Fossil Fuels Petroleum

Petroleum, or crude oil, is a complex mixture of thousands of compounds, most of which are hydrocarbons; the proportions of the compounds vary from one oil field to another. The most abundant type of hydrocarbon usually is the alkane series, which can be generically designated by the formula CnH2n+2- In petroleum, the alkane molecules vary greatly, from the simple methane, CH4 (i.e., n = 1), to molecules having almost 100 carbons. Most of the alkane molecules in crude oil are of two structural types: One type is simply a long, continuous chain of carbons; the other has one main chain and only short branches—e.g., 3-methylhexane.

Petroleum also contains substantial amounts of cycloalkanes, mainly those with five or six carbons per ring, such as the C6H12 systems methyl' cyclopentane and cyclohexane:

methylcyclopentane cyclohexane methylcyclopentane cyclohexane

Petroleum contains some aromatic hydrocarbons, principally benzene and its simple derivatives in which one or two hydrogen atoms have been replaced by methyl or ethyl groups. Recall from Chapter 4 that toluene is benzene with one hydrogen replaced by one methyl group and that the xylenes are the three isomers having two methyl groups.


Deduce the structures of all the trimethylated benzenes. [Hint: For each of the three dimethylated benzenes, draw all the structures corresponding to placement of a third methyl group. Inspect each pair of structures you draw to eliminate duplicates.]

It is the component of petroleum containing these aromatic hydrocarbons that is most toxic to shellfish and other fish when an oil spill occurs in an ocean, whether from an oil tanker or from an offshore oil well. Higher-molecular-weight hydrocarbons form sticky, tar-like blobs that adhere to birds, sea mammals, rocks, and other objects that the oil encounters.

Petroleum is found in certain rock formations in the ground and is pumped to the surface in oil wells. As it exits from the ground, crude oil is not a very useful substance because it is a mixture of so many compounds. To gain utility, it must first be separated into components, each of which has several particular uses.

The liquid compounds that are present in crude oil consist of hydrocarbons containing from 5 to about 20 carbon atoms each. Although no attempt is usually made to isolate individual compounds from the mixture, crude oil is separated into a number of fractions—different liquid solutions whose components all boil within a relatively small temperature range. This separation of oil into fractions is accomplished by a process called distillation, which involves the vaporization by boiling of a liquid mixture, followed by the cooling of the vapor in order to cause its condensation back to the liquid state; it is described in Box 7-1. Because of the different boiling points of the compounds, it is possible to separate the mixture into components. Each day, a total of about 10 billion liters of crude oil are distilled by this procedure in hundreds of petroleum refineries located around the world.

In addition to hydrocarbons, petroleum also contains some sulfur (up to 4%) in the form of compounds: hydrogen sulfide gas, H2S, and organic sulfur compounds that are alcohol and ether analogs in which an S atom has replaced the oxygen. These substances are much more readily removed from oil than is the sulfur from coal, making petroleum products inherently cleaner. Diesel fuel distilled from petroleum contains a higher percentage of residual sulfur than does gasoline, and the residue from the distillation contains the highest sulfur concentration of all—as well as most of the metals vanadium and nickel from the original crude oil, usually at levels of several parts per million. Sulfur that is present in fuels generally is converted during the fuel's combustion into sulfur dioxide, which is a serious pollutant if released into the air (Chapter 3). Some organic compounds of nitrogen also occur in petroleum and are the source of the "fuel NO" (Chapter 3) formed when gasoline and diesel oil burn.

Petroleum and fuels made from it, such as gasoline and diesel oil, have the great advantage that they are energy-dense liquids that are convenient, relatively safe to use, and relatively cheap to produce. Virtually all nonelectrical transportation systems in both the developed and developing worlds are based on cheap petroleum fuels. The possibility of switching to alternative fuels for transportation is discussed in detail in Chapter 8. It will be much more difficult to switch from oil to other chemical feedstocks for the production of pharmaceuticals and polymers once oil runs out.

Although it took nature about half a billion years to create the world's supply of petroleum, humans will probably have used almost all of it during the 200-year period that started just before the end of the nineteenth century. Indeed, the production of petroleum in the lower 48 states of the United States has already peaked. In commerce, petroleum is measured in barrels, each of which is equivalent to 159 liters, or 42 U.S. gallons. Current annual

BOX 7-1

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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