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that are based on the benzene ring structure are called aromatic
hydrocarbons. To denote the six-carbon ring with delocal- Condenser
ized electrons, benzene is represented by the symbol shown in
Figure 12.9B.
The circle in the six-sided benzene symbol represents the Gas
delocalized electrons. Figure 12.9B illustrates how this benzene
Gasoline
ring symbol is used to show the structural formula of some aro-
Kerosene
matic hydrocarbons. You may have noticed some of the names
on labels of paints, paint thinners, and lacquers. Toluene and
Heating oil
the xylenes are commonly used in these products as solvents. A
benzene ring attached to another molecule or functional group Lubricating oil
is given the name phenyl.

12.3 PETROLEUM
Crude oil
Petroleum is a mixture of alkanes, cycloalkanes, and some aro- vapors
matic hydrocarbons. The origin of petroleum is uncertain, but from
heater
it is believed to have formed from the slow decomposition of
buried marine life, primarily microscopic plankton and algae
in the absence of oxygen (i.e., anaerobic). Time, temperature,
pressure, and perhaps bacteria are considered important in the
Steam
formation of petroleum. As the petroleum formed, it was forced
through porous rock until it reached a rock type or rock struc- Residue
ture that stopped it. Here, it accumulated to saturate the porous (asphalt, tar)
rock, forming an accumulation called an oil fi eld. The composi- FIGURE 12.10 Fractional distillation is used to separate
tion of petroleum varies from one oil field to the next. The oil petroleum into many products. This simplified illustration shows
from a given field might be dark or light in color, and it might how the about 9 m (30 ft) tower is used to separate the different
have an asphalt or paraffin base. Some oil fields contain oil with “fractions” by differences in their boiling points.
a high quantity of sulfur, referred to as “sour crude.” Because of
such variations, some fields have oil with more desirable quali-
ties than oil from other fields. within a range of carbon atoms per molecule (Figure 12.11).
Early settlers found oil seeps in the eastern United States The products, their boiling ranges, and ranges of carbon atoms
and collected the oil for medicinal purposes. One enterprising per molecule are listed in Table 12.3.
oil peddler tried to improve the taste by running the petroleum The hydrocarbons that have one to four carbon atoms (CH 4
through a whiskey still. He obtained a clear liquid by distilling to C 4 H 10 ) are gases at room temperature. They can be pumped
the petroleum and, by accident, found that the liquid made an from certain wells as a gas, but they also occur dissolved in
excellent lamp oil. This was fortunate timing, for the lamp oil crude oil. Natural gas is a mixture of hydrocarbon gases, but it
used at that time was whale oil, and whale oil production was is about 95 percent methane (CH 4 ). Propane (C 3 H 8 ) and butane
declining. This clear liquid obtained by distilling petroleum is (C 4 H 10 ) are liquefied by compression and cooling and are sold
today known as kerosene. as liquefied petroleum gas, or LPG. LPG is used where natural
Wells were drilled, and crude oil refineries were built to gas is not available for cooking or heating and is widely em-
produce the newly discovered lamp oil. Gasoline was a by- ployed as a fuel in barbecue grills and camp stoves.
product of the distillation process and was used primarily as a Gasoline is a mixture of hydrocarbons that may have 5 to
spot remover. With Henry Ford’s automobile production and 12 carbon atoms per molecule. Gasoline distilled from crude
Edison’s electric light invention, the demand for gasoline in- oil consists mostly of straight-chain molecules not suitable for
creased, and the demand for kerosene decreased. The refineries use as an automotive fuel. Straight-chain molecules burn too
were converted to produce gasoline, and the petroleum industry rapidly in an automobile engine, producing more of an ex-
grew to become one of the world’s largest industries. plosion than a smooth burn. You hear these explosions as a
Crude oil is petroleum that is pumped from the ground, a knocking or pinging in the engine, and they indicate poor ef-
complex and variable mixture of hydrocarbons with an upper ficiency and could damage the engine. On the other hand,
limit of about 50 carbon atoms. This thick, smelly black mixture branched-chain molecules burn slower by comparison, with-
is not usable until it is refined, that is, separated into usable out the pinging or knocking explosions. The burning rate of
groups of hydrocarbons called petroleum products. Petroleum gasoline is described by the octane number scale. The scale
products are separated from crude oil by distillation, and any is based on pure n-heptane, straight-chain molecules that
particular product has a boiling point range, or “cut,” of the are assigned an octane number of 0 and a multiple branched
distilled vapors (Figure 12.10). Thus, each product, such as isomer of octane that is assigned an octane number of 100
gasoline, heating oil, and so forth, is made up of hydrocarbons (Figure 12.12). Most gasolines have an octane rating of 87,

12-7 CHAPTER 12 Organic Chemistry 305