HYDROCARBONS
In chemical terms a hydrocarbon is a compound containing only the elements carbon and
hydrogen. Petroleum and natural gas are themselves often referred to as "hydrocarbons," but
that usage is incorrect trom the chemist's point of view because those materials often contain
substantial amounts of nitrogen, sulfur, oxygen, trace metals, and other elements. In this chapter
we restrict the usage of the term hydrocarbon to the standard chemical one; elsewhere in this
text usage will vary, as it does in the real world.
Examples of hydrocarbons are methane, ethane, and cyclohexane, whose structures are shown
below.
In each of these compounds, and indeed in every carbon compound (except a few highly
unstable ones created only in laboratories), every carbon atom forms four bonds. Similarly,
hydrogen always forms one bond; oxygen and sulfer, two bonds; and nitrogen, three bonds.
Carbon atoms like to form bonds with each other, creating long chains and ring structures. This
unique property of carbon is responsible for the existence of literally millions of different
organic compounds. Writing the detailed structure of a simple molecule like methane is no
problem, especially if one has to do it only occasionally. If one wants to draw large molecules,
however, the explicit inclusion of every atom and every bond becomes extremely tedious.
Several different types of shorthand have therefore developed to facilitate drawing organic
molecules.
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One common convention is to represent all the hydrogen atoms attached to a given carbon atom
by a single H, using a subscript on the H to denote the total number of hydrogens around that
atom. The structures of methane and ethane are thus represented by CH4 and CH3CH3
respectively. We can make other logical simplifications for longer carbon chains.
The following representations of n-pentane are equivalent:
CH3CH2CH2CH2CH3 or
CH3(CH2)3CH3.
An even quicker shorthand that uses no letters at all has evolved. Each carbon atom is
represented by a point, and carbon-carbon bonds are shown as lines connecting those points.
Hydrogen atoms and bonds to hydrogen atoms are not shown at all. Because we know that each
carbon atom forms four bonds and each hydrogen atom forms one bond, simple inspection
shows how mant' hydrogen atoms each carbon atom must have. For example, n-pentane and
cyclohexane are represented by the line structures shown below.
n-pentane
cyclohexane
The zigzag configuration illustrated for n-pentane is adopted to show clearly each carbon atom.
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The simplest series of hydrocarbons has linear structures; these molecules are called n-alkanes
or nparains. The letter n stands for normal, and indicates that there is no branching in the carbon
chain. We have ahready encountered n-pentane; the names of the other nine simplest n-alkanes
are given in the following table. Note that the name of each compound ends in -ane, as in
"alkane." The first four names are irregular, but the prefixes denoting the number of carbon
atoms in the other alkanes are derived from Greek numbers.
Names and formulas of the ten smallest n-alkanes
Methane
Ethane
Propane
Butane
Pentane
Hexane
Heptane
Octane
Nonane
Decane
CH4
C2H6
C3H8
C4H10
C5H12
C6H14
C7H16
C8H18
C9H20
C10H22
CH4
CH3CH3
CH3CH2CH3
CH3 (CH2)2
CH3
(CH2)3
CH3
CH3 (CH2)4
CH3
CH3
(CH2)5
CH3
CH3 (CH2)6
CH3
CH3 (CH2)7
CH3
CH3
CH3 (CH2)8
CH3
Carbon atoms need not always bond together in a linear arrangement. Branching can occur,
giving rise to a vast number of possible structures.
The term methyl, which we used earlier, is the adjectival form of the word methane. In the case
of 2-methylhexane (C7H16) the basic structure is hexane; a CH3 (methyl) group is attached to
the second carbon atom. Other adjectival forms are made by dropping the -ane ending and
adding yl (for example, ethyl and propyl).
Among the most important branched hydrocarbons in organic geochemistry are the isoprenoids.
Regular isoprenoids consist of a straight chain of carbon atoms with a methyl branch on every
fourth carbon. Isoprenoids ranging in length from six to forty carbon atoms have been found in
petroleum and rocks.
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We have also seen that carbon atoms can be arranged in rings. These cyclic compounds (called
naphthenes) are named by counting the number of carbon atoms in the ring and attaching the
prefix cyclo.
All the compounds mentioned above are called saturated hydrocarbons or saturates, because
they are saturated with respect to hydrogen. That is, no more hydrogen can be incorporated into
the molecule without breaking it apart.
