General Chemistry Principles, Patterns, and Applications, 2011

Lipids
Lipids (from the Greek lipos, meaning “fat” or “lard”) are characterized by their insolubility in water. They
form a family of compounds that includes fats, waxes, some vitamins, and steroids. Fatty acids, the
simplest lipids, have a long hydrocarbon chain that ends with a carboxylic acid functional group. In
saturated fatty acids, the hydrocarbon chains contain only C–C bonds, so they can stack in a regular array
(part (a) in Figure 24.23 "Fatty Acids, the Simplest Class of Lipids"). In contrast, unsaturated fatty acids
have a single double bond in the hydrocarbon chain (monounsaturated) or more than one double bond
(polyunsaturated). These double bonds give fatty acid chains a kinked structure, which prevents the
molecules from packing tightly (part (b) in Figure 24.23 "Fatty Acids, the Simplest Class of Lipids"). As a
result of reduced van der Waals interactions, the melting point of an unsaturated fatty acid is lower than
that of a saturated fatty acid of comparable molecule mass, thus producing an oil rather than a solid. (For
more information on van der Waals interactions, see Chapter 11 "Liquids", Section 11.2 "Intermolecular
Forces".) Fish oils and vegetable oils, for example, have a higher concentration of unsaturated fatty acids
than does butter.
Figure 24.23 Fatty Acids, the Simplest Class of Lipids
Fatty acids are composed of a long chain that terminates in a carboxylic acid functional group. (a) Molecules of
saturated fatty acids, which contain no carbon–carbon double bonds, can stack in a regular array. (b) Molecules of
unsaturated fatty acids, which contain one or more cis carbon–carbon double bonds, have kinked structures that
cannot pack closely together.
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