Nonpolar molecules tend to interact with other

Nonpolar molecules tend to interact with other nonpolar
molecules. For example, molecules containing only hydrogen
and carbon atoms—called hydrocarbon molecules —are nonpolar.
(Compare the electronegativities of hydrogen and carbon in
Table 2.2 to see why.) In water these molecules tend to aggre-
gate with one another rather than with the polar water mol-
ecules. Therefore, nonpolar molecules are known as
hydro-
phobic (“water-hating”), and the interactions between them
are called hydrophobic interactions ( FIGURE 2.6B ). Hydrophobic
substances do not really “hate” water—they can form weak
interactions with it, since the electronegativities of carbon and
hydrogen are not exactly the same. But these interactions are
far weaker than the hydrogen bonds between the water mol-
ecules, so the nonpolar substances tend to aggregate.
Water
is polar.
Polar molecules are
attracted to water.
Nonpolar molecules are
more attracted to one
another than to water.
(A) Hydrophilic
(B) Hydrophobic
δ
δ
δ
+
δ
–
δ
–
δ
+
The concepts of chemical bonding and electronegativ-
ity (see Table 2.2) allow us to predict whether a mol-
ecule will be polar or nonpolar, and how it will interact
with water. Typically, a difference in electronegativity
greater than 0.5 will result in polarity. For each of the
bonds below, indicate:
1. Whether the bond is polar or nonpolar
2. If polar, which is the d + end
3. How a molecule with the bond will interact with
water (hydrophilic or hydrophobic).
N—H C—H C==O C—N
O—H C—C H—H O—P
Atoms interact and form molecules
APPLY THE CONCEPT
FIGURE 2.6 Hydrophilic and Hydrophobic
(A) Molecules with
polar covalent bonds are attracted to polar water (they are hydro-
philic). (B) Molecules with nonpolar covalent bonds show greater
attraction to one another than to water (they are hydrophobic).
The color convention in the models shown here (gray, H; red, O;
black, C) is often used.

5.2
2.2
Atoms Interact and Form Molecules 23
Functional groups confer specific properties to
biological molecules
Certain small groups of atoms, called
functional groups
, are
consistently found together in very different biological mol-
ecules. You will encounter several functional groups repeat-
edly in your study of biology ( FIGURE 2.7 ). Each functional
group has specific chemical properties, and when attached
to a larger molecule, it confers those properties on the larger
molecule. One of these properties is polarity. Can you deter-
mine which functional groups in Figure 2.7 are the most polar?
The consistent chemical behavior of functional groups helps us
understand the properties of the molecules that contain them.
Biological molecules often contain many different functional
groups. A single large protein may contain hydrophobic, polar,
and charged functional groups. Each group gives a different
specific property to its local site on the protein, and it may in-
teract with another group on the same protein or with another
molecule. Thus, the functional groups determine molecular
shape and reactivity.