Natural Product Biomolecules - Organic Chemistry Concepts: An EFL Approach - page 157

If there is only one C ] C double bond, the fatty acid is monounsaturated. If
there is more than one C ] C double bond, the fatty acid is polyunsaturated.
From Table 8.1 , you can see that the melting point of fatty acids depends on
both the length of the carbon chain and the number of C ] C double bonds.
The presence of cis double bonds strongly affects the efficiency of the hydro-
carbon chains to pack together. This weakens the intermolecular attractions
and lowers the melting point. The high percentage of non-polar hydrocar-
bon material means that fatty acids are not soluble in water and other polar
solvents.
Except for the carboxylic acid group and C ] C double bonds, fatty acids do not
have any other functional groups. As a result, fatty acids undergo the reactions
which are expected from these groups. Therefore, simple acid-base reactions,
acyl conversions such as esterification and hydrolysis, and C ] C additions such
as halogenation and hydrogenation occur.
8.3.2 Fats and Oils
This is the biggest group of naturally occurring lipids. Fats usually come from
animal sources and are solids at room temperature. Oils usually come from
plants and are liquids at room temperature. Fats and oils are all triesters based
on three molecules of a fatty acid, with glycerol as the common alcohol.
Figure 8.11 shows esterification to give a triglyceride.
Simple triglycerides have only one type of fatty acid. Mixed triglycerides have
more than one type of fatty acid. Again, the presence of C ] C double bonds
in the fatty acid gives products with lower melting points. Fats are usually
solids because they have a high percentage of saturated fatty acids. Oils are
usually liquids because they have a relatively higher percentage of unsatu-
rated fatty acids.
FIGURE 8.11
Triglycerides by esterification.
The functional group reactions of triglycerides are similar to those of the simple
alkenes and esters seen earlier. For example, hydrogenation (Section 7.3.2) of
unsaturated vegetable oils changes them to the more saturated solid or semi-
solid fats. The most common reaction of the triglyceride ester groups is base
hydrolysis to give soaps. This is often called saponification.