Geosynthesis of organic compounds.
Part V Ð methylation of alkylnaphthalenes
Trevor P. Bastow *, Robert Alexander, Steven J. Fisher, Raj K. Singh,
Ben G.K. van Aarssen, Robert I. Kagi
Australian Petroleum CRC/Centre for Petroleum and Environmental Organic Geochemistry, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
Received 24 August 1999; accepted 6 March 2000 (returned to author for revision 29 November 1999)
Abstract
Several crude oils and rock extracts with high concentrations of 1,6-dimethylnaphthalene, 1,2,5-trimethylnaphtha-lene, 1,2,3,5-tetramethylnaphtha1,2,5-trimethylnaphtha-lene, 1,2,3,5,6-pentamethylnaphthalene and 6-isopropyl-2-methyl-1-(4-methylpentyl)-naphthalene are also shown to contain enhanced distributions of their corresponding methylated counterparts. Of these methylated counterparts, 1,2,3,5,6,7-hexamethylnaphthalene has been synthesised and is identi®ed in sedimentary material for the ®rst time. In laboratory experiments carried out under mild conditions with a methyl donor in the presence of a clay catalyst, each of the parent alkylnaphthalenes was shown to be substituted in preferential positions. The distributions of these products from laboratory methylation experiments are similar to the distributions found in sedimentary material. These observations have been interpreted as evidence for a sedimentary electrophilic aromatic methylation process.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Methylation; Alkylation; Geosynthesis; Alkylnaphthalenes; Clay catalysis; Electrophilic aromatic substitution; 1,2,3,5,6,7-Hexamethylnaphthalene
1. Introduction
Sedimentary methylation processes have been proposed to account for the occurrence of numerous methyl sub-stituted aromatic hydrocarbons with carbon skeletons that are not obviously derived from common natural product precursors (Radke et al., 1982a). The formation of alkylaromatics by alkylation processes has been demonstrated for alkylphenols (Ioppolo-Armanios et al., 1995), benzenes (Ellis et al., 1995; He et al., 1995), naphthalenes (He et al., 1992, 1994; Bastow et al., 1996), phenanthrenes (Smith et al., 1994, 1995; Alexander et al., 1995), anthracenes (Smith et al., 1994, 1995) and pyrene (Derbyshire and Whitehurst, 1981; Smith et al.,
1994, 1995). Further evidence to support an alkylation process comes from the identi®cation of 6-isopropyl-2,4-dimethyl-1-(4-methylpentyl)naphthalene (4-Me-iP-iHMN) as a methylation product of 6-isopropyl-2-methyl-1-(4-methylpentyl)naphthalene (iP-iHMN) (Ellis, 1994; Ellis et al., 1996; Bastow et al., 1999). In this paper we show evidence for the formation of methylated alkylnaphtha-lenes via sedimentary methylation processes.
Dimethylnaphthalenes (DMNs), trimethylnaphthalenes (TMNs), tetramethylnaphthalenes (TeMNs) and penta-methylnaphthalenes (PMNs) are common constituents of petroleum and fossil fuels. The presence of abundant 1,6-DMN (Radke et al., 1990, 1994; van Aarssen et al., 1992), 1,2,5-TMN (PuÈttmann and Villar, 1987; Strachan et al., 1988; Killops, 1991; Ellis et al., 1996), 1,2,7-TMN (Strachan et al., 1988; Forster et al., 1989), 1,2,5,6-TeMN (PuÈttmann and Villar, 1987; Killops, 1991; Ellis et al., 1996), 1,2,3,5-TeMN (Alexander et al., 1992, 1993), 1,2,3,5,6-PMN (Bastow et al., 1998) and iP-iHMN (Ellis, 1994; Ellis et al., 1996; Bastow et al., 1999) in low
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* Corresponding author. Tel.: 8-9266-3819; fax; +61-8-9266-2300.
maturity sedimentary organic matter has led to the sug-gestion that these compounds originate from terpenoids derived from microbial and land plant sources. However, with increasing maturity the relative abundances of these isomers are diminished until at high maturity the isomer distributions re¯ect their relative stabilities (Alexander et al., 1985, 1986, 1993; Strachan et al., 1988; van Duin et al., 1997; van Aarssen et al., 1999). This change in the relative abundances of isomers with increasing maturity has been attributed to ring isomer-isation and transalkylation processes (Strachan et al., 1988; Alexander et al., 1993; Bastow et al., 1998; van Aarssen et al., 1999). A recent study by van Aarssen et al. (1999) has suggested that the distributions of TMNs, TeMNs and PMNs are closely related as a result of isomer-isation and transalkylation reactions and it has been proposed that the methyl groups involved in these reactions form a `methyl pool' to which every compound in a crude oil has access.
