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The Chemistry and Technology of Petroleum
There is also the premise that the naphthene ring systems carry alkyl chains that are generally
shorter than the alkyl substituents carried by aromatic systems. There are indications from spectroscopic studies that the short chains (methyl and ethyl) appear to be characteristic substituents
of the aromatic portion of the molecule, whereas a limited number (one or two) of longer chains
may be attached to the cycloparaffin rings. The total number of chains, which is in general four to
six, as well as their length, increases according to the molecular weight of the naphthenoaromatic
compounds.
In the asphaltene constituent, free condensed naphthenic ring systems may occur but general
observations favor the occurrence of combined aromatic-naphthenic systems that are variously substituted by alkyl systems. There is also general evidence that the aromatic systems are responsible
for the polarity of the asphaltenes constituents. The heteroatoms are favored to occur on or within
the aromatic (pseudo-aromatic) systems (Chapters 11 and 12).
8.3.1.3 Aromatic Hydrocarbons
The concept of the occurrence of identifiable aromatic systems in nature is a reality and the occurrence of monocyclic and polycyclic aromatic systems in natural product chemicals is well documented (Sakarnen and Ludwig, 1971; Durand, 1980; Weiss and Edwards, 1980). However, one
source of aromatic systems that is often ignored is petroleum (Eglinton and Murphy, 1969; Tissot
and Welte, 1978; Brooks and Welte, 1984). Therefore, attempts to identify such systems in the
nonvolatile constituents of petroleum should be an integral part of the repertoire of the petroleum
chemist as well as the domain of the natural product chemist.
Crude oil is a mixture of compounds, and aromatic compounds are common to all petroleum
and it is the difference in extent that becomes evident upon examination of a series of petroleum. By
far, the majority of these aromatics contain paraffinic chains, naphthene rings, and aromatic rings
side by side.
There is a general increase in the proportion of aromatic hydrocarbons with increasing molecular
weight. However, aromatic hydrocarbons without the accompanying naphthene rings or alkyl-substituted
derivatives seem to be present in appreciable amounts only in the lower petroleum fractions. Thus, the
limitation of instrumentation notwithstanding, it is not surprising that spectrographic identification of
such compounds has been concerned with these low-boiling aromatics.
All known aromatics are present in gasoline fractions but the benzene content is usually
low compared to the benzene homologues, such as toluene and the xylene isomer. In addition to
the 1- and 2-methylnaphthalenes, other simple alkylnaphthalenes have also been isolated from
crude oil. Aromatics without naphthene rings appear to be relatively rare in the heavier fractions
of petroleum (e.g., lubricating oils). In the higher molecular weight fractions, the rings are usually
condensed together. Thus components with two aromatic rings are presumed to be naphthalene
derivatives and those with three aromatic rings may be phenanthrene derivatives. Currently, and
because of the consideration of the natural product origins of petroleum, phenanthrene derivatives
are favored over anthracene derivatives.
In summation, all hydrocarbon compounds that have aromatic rings, in addition to the presence
of alkyl chains and naphthenic rings within the same molecule, are classified as aromatic compounds. Many separation procedures that have been applied to petroleum (Chapter 9) result in the
isolation of a compound as an aromatic even if there is only one such ring (i.e., six carbon atoms)
that is substituted by many more than six nonaromatic carbon atoms.
It should also be emphasized that in the higher boiling point petroleum fractions, many polycyclic structures occur in naphthenoaromatic systems. The naphthenoaromatic hydrocarbons, together
with the naphthenic hydrocarbon series, form the major content of higher boiling point petroleum
fractions. Usually, the different naphthenoaromatic components are classified according to the number of aromatic rings in their molecules. The first to be distinguished is the series with an equal
number of aromatic and naphthenic rings. The first members of the bicyclic series C 9 –C 11 are the
simplest, such as the 1-methyl-, 2-methyl-, and 4-methylindanes and 2-methyl- and 7-methyltetralin.
