The Science Book

A CENTURY OF PROGRESS 163

Kekulé used the concept of valency to describe the

bonds that are formed between atoms to make various

molecules. Here, each bond is represented by a line.

a valency of two. Then, in 1858,
British chemist Archibald Couper
suggested that bonds were formed
between self-linking carbon atoms,
and that molecules were chains of
atoms bonded together. So water,
which was known to consist of two
parts of hydrogen to one of oxygen,
could be represented as H 2 O, or
H–O–H, where “–” signifies a bond.
Carbon has a valency of four,
making it tetravalent, so a carbon
atom can form four bonds, as in
methane (CH 4 ), where the hydrogen
atoms are arranged in a tetrahedron
around the carbon. (Today, chemists
think of a bond as representing a
pair of electrons shared between
the two atoms, and the symbols H,
O, and C as representing the central
part of the appropriate atom.)
Couper was working at the time
at a laboratory in Paris. Meanwhile,
in Heidelberg, Germany, August
Kekulé had come up with the same
idea, announcing in 1857 that
carbon has a valency of four, and
early in 1858 that carbon atoms can
bond to one another. Publication of
Couper’s paper had been delayed,
allowing Kekulé to publish a month
before him and claim priority for
the idea of self-bonding carbon
atoms. Kekulé called the bonds
between atoms “affinities,” and
explained his ideas in greater
detail in his popular Textbook of
Organic Chemistry, which first
appeared in 1859.

Carbon compounds
Figuring out theoretical models
based on evidence from chemical
reactions, Kekulé declared that
tetravalent carbon atoms could link
together to form what he called a
“carbon skeleton,” to which other
atoms with other valencies (such as

hydrogen, oxygen, and chlorine)

could bond. Suddenly, organic

chemistry began to make sense,

and chemists assigned structural

formulae to all kinds of molecules.

Simple hydrocarbons such as

methane (CH 4 ), ethane (C 2 H 6 ), and

propane (C 3 H 8 ) were now seen to be

chains of carbon atoms where the

spare valencies were occupied by

hydrogen atoms. Reacting such a

compound with, say, chlorine (Cl 2 )

produced compounds in which one

or more of the hydrogen atoms were

replaced by chlorine atoms, making

compounds such as chloromethane

or chloroethane. One feature of this

substitution was that chloropropane

came in two distinct forms, either

1-chloropropane or 2-chloropropane,

depending on whether the chlorine

was attached to the middle carbon

atom or one of the end carbon atoms

See also: Robert Boyle 46–49 ■ Joseph Black 76–77 ■ Henry Cavendish 78–79 ■ Joseph Priestley 82–83 ■
Antoine Lavoisier 84 ■ John Dalton 112–13 ■ Humphry Davy 114 ■ Linus Pauling 254–59 ■ Harry Kroto 320–21

(see the diagram above). Some

compounds need double bonds to

satisfy the valencies of the atoms:

the oxygen molecule (O 2 ), for

example, and the molecule of

ethylene (C 2 H 4 ). Ethylene reacts

with chlorine, and the result is

not substitution but addition. The

chlorine adds across the double

bond, to make 1,2 dichloroethane

(C 2 H 4 Cl 2 ). Some compounds even

have triple bonds, including the

nitrogen molecule (N 2 ) and

acetylene (C 2 H 2 ), which is highly

reactive, and used in oxyacetylene

welding torches.

Benzene, however, remained

a puzzle. It turned out to have

the formula C 6 H 6 , but is much

less reactive than acetylene,

even though both compounds

have equal numbers of carbon

and hydrogen atoms. Devising a ❯❯

O = O

Oxygen

N = N

Nitrogen

Water

O

HH

C

H

HH

H

Methane Ethane

C

H

H

H

C

H

H

C

H

H

H

Propane

C

H

H

H

C

H

H

C

H

H

C

H

H

H

1-Chloropropane

C

H

Cl

H

C

H

H

C

H

H

C

H

H

H

2-Chloropropane

C

H

H

H

C

H

H

C

Cl

H

C

H

H

H