In this paper we show that crude oils and rock extracts with a high relative abundance in alkyl-naphthalenes likely to be derived from natural product origins, have a correspondingly high abundance of their methylated counterparts, and suggest that this is caused by sedimentary methylation of these alkylnaphthalenes.
2. Experimental
2.1. Samples
Information about the samples is given in Table 1.
2.2. Reference compounds
1,6-DMN was purchased from Sigma. 1,2,3,5-TeMN, 1,2,3,5,6-PMN, iP-iHMN and 4-Me-iP-iHMN were available from previous studies (Alexander et al., 1993; Bastow et al., 1998; Ellis et al., 1996; Bastow et al., 1999). A sample of 1,2,5-TMN was provided by Dr P. Forster.
1,2,3,5,6,7-Hexamethylnaphthalene (1,2,3,5,6,7-HMN) was prepared using the synthetic scheme reported by Araki and Mukaiyama (1974). 1,2,3-Trimethylbenzene (13 mmol) in glacial acetic acid (50 ml) at 0C, was
treated with bromine (15 mmol), and the mixture was allowed to warm to room temperature over a period of 1 h yielding 1-bromo-2,3,4-trimethylbenzene. The Grignard reagent prepared from 1-bromo-2,3,4-trimethylbenzene (8 mmol) and magnesium (5 mmol) was added to 2,3-dimethylsuccinic anhydride (4 mmol) in tetrahydrofuran (THF). The resulting keto carboxylic acid (2 mmol) was reduced using zinc amalgam (Martin, 1942) to give the carboxylic acid. The carboxylic acid (1 mmol) was cyclised to 2,3,5,6,7-pentamethyltetralone by treatment with polyphosphoric acid (PPA). The tetralone (0.5 mmol) was methylated using methylmagnesium iodide and the resultant tertiary alcohol (0.4 mmol) was con-verted to the alkyltetralin by dehydration in a mixture of sulfuric acid and glacial acetic acid. This alkyltetralin was then treated with activated platinum (5%) on carbon at 280C to yield 1,2,3,5,6,7-HMN as a white solid (m.p.
172±174C) with an overall yield of 8%. The1H-NMR spectra and melting point of this compound corre-sponded well with those reported by Chen et al. (1973) (m.p. 173±174C; 1H-NMR (CDCl
3): d 2.34, s, 6H, Ar(C2, C6)-CH3; 2.44, s, 6H, Ar(C3, C7)-CH3; 2.57, s, 6H, Ar(C1, C5)-CH3; 7.64, s, 2H, Ar(C4, C8)-H).
2.3. Nuclear magnetic resonance (NMR) data
All measurements were carried out in a solution of chloroform-d using a Varian Gemini-200 spectrometer operating at 200 MHz for1H (ppm relative to CDCl3at 7.25 ppm) and 50 MHz for13C (ppm relative to CDCl
3 at 77.0 ppm).
1H-NMR data for 1,2,3,5,6,7-HMN: d 2.37, s, 6H,
Ar(C2, C6)-CH3; 2.47, s, 6H, Ar(C3, C7)-CH3; 2.57, s, 6H, Ar(C1, C5)-CH3; 7.67, s, 2H, Ar(C4, C8)-H.
