Examples of Hydrocarbons Are Methane, Ethane, and Cyclohexane, Whose Structures Are Shown Below

HYDROCARBONS

In chemical terms ahydrocarbon 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 termhydrocarbon 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.

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.

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

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.

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.

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 arebiogenic and others are formed during diagenesis.

Many of the heterocompounds present in organisms are converted to hydrocarbons during diagenesis and catagenesis.

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 ofvarying 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 isan 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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