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number of double bonds, melting point,
and solubility at 30°C are shown in Table 1. A saturated lauric
acid (Latin laurus, laurel plant) with 12 carbons is named 12:0
(n-dodecanoic acid), and a saturated 16-carbon palmitic acid (Greekpalma,
palm tree) is named 16:0 (n-hexadecanoic acid). The 18-carbon
oleic acid with one double bond
is
18: 1. The positions of double bonds are indicated with
superscript numbers, e.g., a 20-carbon fatty acid with one double
bond between C-9 and C-10, (C-l being the carboxyl carbon) and
another between C-12 and C-13, is designated as 20:2 (L19,12).
Most commonly occurring fatty acids have even numbers of carbon
atoms in a linear chain of 12 to 24 carbons (Table 1). The even
number of carbon atoms in these compounds reflects the mode of
their synthesis, which involves condensation of 2-C acetate units.
Most monounsturated fatty acids have their double bond between C-9
and C-10, the other double bonds being usually after C-12 and C-15
positions and are in the cis configuration.
The
physical properties of the fatty acids and their derivatives are
influenced by the length and degree of unsaturation of the
hydrocarbon chain, the nonpolar hydrocarbon chain primarily
acccounting for their poor solubility in water. The water
solubility decreases with an increase in the length of the fatty
acyl chain and a decrease in the number of double bonds. The polar
carboxylic acid group accounts for the slight solubility of
short-chain fatty acids in water.
The
length and degree of unsaturation of the hydrocarbon chain of
fatty acids and lipids also influence their melting points (Table
1). At the ambient temperature (25°C), the saturated fatty acids
from C-12 to C-24 have a waxy consistency, whereas the unsaturated
fatty acids of the same lengths are oily liquids. Free rotation
around each C-CG bgnd in the fully saturated fatty acids enables
great flexibility and more stable conformation of the fully
extended hydrocarbon chain with minimum stearic hindrance from the
neighboring atoms. Such molecules can pack together tightly as the
result of van der Waals forces with the atoms of neighboring
molecules. The presence of one or more cis double bonds interferes
with this tight packing and results in less stable aggregates. The
lower melting points of the unsaturated fatty acids can be
attributed to the lower energy required to disorder the poorly
ordered arrays of fatty acids due to the presence of double
bonds
.
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