 ( C l l ) and 212 (C12-acid e s t e r ) . F o r each m o l e c u l a r weight s e v e r a l i s o m e r s a r e present; the m a j o r compound of this class is one of the i s o m e r s of MW-184 (see e s t e r 13 fraction 2, Fig.SC). Mass s p e c t r a l data on cyclic e s t e r s a r e p r e s e n t e d in some detail in the work of Haug (1967). High-resolution m a s s m e a s u r e m e n t s on many isolated fractions c o n f i r m e d the composition of m o l e c u l a r and f r a g m e n t ions, but these data a r e insufficient to p r e s e n t r e a s o n a b l e s t r u c t u r a l proposals for these compounds. Cyclic e s t e r s a r e components of g.l.c. peaks 7, 9, 10, 12, 13, 15, 17-19, and 2 1 - 2 4 in Fig.1. P e a k 21 of that c h r o m a t o g r a m contains a cyclic C l l - acid e s t e r (MW-198) and two apparently bicylcic components of MW-196 ( C l l ) and 9.10 (C12); an additional i s o m e r of MW-210 (C12 -acid e s t e r ) was p r e s e n t in peak 24. By h i g h - r e s o l u tion mass s p e c t r o m e t r y , the compositions of C12H2002 and C13H2202 w e r e obtained as r e q u i r e d for bicyelic carboxylic e s t e r s . Mono-unsaturated acids T h r e e compounds w e r e isolated from g.l.c, peaks 9, 11 and 14 which on m a s s s p e c t r o m e t r i c examination proved to be i s o m e r i c with cyclic e s t e r s , with m o l e c u l a r weights of 170 (C9-acid) and 184 ( C l o - a c i d s - two compounds), but exhibiting relatively intense M-32 peaks ( m o r e intense than M-31). T h e s e compounds a r e , t h e r e f o r e , c o n s i d e r e d to be e s t e r s of acyclic m o n o - s a t u r a t e d acids. However, since they could not be c o r r e l a t e d directly with authentic mono*unsaturated e s t e r s , the identification of compound type must r e m a i n quite tentative. Chem. Geol.. 7 (1971) 213-236

229

Summary Table I provides a brief s u m m a r y of all results discussed above. TABLE I Summary of the types and molecular weight range of acids obtained from extracts of oil shale from the Green River Formation Acids structural type

MW range of methyl esters

Carbon number range of acids

Normal Isoprenoidal Other ~-methyl branched Methyl keto ,co -diearboxylie Branched dicarboxylic Benzoic

144-200 172, 186 172 214-242 258-342 258,300,314 150-178

Phenylalkanoic

192-234

Naphthoic

200-242

Cycloalkylaromatic Cyclic

204- 232 156-212

Bicyclic Unsaturated (?)

196, 210 170, 184

C7-Cll C9, C10 C9 Cll-C14 C12-C18 C12 , C15 , C16 C8, C9, C10 several isomers of each Cll-C:L 4 several isomers of each C12 (two isomers) C13-C15 C12-C 14 C8-C12 several isomers of each Cll, C12 (two compounds) C9, C10

DISCUSSION The isolation and c h a r a c t e r i z a t i o n of dicarboxylic acids and of oxocarboxylic acids is one of the interesting findings of this study. The significance of the p r e s e n c e of these compounds in the sediment r e m a i n s o b s c u r e at this time, since both of these compound c l a s s e s a r e r e l a t i v e l y r a r e in nature (Htlditch, 1956; Eglinton and Hamilton, 1963). DicarboxyUc acids could a r i s e f r o m a specific organism, since Eglinton's group has r e p o r t e d the p r e s e n c e of these acids (C8-C22) in a Carboniferous sediment, torbanite (Douglas et al., 1969b), and subsequently found a s i m i l a r range of dibasic acids in the f r e s h w a t e r alga Botryococcus braunii (Douglas et al., 1969a), the o r g a n i s m believed mainly responsible for the organic m a t t e r in torbanite. Dicarboxylic acids have also been r e p o r t e d in both T a s m a n t a n ( P e r m i a n age) and Alaskan (Late J u r a s s i c to E a r l y C r e t a c e o u s age) t a s m a n i t e (Burlingame et al., 1969b), two related ancient sediments made up a l m o s t entirely of c o m p r e s s e d d i s c s of Tasmanites Newton (1875) which a r e thought to be fossil g r e e n algae with close biological affinities to the p r e s e n t day m a r i n e o r g a n i s m s Pachysphera pelagica Osterfeld (1899) and other s p e c i e s of Pachysphera (Wall, 1962). A s i m i l a r investigation of the acidic constituents of p r e s e n t day algae of the type known to o c c u r in the Green R i v e r F o r m a t i o n has not yet been undertaken, but obviously would be d e s i r a b l e in the light of these findings. Dicarboxylic acids could have a r i s e n also by m i c r o b i a l 230

