Organic Chemistry

Section 15.8 How Benzene Reacts 605

THE TOXICITY OF BENZENE

Although benzene has been widely used in chem-

ical synthesis and has been frequently used as a

solvent, it is toxic. Its major toxic effect is on the central ner-

vous system and on bone marrow. Chronic exposure to benzene

causes leukemia and aplastic anemia. A higher-than-average

incidence of leukemia has been found in industrial workers

with long-term exposure to as little as 1 ppm benzene in the at-

mosphere. Toluene has replaced benzene as a solvent because,

although it is a central nervous system depressant like benzene,

it does not cause leukemia or aplastic anemia. “Glue sniffers”

seek the narcotic central nervous system effects of solvents

such as toluene. This can be a highly dangerous activity.

An aryl group (Ar) is the general term for either a phenyl group or a substituted
phenyl group, just as an alkyl group (R) is the general term for a group derived from an
alkane. In other words, ArOH could be used to designate any of the following phenols:

OH OH

Br

OH OH

OCH 3

CH 2 CH 3

PROBLEM 15

Draw the structure of each of the following compounds:

a. 2-phenylhexane c. 3-benzylpentane

b. benzyl alcohol d. bromomethylbenzene

15.8 How Benzene Reacts

As a consequence of the electrons above and below the plane of its ring, benzene is
a nucleophile. It will, therefore, react with an electrophile When an electrophile
attaches itself to a benzene ring, a carbocation intermediate is formed.

This should remind you of the first step in an electrophilic addition reaction of an
alkene: A nucleophilic alkene reacts with an electrophile, thereby forming a carboca-
tion intermediate (Section 3.6). In the second step of an electrophilic addition reaction,
the carbocation reacts with a nucleophile to form an addition product.

RCH CHR + Y+

carbocation

intermediate product of electrophilicaddition

Y

CHR

Z−

RCH

+

Y

RCH CHR

Z

1 Z- 2

Y+ Y

+ H

+

carbocation

intermediate

1 Y+ 2.

p