Page 152 - Encyclopedia of Chemical Compounds 3 Vols

Z
BENZENE

the double bonds in benzene do not act like double bonds
in other compounds was explained by the German chemist
Johannes Thiele (1860–1935), who suggested that the bonds
in benzene shift back and forth between single and double
bonds so rapidly that they are not able to behave like typical
double bonds. Chemists now use a variety of chemical for-
mulas for representing the character of chemical bonds in
benzene.
Benzene is a very popular raw material for a variety of
industrial chemical reactions. In 2004, U.S. manufacturers
produced 8.8 million metric tons (9.7 million short tons) of
benzene, placing it in twelfth place among all chemicals
made in the United States that year.

HOW IT IS MADE
At one time, benzene was obtained from coal tar, the
thick gooey liquid left over after soft coal is converted to
coke. This method has now been largely replaced by a variety
of methods that use crude oil or refined petroleum as a raw
material. In the most popular of these methods, toluene
(C 6 H 5 CH 3 ) from petroleum is heated over a catalyst of plati-
num metal and aluminum oxide (Al 2 O 3 ). The toluene loses its
methyl group (-CH 3 ), leaving benzene as the primary product.
Other methods are available for changing the molecular
structure of hydrocarbons found in petroleum and convert-
ing them to benzene.

COMMON USES AND POTENTIAL HAZARDS
By far the most important use of benzene is as a raw
material in the synthesis of other organic compounds.
More than 90 percent of the benzene produced in the
United States is used to make ethylbenzene (55 percent),
cumene (24 percent), and cyclohexane (12 percent). The
first two compounds rank fifteenth and twentieth, respec-
tively, among all chemicals produced in the United States
each year. Another five percent of benzene production goes
to the synthesis of a large variety of other organic com-
pounds, including nitrobenzene, chlorobenzene, and maleic
anhydride, a raw material for the manufacture of plastics.
Smaller amounts of benzene are used as a solvent for

CHEMICAL COMPOUNDS 101