Chem. Geol., 7 (1971) 213-236

oxidation of p r e f o r m e d monocarboxylic acids or by diterminal oxidation of a p p r o p r i a t e hydrocarbons, a p r o c e s s which has been demonstrated with a strain of Corynebacterium (Kester and F o s t e r , 1963) and the y e a s t T o r u l o p sis g r o p e n g i e s s e r e i (Jones and Howe, 1968). The p r o c e s s appears to involve the sequence: alkane ~llmne-1 -al ~alkanoic acid----~o-hydroxyalkanoic acid---,~ ,w-alkanedioic acid. L a s t l y , such compounds could be f o r m e d , of c o u r s e , by some oxidative diagenetic degradation of h y d r o c a r b o n or acid constituents of the shale. Oxidation of suitable i s o - a c i d s would s e e m the m o s t straightforward route to dicarboxylic acids of this type~ although again suitable 2-methylalkanes may be the ultimate p r e c u r s o r s which, by m i c r o b i a l oxidations, a r e c o n v e r t e d to the corresponding b r a n c h e d c%o~-dicarboxylic acids (Jones, 1968). T h e p r e s e n c e of iso-acids in this shale, originally r e p o r t e d by Leo and P a r k e r (1966), has been confirmed by m o r e r e c e n t studies (Murphy et al., 1969). A discussion of the origin of methyl keto acids p r e s e n t s s i m i l a r difficulties. Quite uncommon as natural compounds (Egiinton and Hamilton, 1965), t h e i r p r e s e n c e is of g r e a t i n t e r e s t if they can be r e l a t e d to specific o r g a n i s m s . P e r h a p s m o r e likely, they could be the products of m i c r o b i o logical oxidation p r o c e s s e s , since the c o n v e r s i o n of alkanes (as well as alkenes and alkanoic acids) to (o~-l)-hydroxyalkanoic acids (of the same or s h o r t e r chain length) is a known pathway for the y e a s t T. g r o p e n g i e s s e r i (Jones and Howe, 1968). Oxidation of the hydroxyl function introduced at the penultimate carbon of the chain to the ketone (enzymatically or nonenzymatically) would complete the p r o c e s s . Initial/3 -oxidation of d i c a r boxylic acids on either end followed by decarboxylation could be advanced as an alternative route. (An analogous c o n v e r s i o n appears to be the m e c h a n i s m by which certain fungi - e.g., A s p e r g i l l u s , Penicillium, Neurospora - metabolize saturated carboxyUc acids to methyl ketones, F r a n k e and Heinen, 1958.) Aliphatic ketones (C13-C16 ) - apparently predominantly 2-alkanones have been obtained f r o m a shale oil distillate ( 2 8 0 - 3 0 5 o c ) of the Green River F o r m a t i o n (Itda et al., 1966) and long chain methylketones have been r e p o r t e d in soil samples (Morrison and Bick, 1966). The o c c u r r e n c e of t h e s e compounds in the shale - which, of c o u r s e , could be d e r i v e d from the c o r r e sponding (w-1)-oxoalkanoic acids - suggests that an investigation of ketones, alcohols and hydroxy acids might furnish r a t h e r interesting data b e a r i n g on the diagenetic relationship of these compound types and t h e i r ultimate source. Such a study appears d e s i r a b l e also in view of the r a t h e r f r a g m e n t a r y information currently available on the distribution of these s t r u c t u r a l c l a s s e s in sediments (cf. Hoering, 1968 and Sever and P a r k e r , 1969, have v e r y recently r e p o r t e d the p r e s e n c e of s a t u r a t e d alcohols ranging f r o m C12 to C22 in the Green River Shale). Only a limited range of oxo-acids could be shown to o c c u r in this s e d i ment (Cll -C14 ) by d i r e c t isolation methods, but subsequent high-resolution m a s s s p e c t r a l analyses of total fractions obtained by improved extraction methods revealed a b r o a d e r range (C5-C15) (Burlingame and Simoneit, 1968a, 1969; Burlingame et al., 1969a; R i c h t e r et al., 1969). Similar m a s s s p e c t r o m e t r i c data (Burlingame and Simoneit, 1968a, 1969a; Richter et al., 1969) revealed dicarboxylic acids ranging f r o m CS to C15, whereas in the p r e s e n t study all normal dibasic acids c o m p r i s i n g the s e r i e s C12-C18 w e r e isolated (in addition to two branched compounds). Chem. Geol., 7 (1971) 213-236