13C-NMR data for 1,2,3,5,6,7-HMN:d14.98, 16.11,
22.05, Ar(C1, C2, C3, C5, C6, C7)-CH3; 121.95, Ar(C4,
Table 1
Geological and geochemical data for the crude oils and rock extractsa
Sample Location
Name Type Country Basin Age or probable age
of oil source rock
MPI-1 TNR-1 PNR %MR
Dullingari-26 Crude oil Australia Eromanga Jurassic 0.4 0.4 0.21 7.3
GK 1959m Rock extract Indonesia S. Sumatra Tertiary 0.6 0.6 0.18 13.0
GK 2026m Rock extract Indonesia S. Sumatra Tertiary 0.7 0.6 0.20 12.4
Moorari-4 Crude oil Australia Eromanga Jurassic 0.2 0.3 0.15 8.0
Moorari-3 71080 Rock extract Australia Eromanga Jurassic 0.3 0.4 0.12 11.8
PD130A Rock extract Australia Canning Devonian 0.3 0.5 0.13 ±
C8); 130.05, 130.56, 131.67, 133.93, Ar(C1,C2,C3,C4a, C5,C6,C7,C8a).
2.4. Sample preparation
Rock samples were air dried and crushed to a ®ne powder using a Tema mill and extracted ultrasonically with dichloromethane:methanol (95:5). The solvent extract was recovered by ®ltration and the solvent was carefully removed to yield the soluble organic matter (SOM).
2.5. Isolation of aromatic hydrocarbon fractions
In a typical separation, a crude oil or sediment extract was subjected to liquid chromatography as follows. Glass columns (401.2 cm i.d.) were packed with activated silica gel (6 g, Merck, particle size 0.063±0.200 mm for chromatography) as a slurry inn-pentane. The sample (50±100 mg) inn-pentane was introduced to the top of the column to produce a concentrated band. Aliphatic hydro-carbons were then eluted under gravity with n-pentane (40 ml), aromatic hydrocarbons withn -pentane/dichloro-methane (35:5, 40 ml), and polar compounds with dichloromethane:methanol (1:1, 30 ml). The aromatic fractions were obtained by careful removal of the sol-vent. The residue was dissolved in hexane to provide a sample ready for GC±MS analysis.
2.6. Analysis by gas chromatography±mass spectrometry (GC±MS)
Two systems were used for GC±MS analysis. One, a Hewlett-Packard 5973 MSD, interfaced with a 6890 gas chromatograph was ®tted with a fused-silica open tub-ular column (SGE, Australia, 50 m0.22 mm i.d.) coated with BP-20 stationary phase (0.25mm). The GC oven temperature was programmed from 50 to 260C at
3C minÿ1. Samples for analysis were dissolved in hexane and injected on-column using a HP 6890 autosampler at an oven temperature of 50C. Helium was used as carrier
gas at a constant pressure of 22 psi. Typical MSD con-ditions were: ionisation energy 70 eV; source tempera-ture 230C; electron multiplier voltage 1800 V.
The other system was a Hewlett-Packard 5971 MSD interfaced with a 5890 Series II gas chromatograph ®tted with a DB-5 (60 m0.25 mm i.d., phase thickness 0.25
mm, J&W Scienti®c) fused silica open tubular column. Samples for analysis were dissolved in hexane and injected on-column using a HP 5890 autosampler at an oven temperature of 60C. Helium was used as carrier
gas at a constant ¯ow of 1.28 ml minÿ1 and the GC oven was temperature programmed from 60 to 300C at
3C minÿ1. Typical MSD conditions were: ionisation energy 70 eV; source temperature 200C; electron
mul-tiplier voltage 2200 V.
Individual alkylnaphthalenes in crude oils and sedi-ment extracts were identi®ed by mass chromatography on the basis of mass spectra and by co-chromatography with authentic reference compounds.
2.7. Methylation experiments
The aluminum montmorillonite used in the study was material prepared for an earlier study (Alexander et al., 1984). Reagents were used in the following proportions; alkylnaphthalenes (1 mg), and hexamethylbenzene (3 mg), pentadecane (0.5 mg)(normalisation standard) and ®nely divided aluminum montmorillonite (10 mg). These reagents were placed in small (1 ml) Pyrex glass ampoules and ¯ushed with dry nitrogen. The ampoules were evacuated, sealed and then heated to 120C for
various times. The contents were extracted with dichloromethane (2 ml), the solvent was removed by careful distillation, and hexane (1.8 ml) containing the external standard hexadecane (0.5 mg) was added. An aliquot was then subjected to analysis using GC±MS techniques.
