Page 100 - Physical chemistry understanding our chemical world

CREATING FORMAL CHEMICAL BONDS 67

Nevertheless, even today, we often discuss the bonding of organic compounds in
terms of Lewis structures and valence bond theory.

Whyispetroleum gelsosoft?

Properties of covalent compounds

Clear petroleum gel is a common product, comprising a mixture of simple hydro-
carbons, principally n-octadecane (III). It is not quite a solid at room temperature;
neither is it really a liquid, because it is very viscous. We call it a gel. Its principal
applications are to lubricate (in a car) or to act as a water-impermeable barrier (e.g.
between a baby and its nappy, or on chapped hands).

(III)

We saw on p. 52 how methane is a gas unless condensed by Molecules made of only
compression at high pressure or frozen to low temperatures. But
one element are called
octadecane is neither a solid nor a gas. Why? ‘homonuclear’, since
There are several, separate types of interaction in III: both cova- homo is Greek for
lent bonds and dipoles. Induced dipoles involve a partial charge, ‘same’. Examples of
−
+
which we called δ or δ , but, by contrast, covalent bonds involve homonuclear molecules
whole numbers of electrons. A normal covalent bond, such as that are H 2 , N 2 , S 8 and ful-
between a hydrogen atom and one of the carbon atoms in the back- lerene C 60 .
bone of III, requires two electrons. A ‘double bond’ consists simply
of two covalent bonds, so four electrons are shared. Six electrons
are incorporated in each of the rare instances of a covalent ‘triple Even a covalent bond
bond’. A few quadruple bonds occur in organometallic chemistry, can possess a perma-
but we will ignore them here. nent induced dipole.
Most covalent bonds are relatively non-polar. Some are com-
pletely non-polar: the diatomic hydrogen molecule is held together
with two electrons located equidistantly from the two hydrogen
nuclei. Each of the two atoms has an equal ‘claim’ on the elec- Covalent compounds
trons, with the consequence that there is no partial charge on the tend to be gases or liq-
atoms: each is wholly neutral. Only homonuclear molecules such uids. Even when solid,
as H 2 ,F 2 ,O 2 or N 2 are wholly non-polar, implying that the major- they tend to be soft.
ity of covalent bonds do possess a slight polarity, arising from an But many covalent
unequal sharing of the electrons bound up within the bond. compounds are only
solid at lower temper-
We see the possibility of a substance having several types of
atures and/or higher
bond. Consider water for example. Formal covalent bonds hold
pressures, i.e. by max-
together the hydrogen and oxygen atoms, but the individual water
imizing the incidence of
molecules cohere by means of hydrogen bonds. Conversely, paraf-
induced dipoles.
fin wax (n-C 15 H 32 ) is a solid. Each carbon is bonded covalently