231

The isolation of the C 9 and C 10 isoprenoidal acids is unexceptional, since the o c c u r r e n c e of this c l a s s of compounds in this oil shale has been e s t a b l i s h e d by E glinton and c o w o r k e r s (Eglinton et al., 1966b; M a c L e a n et al., 1968; Douglas et al., 1969b), who identified all acids of the s e r i e s C14-C21 with the exception of C18. By oxidation of the k e r o g e n m a t r i x derived f r o m the G r e e n R i v e r F o r m a t i o n , B u r l i n g a m e and Simoneit (1968b, 1969) obtained p r i s t a n i c and phytanic acids, and the p r e s e n c e of s e v e r a l isoprenoidal a c i d s (C14, C15, C19, C20 ) in p e t r o l e u m has been e s t a b l i s h e d s e v e r a l y e a r s ago (Cason and G r a h a m , 1965). T h e p r e s e n t r e s u l t s thus extend the l o w e r l i m i t of this r a n g e to C9, where, however, the o c c u r r e n c e of the group f r o m C l l to C13 r e m a i n s yet to be established. It is p e r h a p s of i n t e r e s t to note h e r e that the C10 isoprenoid acid was not found in a California p e t r o l e u m (Cason and Khodair, 1967b), although the C l l h o m o logue was p r e s e n t . A d i s c u s s i o n a s to the origin of a r o m a t i c acids a p p e a r s p r e m a t u r e at this stage, since unambiguous s t r u c t u r a l c h a r a c t e r i z a t i o n s - the n e c e s s a r y p r e r e q u i s i t e for any p r o p o s a l s of this kind - is not fulfilled by this study. Obviously, m a n y of t h e s e could a r i s e by degradation of m o n o - and b i c y c l i c mono- and s e s q u i t e r p e n o i d s (or even l a r g e r m o l e c u l e s ) , which a r e quite abundant in m a n y higher plant s p e c i e s . F o r example, e s t e r 30 f r a c t i o n 1, for which s t r u c t u r e XV m a y be suggested on the b a s i s of its m a s s s p e c t r u m , (Fig.4D) might be one end p r o d u c t of degradation of c a r o t e n o t d s . S i m i l a r

~COOCH~ XV a r o m a t i z a t i o n r e a c t i o n s of p r e f o r m e d alicyclic s y s t e m s could account f o r a v a r i e t y of other a r o m a t i c acids; a l t e r n a t i v e l y , the p o s s i b i l i t y of cyclization of acyclic (unsaturated) compounds to m o n o c y c l i c and a r o m a t i c p r o d u c t s m u s t be kept in mind. ( F o r e x a m p l e , the r a t h e r facile c h e m i c a l c y c l i z a t i o n s of functionalized olefins to m o n o - , b i - and even polycyclic s y s t e m s - cf. review by Johnson (1968) - m a y have t h e i r diagenetic analogues.) Quite evidently, r i g o r o u s c h a r a c t e r i z a t i o n of m e m b e r s of this compound c l a s s can furnish much information on diagenetic p r o c e s s e s , p a r t i c u l a r l y when c o m bined with a n a l y s e s on r e l a t e d s u b s t a n c e s - phenolsp a r o m a t i c k e t o n e s and hydrocarbons. Such an effort will not only r e q u i r e r i g o r o u s l y p u r i f i e d s a m p l e s , but a l s o extensive collection and i n t e r p r e t a t i o n of m a s s s p e c t r a of authentic compounds and the application of other physical t e c h n i q u e s especially n u c l e a r magnetic r e s o n a n c e and i n f r a r e d s p e c t r o s c o p y , a s s u m i n g sufficient amounts a r e available. A final c o m m e n t m a y be m a d e on the distribution of a r o m a t i c acids. All acids up to m o l e c u l a r weight 1'/8 a p p e a r to be d e r i v a t i v e s of benzoic acid. Phenylacetic acid, phenylpropionic acid and substituted p h e n y l a c e t i c acids w e r e not found. Compounds with longer acid side chains fall into the m o l e c u l a r weight r a n g e of 192 to 234, and the side chain then a p p e a r s to contain at l e a s t t h r e e c a r b o n a t o m s . The significance of this finding r e m a i n s obscure at p r e s e n t ~ since it could be m e r e l y a consequence of our e x t r a c t i o n and analysis p r o c e d u r e (see below). 232