3. Results and discussion
3.1. Laboratory methylation of alkylnaphthalenes
Laboratory experiments were carried out to show that methyl derivatives of alkylnaphthalenes could be formed via clay-catalysed methylation reactions. Samples of alkylnaphthalenes were heated with hexamethylbenzene (methyl donor) and aluminum montmorillonite at 120C
1979). Furthermore, positions adjacent to alkyl substituents were found to be unfavourable to EAS due to steric hindrance, with peri positions (e.g. positions 1 and 8) having the greatest steric hindrance and positions adjacent
to multiple alkyl substituents more sterically hindered than those located adjacent to a single alkyl substituent. The positions with electronic activation to EAS for the compounds used are marked with asterisks in Fig. 6.
In the case of 1,6-DMN the positions activated to EAS are positions 2, 4, 5 and 7. However, only the 4 position is devoid of an adjacent methyl group, making this the position activated to EAS with least steric hin-drance. Therefore, 1,4,6-TMN should be the major TMN formed from the methylation of 1,6-DMN. This is a fact substantiated by the experimental results shown in Fig. 1a. Clearly, the major methylation product is 1,4,6-TMN with only minor amounts of 1,2,6-TMN and 1,6,7-TMN formed.
For 1,2,5-TMN, positions 4 and 7 are activated to EAS by only one methyl substituent and positions 3, 6 and 8 are activated by two methyl substituents. Position 7 is the only position devoid of an adjacent methyl group, resulting in this position having the least steric hindrance. Therefore, it is not surprising that 1,2,5,7-TeMN is the major product (1,2,5,6-1,2,5,7-TeMN is a less signi®cant product) from the methylation of 1,2,5-TMN (Fig. 2a). Further evidence that the position of methyl-ation represents a compromise between activmethyl-ation to
EAS and steric hindrance is shown from the comparison of the methylation products of 1,2,5-TMN and 1,2,3,5-TeMN. It is evident from Fig. 6c that position 7 of 1,2,3,5-TeMN is now activated by two methyl sub-stituents (positions 1 and 3), and is still free of adjacent methyl substituents. Therefore, position 7 is the site with greatest activation to EAS with the least steric hin-drance. This hypothesis agrees well with the products formed from the methylation of 1,2,3,5-TeMN (Fig. 3a) where 1,2,3,5,7-PMN was the major product, and 1,2,3,5,6-PMN was a minor product. Clearly, from the comparison of the methylation products of 1,2,5-TMN and 1,2,3,5-TeMN, a methyl substituent in one naphth-alene ring can eect the activation of positions to EAS in the other ring. From this comparison it is also apparent that the steric hindrance caused by an adjacent methyl
substituent can eect the orientation of methylation as strongly as electronic eects resulting from alkyl sub-stituents.
Similar reasoning was used to explain the methylation products of 1,2,3,5,6-PMN (Fig. 6d). Of the three vacant positions, two (positions 4 and 8) are locatedperi to existing methyl substituents. This leaves only the 7 position available to methylation, a preferred position for methylation according to the results (Fig. 4a).
Again the 1,6-alkyl substition pattern of iP-iHMN results in a similar substitution pattern to that outlined for 1,6-DMN. Positions 4, 5 and 7 are activated by the two alkyl substituents. Of these positions only the 4 position has no adjacent alkyl substituents, and posi-tions 5 and 7 are located adjacent to an isopropyl sub-stituent resulting in greater steric hindrance for these
two positions. The other two vacant positions (3 and 8) are activated by a single methyl substituent and both experience steric hindrance from adjacent alkyl groups substituents. Hence, position 4 is the preferred position of attack resulting in the formation of 4-Me-iP-iHMN.
The methylation reagent (HMB) used in these laboratory methylation reactions was chosen because of its labile methyl groups. We do not suggest that this is the actual methylating agent in sedimentary material. Potential methylating reagents in sedimentary material may be compounds containing gem and angular methyl groups, which on aromatisation are likely to produce active methyl groups. Other sources for methylation reagents may include alkylaromatics undergoing acid catalysed dealkylation. Also, during the coali®cation of lignin the methyl of methoxyl groups is released (Botto et al., 1989). However, the source of the methylating reagents in sedimentary matter is uncertain.