Another important group of hydrocarbons is the unsaturates, which, in contrast, are able to
combine with additional hydrogen. Many unsaturated compounds have carbon-carbon double
bonds; these compounds are called alkenes. Examples are ethene (C2H4) , propene (C3H6),
and cyclohexene (C6H10), the structures of which are shown below. They are named in a similar
manner to the alkanes, except that the ending -ene indicates the presence of a double bond.
Pro(yl)ene
eth(yl)ene
cyclohexane
Because alkenes are highly reactive, they do not long persist in geologic environments. In the
laboratory they are readily converted to alkanes by the addition of hydrogen in the presence of a
catalyst. By hydrogenation ethene thus reacts to form ethane.
A variety of reactions, including hydrogenafion, converts alkenes to alkanes and cyclic
compounds during diagenesis.
Aromatics form an extremely important class of unsaturated hydrocarbons. At first glance
aromatics appear to be nothing more than cyclic alkenes containing several double bonds, but
they actually have completely different chemical properties from alkenes and are unusually
stable. Although they are unsaturated, they do not add hydrogen easily. Their stability permits
aromatics to be important constituents of oils and sediments.
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Aromatics possess a system of alternating single and double bonds within a cyclic structure. A
simplified notation for drawing these molecules permits us to represent the double-bond system
by a circle within the ring. The circle indicates that the electrons in the double bonds are
delocalized; that is, they are free to move throughout the cyclic system instead of being held
between two particular carbon atoms. It is this delocalization of electrons which makes aromatic
compounds very stable.
Some aromatic molecules are very large. Polycyclic aromatic hydrocarbons having fused ring
structures are quite common. The extreme case is graphite, which is an almost-endless sheet of
aromatic rings.
The hydrocarbons we discussed so far are relatively simple molecules. Although they are very
important constituents of petroleum, these compounds are quite different from the majority of
the organic molecules found in living organisms. Most biological molecules are larger and more
complex than the simple hydrocarbons; the majority contain oxygen, nitrogen, phosphorus,
sulfur, or other elements.
The hydrocarbons present in petroleum are mostly the end products of extensive degradation of
biogenic molecules. In fact, some complex hydrocarbons that are found in fossil organic
material can be related directly to individual biological precursors.
NON HYDROCARBONS
Atoms other than hydrogen and carbon that occur in petroleum, bitumen, and kerogen are called
heteroatoms; the compounds in which they occur are called heterocompounds.
Heterocompounds are also called NSO compounds, because the most common heteroatoms
are nitrogen, sulfur, and oxygen.
Fossil organic matter often contains a vide variety of heterocompounds, of which some are
biogenic and others are formed during diagenesis.
Many of the heterocompounds present in organisms are converted to hydrocarbons during
diagenesis and catagenesis.
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Many common NSO compounds are not directly related to biogenic precursors. Among the
most important NSO compounds are the asphaltenes, which are large, highly aromatic
materials of varying structure. They have many characteristics in common with kerogen, but
asphaltene molecules are smaller and more aromatic than most kerogens.
Many nonhydrocarbon molecules common to living organisms are also present in sediments.
Among these are lignin, carbohydrates, and amino acids.
Lignin is an important component of wood, providing much of the structural support for large
land plants. It is a polymer consisting of many repetitions and combinations of three basic
aromatic subunits.
Lignin monomers are linked topether to form molecules having molecular weights from 3000
to 10,000 atomic mass units. Upon decomposition lignin forms phenolic compounds, which are
aromatics having a hydroxyl group (OH) attached. Because phenols are potent bactericides,
lignin is rather resistant to degradation, and thus tends to become concentrated as other organic
matter is decomposed.
Carbohydrates include starch, sugars, and cellulose; the latter is the most abundant organic
compound in the biosphere. Like lignin, it is an important constituent of terrestrial organic
matter. Although cellulose is quite resistant to decomposition under some conditions, most
carbohydrates are attacked readily by microorganisms.
Lignin and cellulose are major constituents of humic coals. Amino acids are the building
blocks of proteins. They are rapidly metabolized by virtually all organisms, however, and thus
are seldom preserved in sediments (exceptions occur in shell material and in bones, where small
amounts of preserved amino acids can be used to date specimens)
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