The Science Book

164 AUGUST KEKULÉ

linear structure that was not
highly reactive was a real
conundrum. There clearly had to be
double bonds, but how they were
arranged was a mystery.
Furthermore, benzene reacts
with chlorine not by addition (like
ethylene) but by substitution: a
chlorine atom replaces a hydrogen
atom. When one of benzene’s
hydrogen atoms is substituted
by a chlorine atom, the result is
only a single compound C 6 H 5 Cl,
chlorobenzene. This seemed to
show that all the carbon atoms
were equivalent, since the chlorine
atom might be attached to any
one of them.

Benzene rings

The solution to the puzzle of

benzene’s structure came to Kekulé

in 1865 in a dream. The answer

was a ring of carbon atoms, a ring

in which all six atoms were equal,

with a hydrogen atom bonded to

each one. This meant that the

chlorine in chlorobenzene could be

attached anywhere around the ring.

Further support for this theory

came from substituting hydrogen

twice, to make dichlorobenzene

(C 6 H 4 Cl 2 ). If benzene is a six-

membered ring with all the carbon

atoms equal, there should be three

distinct forms, or “isomers,” of this

compound—the two chlorine atoms

could be on adjacent carbon atoms,

on carbon atoms separated by one

other carbon, or at opposite ends of

the ring. This turned out to be the

case, and the three isomers were

named ortho-, meta-, and para-

dichlorobenzene respectively.

Establishing symmetry

An unsolved mystery still remained

over the observed symmetry of

the benzene ring. To satisfy

its tetravalency, each carbon atom

should have four bonds to other

atoms. This meant that they all

had a “spare” bond. At first,

Kekulé drew alternating single

and double bonds around the

ring, but when it became

apparent that the ring had to

be symmetrical, he suggested

that the molecule oscillated

between the two structures.

The electron was not discovered

until 1896. The idea that bonds

form through the sharing of

electrons was first proposed by

American chemist G. N. Wilson in

1916. In the 1930s, Linus Pauling
      then used quantum mechanics
      to explain that the six spare
      electrons in the benzene ring are
      not localized in double bonds, but

Kekulé suggested that double and single bonds between carbon
atoms in a benzene ring alternated (left). Two chlorine atoms can
substitute for two of the hydrogen atoms in three different ways (right).

Benzene C 6 H 6

C

H

H

H

C

CC

CC

H

H H

C

H

H

H

C

CC

CC

H

H H

Ortho-dichlorobenzine

C

Cl

H

H

C

CC

CC

Cl

H H

Meta-dichlorobenzine

C

H

Cl

H

C

CC

CC

Cl

H H

Para-dichlorobenzine

C

H

H

H

C

CC

CC

Cl

H Cl

This image of a hexabenzocoronene

molecule was captured using an atomic

force microscope. It is 1.4 nanometers

in diameter and shows carbon–carbon

bonds of different lengths.