Page 12 - Mechanism and Theory in Organic Chemistry

Chapter 1

THE COVALENT

BOND

Because the covalent bond is of central importance to organic chemistry, we
begin with a review of bonding theory. Later, in Chapter 10, we shall return to
develop certain aspects of the theory further in preparation for the discussion of
pericyclic reactions.

1.1 MODELS OF CHEMICAL BONDING
Understanding and progress in natural science rest largely on models. A little
reflection will make it clear that much of chemical thinking is in terms of models,
and that the models useful in chemistry are of many kinds. Although we cannot
see atoms, we have many excellent reasons for believing in them, and when we
think about them we think in terms of models. For some purposes a very simple
model suffices. Understanding stoichiometry, for example, requires only the idea
of atoms as small lumps of matter that combine with each other in definite pro-
portions and that have definite weights. The mechanism by which the atoms are
held together in compounds is not of central importance for this purpose. When
thinking about stereochemistry, we are likely to use an actual physical model con-
sisting of small balls of wood or plastic held together by springs or sticks. Now the
relative weights of atoms are immaterial, and we do not bother to reproduce
them in the model; instead we try to have the holes drilled carefully so that the
model will show the geometrical properties of the molecules. Still other models
are entirely mathematical. We think of chemical rate processes in terms of sets of
differential equations, and the details of chemical bonding require still more ab-
stract mathematical manipulations. The point to understand is that there may be
many ways of building a model for a given phenomenon, none of which is com-