3.2. Methylation in sediments
Crude oils and sediment extracts contain isomers of methylated naphthalenes that have no obvious natural product origin. The formation of these compounds has been suggested to be from processes such as isomerisa-tion and transalkylaisomerisa-tion. Therefore, these methylated naphthalenes are present in all crude oils and sediments that have some input of alkylnaphthalenes from natural products. However, when these alkylnaphthalenes of natural product origins are in high abundance the dis-tributions of methylated naphthalenes formed directly from these alkylnaphthalenes are likely to be enhanced relative to the normal levels formed from other isomer-isation and transalkylation reactions.
The co-occurrence in sedimentary organic matter of anomalously high abundances of certain alkylnaphtha-lenes and their methylated counterparts suggests a link,
possibly involving sedimentary methylation. To investi-gate this relationship we have analysed low maturity samples with anomalously high abundances of 1,6-DMN, 1,2,5-TMN, 1,2,3,5-TeMN, 1,2,3,5,6-PMN and iP-iHMN and obtained relative abundance data for their corre-sponding methylated counterparts such that an enhance-ment can be observed in the resultant isomer distribution. Low maturity samples were used in this study, because these samples still retain an abundance of alkyl-naphthalenes derived from natural product precursors that may be depleted with maturity (Alexander et al.,
1985, 1986, 1993; Strachan et al., 1988; van Aarssen et al., 1999). In the case of the 1,6-DMN, two samples (GK 1959 m and PD 130A) were selected for compar-ison. The laboratory experiments for the methylation of 1,6-DMN (Fig. 1a) showed that 1,4,6-TMN was the predominant methylation product. Comparison of this methylation reaction product to a sedimentary sample
Fig. 6. The positions of alkylnaphthalenes activated to EAS () for (a) 1,6-DMN, (b) 1,2,5-TMN, (c) 1,2,3,5-TeMN, (d) 1,2,3,5,6-PMN and (e) iP-iHMN. Each indicates the ring positions activated by each of the alkyl group(s) highlighted in bold. Arrow ( ) represents experimentally determined pre-ferred positions of methylation.
Table 2
Compounds identi®ed in crude oils and rock extracts
Peak
(GK 1959 m) with high relative abundance of 1,6-DMN with a sample (PD 130A) that has a lower relative abundance of 1,6-DMN is shown in Fig. 1. From these results it is apparent that the relative abundance of 1,4,6-TMN is enhanced in the TMNs in the sample (GK 1959 m) with the high relative abundance of 1,6-DMN compared to the sample (PD 130A) with a lower relative abundance of 1,6-DMN.
For the sedimentary methylation of 1,2,5-TMN the methyl derivative 1,2,5,7-TeMN is shown for the Moorari-3, 71080 sample and compared with the
corre-sponding peak from the GK 2046 m sample (Fig. 2). Comparison of the TMN and TeMN distributions of these two samples clearly shows a higher relative abun-dance of 1,2,5-TMN in the Moorari sample, which is also associated with an enhancement of 1,2,5,7-TeMN in the TeMNs. While 1,2,5,6-TeMN is also a signi®cant product from the laboratory methylation of 1,2,5-TMN it is dif-®cult to assess the contribution of this reaction product to the 1,2,5,6-TeMN abundance in the Moorari sample since 1,2,5,6-TeMN also has an obvious natural product origin in this sample (van Aarssen et al., 1999).
Results from analysis of crude oils and for the reac-tant 1,2,3,5-TeMN and products from the methylation reaction (1,2,3,5,7-PMN) are shown in Fig. 3. Moorari-4 is a sample exhibiting a high relative abundance of 1,2,3,5-TeMN in the TeMNs and is accompanied by an enhanced 1,2,3,5,7-PMN peak relative to the most stable 1,2,4,6,7-PMN (Bastow et al., 1998) in the PMNs (Fig. 3b). This relative enhancement of 1,2,3,5,7-PMN is apparent when compared with a more common dis-tribution of PMNs in the Dullingari-26 sample (Fig. 3c). In this case a lower abundance of 1,2,3,5-TeMN in the TeMNs is also associated with a lower abundance of 1,2,3,5,7-PMN relative to the stable 1,2,4,6,7-PMN.