The Chemistry and Technology of Petroleum
There is also the premise that the naphthene ring systems carry alkyl chains that are generally
shorter than the alkyl substituents carried by aromatic systems. There are indications from spectroscopic studies that the short chains (methyl and ethyl) appear to be characteristic substituents
of the aromatic portion of the molecule, whereas a limited number (one or two) of longer chains
may be attached to the cycloparaffin rings. The total number of chains, which is in general four to
six, as well as their length, increases according to the molecular weight of the naphthenoaromatic
compounds.
In the asphaltene constituent, free condensed naphthenic ring systems may occur but general
observations favor the occurrence of combined aromatic-naphthenic systems that are variously substituted by alkyl systems. There is also general evidence that the aromatic systems are responsible
for the polarity of the asphaltenes constituents. The heteroatoms are favored to occur on or within
the aromatic (pseudo-aromatic) systems (Chapters 11 and 12).
8.3.1.3 Aromatic Hydrocarbons
The concept of the occurrence of identifiable aromatic systems in nature is a reality and the occurrence of monocyclic and polycyclic aromatic systems in natural product chemicals is well documented (Sakarnen and Ludwig, 1971; Durand, 1980; Weiss and Edwards, 1980). However, one
source of aromatic systems that is often ignored is petroleum (Eglinton and Murphy, 1969; Tissot
and Welte, 1978; Brooks and Welte, 1984). Therefore, attempts to identify such systems in the
nonvolatile constituents of petroleum should be an integral part of the repertoire of the petroleum
chemist as well as the domain of the natural product chemist.
Crude oil is a mixture of compounds, and aromatic compounds are common to all petroleum
and it is the difference in extent that becomes evident upon examination of a series of petroleum. By
far, the majority of these aromatics contain paraffinic chains, naphthene rings, and aromatic rings
side by side.
There is a general increase in the proportion of aromatic hydrocarbons with increasing molecular
weight. However, aromatic hydrocarbons without the accompanying naphthene rings or alkyl-substituted
derivatives seem to be present in appreciable amounts only in the lower petroleum fractions. Thus, the
limitation of instrumentation notwithstanding, it is not surprising that spectrographic identification of
such compounds has been concerned with these low-boiling aromatics.
All known aromatics are present in gasoline fractions but the benzene content is usually
low compared to the benzene homologues, such as toluene and the xylene isomer. In addition to
the 1- and 2-methylnaphthalenes, other simple alkylnaphthalenes have also been isolated from
crude oil. Aromatics without naphthene rings appear to be relatively rare in the heavier fractions
of petroleum (e.g., lubricating oils). In the higher molecular weight fractions, the rings are usually
condensed together. Thus components with two aromatic rings are presumed to be naphthalene
derivatives and those with three aromatic rings may be phenanthrene derivatives. Currently, and
because of the consideration of the natural product origins of petroleum, phenanthrene derivatives
are favored over anthracene derivatives.
In summation, all hydrocarbon compounds that have aromatic rings, in addition to the presence
of alkyl chains and naphthenic rings within the same molecule, are classified as aromatic compounds. Many separation procedures that have been applied to petroleum (Chapter 9) result in the
isolation of a compound as an aromatic even if there is only one such ring (i.e., six carbon atoms)
that is substituted by many more than six nonaromatic carbon atoms.
It should also be emphasized that in the higher boiling point petroleum fractions, many polycyclic structures occur in naphthenoaromatic systems. The naphthenoaromatic hydrocarbons, together
with the naphthenic hydrocarbon series, form the major content of higher boiling point petroleum
fractions. Usually, the different naphthenoaromatic components are classified according to the number of aromatic rings in their molecules. The first to be distinguished is the series with an equal
number of aromatic and naphthenic rings. The first members of the bicyclic series C 9 –C 11 are the
simplest, such as the 1-methyl-, 2-methyl-, and 4-methylindanes and 2-methyl- and 7-methyltetralin.