Organic Chemistry

Introduction 337

carbons containing three and four carbons) is a gas that can be liquefied under pres-
sure. This gas is used as a fuel for cigarette lighters, camp stoves, and barbecues. The
fraction that boils at somewhat higher temperatures (hydrocarbons containing 5 to 11
carbons) is gasoline; the next fraction (9 to 16 carbons) includes kerosene and jet fuel.
The fraction with 15 to 25 carbons is used for heating oil and diesel oil, and the
highest-boiling fraction is used for lubricants and greases. The nonpolar nature of
these compounds is what gives them their oily feel. After distillation, a nonvolatile
residue called asphalt or tar is left behind.
The 5- to 11-carbon fraction that is used for gasoline is, in fact, a poor fuel for
internal combustion engines and requires a process known as catalytic cracking to be-
come a high-performance gasoline. Catalytic cracking converts straight-chain hydro-
carbons that are poor fuels into branched-chain compounds that are high-performance
fuels. Originally, cracking (also called pyrolysis) involved heating gasoline to very
high temperatures in order to obtain hydrocarbons with three to five carbons. Modern
cracking methods use catalysts to accomplish the same thing at much lower tem-
peratures. The small hydrocarbons are then catalytically recombined to form highly
branched hydrocarbons.

gasoline

kerosene,

jet fuel

heating oil,

diesel oil

lubricants,

greases

heating

element

natural gas

asphalt, tar

OCTANE NUMBER

When poor fuels are used in an engine, combus-

tion can be initiated before the spark plug fires.

A pinging or knocking may then be heard in the running

engine. As the quality of the fuel improves, the engine is less

likely to knock. The quality of a fuel is indicated by its octane

number. Straight-chain hydrocarbons have low octane num-

bers and make poor fuels. Heptane, for example, with an arbi-

trarily assigned octane number of 0, causes engines to knock

badly. Branched-chain fuels burn more slowly—thereby

reducing knocking—because they have more primary hydro-

gens. Consequently, branched-chain alkanes have high octane

numbers. 2,2,4-Trimethylpentane, for example, does not

cause knocking and has arbitrarily been assigned an octane

number of 100.

The octane rating of a gasoline is determined by comparing

its knocking with the knocking of mixtures of heptane and

2,2,4-trimethylpentane. The octane number given to the

gasoline corresponds to the percent of 2,2,4-trimethylpentane

in the matching mixture. The term “octane number”originat-

ed from the fact that 2,2,4-trimethylpentane contains eight

carbons.

CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 CH 3 CCH 2 CHCH 3

heptane

octane number = 0

2,2,4-trimethylpentane

octane number = 100

CH 3

CH 3

CH 3

FOSSIL FUELS: A PROBLEMATIC

ENERGY SOURCE

We face three major problems as a consequence

of our dependence on fossil fuels for energy. First, fossil fuels

are a nonrenewable resource and the world’s supply is contin-

ually decreasing. Second, a group of Middle Eastern and

South American countries controls a large portion of the

world’s supply of petroleum. These countries have formed a

cartel known as the Organization of Petroleum Exporting

Countries(OPEC), which controls both the supply and the

price of crude oil. Political instability in any OPEC country

can seriously affect the world oil supply. Third, burning fossil

fuels increases the concentrations of and in the

atmosphere. Scientists have established experimentally that

CO 2 SO 2

atmospheric causes “acid rain,”a threat to the Earth’s

plants and, therefore, to our food and oxygen supplies. The at-

mospheric concentration has increased 20% in the last 10

years, causing scientists to predict an increase in the Earth’s

temperature as a result of the absorption of infrared radiation

by (the so-called greenhouse effect). A steady increase in

the temperature of the Earth would have devastating conse-

quences, including the formation of new deserts, massive crop

failure, and the melting of polar ice caps with a concomitant

rise in sea level. Clearly, what we need is a renewable, non-

political, nonpolluting, and economically affordable source

of energy.

CO 2

CO 2

SO 2