Appendix 6 • MHR 565
compounds in organic chemistry and biology.
Hydrocarbon molecules, some of which contain
large numbers of atoms, include carbon–hydrogen
bonds that are slightly polar. However, because
of the shapes of hydrocarbon molecules, these
molecules are either non-polar or very low
in polarity.
Some Biological Applications
The presence or absence of dipoles can have
a great effect on the physical and chemical
properties of covalent molecules. Therefore,
an understanding of dipoles is very important
in explaining many biological processes. Some
examples of how electronegativity is important
in biological processes are given below.
Functional Groups
Some important functional groups are listed in
Figure 1.24 in section 1.4. All of the functional
groups listed contain covalent bonds between
elements with different electronegativities, so
the functional groups contain polar covalent
bonds. The presence of these polar bonds in
biological molecules influences the physical
interactions of the molecules and their
chemical properties.
A hydroxyl group (−OH) contains a single
covalent bond between a highly electronegative
oxygen atom and a less electronegative hydrogen
atom. As in a water molecule, the oxygen atom
is at the slightly negative end of the dipole, and
hydrogen is at the slightly positive end. Hydroxyl
groups can attract some ions and polar molecules
in a type of interaction called a hydrogen bond,
which will be described below.
In a carbonyl group, highly electronegative
oxygen forms a double bond with less
electronegative carbon. Because the carbon
atom is at the more positive end of the dipole,
the carbon atom tends to react with substances
that are electron donors. The oxygen atom at the
more negative end of the dipole tends to react
with substances that are electron acceptors.
A carboxyl group contains both a carbonyl
group and a hydroxyl group connected together.
Because a carboxyl group contains two highly
electronegative oxygen atoms, the positive
charge on the hydrogen atom is larger than in a
hydroxyl group alone. In fact, carboxyl groups
partially dissociate to form hydrogen ions (H+).
Therefore, carboxyl groups are present in
organic acids, such as amino acids. An amino
acid molecule also contains an amino group
(−NH 2 ). In this functional group, the nitrogen
atom is at the slightly negative end of each
nitrogen–hydrogen dipole. The nitrogen atom
can act as an electron donor, for example to a
hydrogen ion. Thus, an amino acid contains a
functional group (a carboxyl group) that can
release hydrogen ions and another functional
group (an amino group) that can react with them.
Hydrogen Bonds
When hydrogen atoms are bonded to small,
highly electronegative atoms, such as oxygen,
a weak force of attraction can exist between
molecules. This force is known as a hydrogen
bond. A hydrogen bond can be formed when a
negative ion (such as Cl−), or a slightly negative
atom in a dipole (such as the oxygen in an −OH
group) attracts a slightly positive hydrogen atom
in another dipole. Therefore, water molecules
form hydrogen bonds with each other, as shown
in Figure A6.4. Hydrogen bonds are only about
one-twentieth as strong as covalent bonds, but
hydrogen bonds have important effects on the
physical and chemical behaviour of many
molecules in biological systems.
Figure A6.4Hydrogen bonds exist between water
molecules and account for many of the physical and
chemical properties of water.
The hydrogen bonds between water molecules
make water an ideal medium for life processes
on Earth. Scientists believe that if there were no
hydrogen bonds present, then water would be a
gas, not a liquid, under the conditions in which
we live. The polarity of water molecules, and
their ability to form hydrogen bonds, explains
hydrogen
bond
δ+
δ−