The methylation reaction of 1,2,3,5,6-PMN (Fig. 4a) shows that 1,2,3,5,6,7-HMN is formed as the only reac-tion product. Corresponding high concentrareac-tions of the substrate 1,2,3,5,6-PMN and product 1,2,3,5,6,7-HMN are shown in Fig. 4b for sample GK 1959 m. The high relative abundance of 1,2,3,5,6-PMN in the PMNs is shown together with the previously unidenti®ed 1,2,3,5,6,7-HMN. This relationship suggests that sedi-mentary methylation has occurred and is the source of 1,2,3,5,6,7-HMN.
Comparison of chromatograms of the experimental methylation product of iP-iHMN (Fig. 5a) with that of the Moorari-4 sample (Fig. 5b) showed evidence for the methylation product of iP-iHMN in the crude oil. This result suggests that a signi®cant amount of sedimentary methylation has occurred to the parent compound.
The co-occurrences of 1,6-DMN, 1,2,5-TMN, 1,2,3,5-TeMN, 1,2,3,5,6-PMN and iP-iHMN with their methyl-ated counterparts in enhanced levels in sedimentary samples suggests a precursorproduct relationship between these compounds, and is strong evidence that methylation is a
geosynthetic process. Further supporting evidence is provided by the fact that the carbon skeleton of these methylated alkylnaphthalenes is uncommon in natural products and the occurrence of 1,2,3,5,6,7-HMN, due to its high level of substitution, has no obvious natural product precursor and is dicult to explain without a methylation process. Additional support that geosynthetic methylation has occurred in the sample suite used in this study is provided by the occurrence of 9-methylretene in most of the samples. The percentage of 9-methylretene compared to retene for the samples used in this study is given in Table 1 (%MR). 9-Methylretene was identi®ed by Alexander et al. (1995) as a methylation product of retene and its presence in all but one of the samples used in this study is supporting evidence that methylation has occurred in these samples.
The results of this study demonstrate that the dis-tributions of methylated alkylnaphthalenes in crude oils and sediments are aected by the distributions of the corresponding class of alkylnaphthalenes. Therefore, these results support the proposal of van Aarssen et al. (1999) that the distributions of TMNs, TeMNs and PMNs in crude oils are closely related due to isomerisa-tion and transalkylaisomerisa-tion reacisomerisa-tions, in which the methyl groups involved in these reactions form a `methyl pool' to which every compound in a crude oil has access. This suggests that the methylating reagents in nature may include the methyl aromatic compounds themselves.
4. Conclusions
1,2,3,5,6,7-Hexamethylnaphthalene has been synthe-sised and identi®ed in sedimentary material. Low maturity crude oils and rock extracts with high relative abundances of 1,6-DMN, 1,2,5-TMN, 1,2,3,5-TeMN, 1,2,3,5,6-PMN and iP-iHMN have been shown to con-tain enhanced levels of their corresponding methyl counterparts 1,4,6-TMN, 1,2,5,7-TeMN, 1,2,3,5,7-PMN, 1,2,3,5,6,7-HMN and 4-Me-iP-iHMN respectively. Laboratory methylation reactions carried out under mild conditions show that these alkylnaphthalenes are readily methylated in speci®c positions and the dis-tributions of the methylated alkylnaphthalenes formed are consistent with those in sedimentary samples. The mechanism for the methylation of the alkylnaphthalenes is consistent with electrophilic aromatic substitution with steric hindrance strongly in¯uencing the reaction products. These results have been interpreted as evidence for a sedimentary electrophilic aromatic methylation process.
Acknowledgements
thanked for the GK well samples used in this paper. Financial support was provided by the Australian Petrol-eum Co-operative Research Centre and from a Curtin University Postgraduate Scholarship. We also thank Dr. C.A. Lewis and an anonymous reviewer for their critical reviews of the paper.
Associate EditorÐS. Rowland
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