Chem. Geol., 7 (1971) 213-236

The r e s u l t s p r e s e n t e d h e r e , although as y e t quite p r e l i m i n a r y , d e r i v e some importance in that, by c o n t r a s t with the r a t h e r general paucity of information on a r o m a t i c acids in shales or p e t r o l e u m , they provide a r a t h e r comprehensive, if tentative, s u r v e y over the range and nature of a r o m a t i c acids. More information is available on cyclic acids, since a n u m b e r of t h e s e have been identified in p e t r o l e u m . In our work no single cyclic acid was definitely identified, and c o r r e l a t i o n s with compounds occuring in p e t r o l e u m a r e thus not possible, although the tentative conclusion that shale and petroleum cyclic acids exhibit s i m i l a r range and distribution a p p e a r s justified. Simple cyclic acids such as cyclopentyl- or cyclohexylalkanoic acids a r e not v e r y common plant constituents (except for c e r t a i n s p e c i e s , e.g., chaulmonrgic acid), suggesting that they a r e , at least in p a r t , f o r m e d by degradation of terpenoidal' p r e c u r s o r s , an assumption which can be s u p p o r t e d by s t r u c t u r a l a r g u m e n t s (i.e., the isolation of 2,2~67trimethylcYclohexylacetic and 2 , 2 , 6 - t r i m e t h y l c y c l o h e x y l carboxylic acids f r o m petroleum). Diagenetie cyclizations of olefinic p r e c u r s o r s , such as alluded to above, could provide another route to these acids (cf. Ansell et al., 1968). A n u m b e r of s i m p l e cyclic acids such as cyclopentylcarboxylic, 2and 3-methylcyclopentylcarboxylic, 2,3-dimethylcyclopentylacetic, cyclohexylacetic and cis and 2tans 2 , 2 , 6 - t r i m e t h y l c y c l o h e x y l c a r b o x y l i c acids have been isolated from various petroleum s o u r c e s p r i m a r i l y due to the efforts of Lochte and c o l l a b o r a t o r s (Lochte and Littmann, 1955). Cason and coworkers m o r e r e c e n t l y have identified t r a n s - 2 , 2 , 6 - t r i m e t h y l c y c l o hexylacetic acid (Cason and Liauw, 1965) and 3 - e t h y l - 4 - m e t h y l c y c l o p e n t y l acetic acid (Cason and Khodair, 1966) in a California p e t r o l e u m . T h e p r e s e n c e of cyclic acids in the G r e e n R i v e r Shale has been noted by R a m s a y (1966) but no individual compounds w e r e isolated. F r o m our r e s u l t s it appears that a C 1 0 - c y c l i c acid is the m a j o r r e p r e s e n t a t i v e of this class; t h e i r range a p p e a r s r e l a t i v e l y n a r r o w (C8-C12) but each homologue r e p r e s e n t s a n u m b e r of i s o m e r s . Complete s t r u c t u r a l elucidation of c y c l i c acids will be a considerable task requiring in many instances the unambiguous synthesis of p r e s u m e d s t r u c t u r e s and c o m p a r i s o n with unknown. At this stage of organic geochemical endeavors such an effort would a p p e a r justified only for the m o r e abundant components of this compound c l a s s . A final comment neects to be m a d e on the range and distribution of compounds identified in this study. The absence of l a r g e r n o r m a l and i s o prenoic acids which a r e known to occur in this sediment (Eglinton et a l . , 1966b) suggests that the p a r t i c u l a r compound distribution found in this study may be a function of the extraction technique used. Exhaustive s o x h let extraction (Burlingame and Simoneit, 1968a) of the shale r e s i d u e utilized in this work showed (by g.l.c, analysis) the p r e s e n c e of lon.g chain acids and digestion by hydrofluoric acid (the technique utilized by Egiinton et al_, 1966b) of the exhaustively e x t r a c t e d m a t e r i a l also gave higher i s o p r e n o i d a l (e.g., pristanic and phytanic acids) and n o r m a l s a t u r a t e d acids. Thus, the distribution of acidic components r e p o r t e d h e r e m o s t certainly does not r e f l e c t t h e i r true distribution in the sediment and does not give a t r u e estimate of t h e i r r e l a t i v e abundance.

Chem. Geol.. 7 (1971) 213-236

233

ACKNOWLEDGEMENT We thank B e r n d S i m o n e i t and M a r t i n Senn f o r t h e c o l l e c t i o n of s h a l e s a m p l e s and D r . D e n n i s S m i t h f o r a s s i s t a n c e in d e t e r m i n i n g h i g h - r e s o l u t i o n m a s s s p e c t r a . We a c k n o w l e d g e g r a t e f u l l y the f i n a n c i a l s u p p o r t f r o m t h e U,S. N a t i o n a l A e r o n a u t i c s and S p a c e A d m i n i s t r a t i o n ( G r a n t N G L 0 5 - 0 0 3 - 0 0 3 ) .

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