Organic Chemistry

Section 26.1 Fatty Acids 003

OMEGA FATTY ACIDS

Omegais a term used to indicate the position of

the first double bond—from the methyl end—in an

unsaturated fatty acid. For example, linoleic acid is called

omega-6 fatty acid because its first double bond is after the sixth

carbon, and linolenic acid is called omega-3 fatty acid because

its first double bond is after the third carbon. Mammals lack the

enzyme that introduces a double bond beyond C-9 (the carboxyl

carbon is C-1). Linoleic acid and linolenic acids, therefore, are

essential fatty acids for mammals. In other words, the acids must

be included in the diet because, although they cannot be synthe-

sized, they are required for normal body function.

linoleic acid linolenic acid

COOH

omega-3

fatty acid

COOH

omega-6

fatty acid

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fewer intermolecular interactions and, therefore, lower melting points than saturated
fatty acids with comparable molecular weights (Table 26.1). The melting points of the
unsaturated fatty acids decrease as the number of double bonds increases. For example,
an 18-carbon fatty acid melts at 69°C if it is saturated, at 13°C if it has one double bond,
at - 5 °Cif it has two double bonds, and at - 11 °Cif it has three double bonds.

oleic acid

an 18-carbon fatty acid

with one double bond

stearic acid an 18-carbon fatty acid

with one double bond

linoleic acid

an 18-carbon fatty acid

with three double bonds

linolenic acid an 18-carbon fatty acid

with three double bonds

PROBLEM 1

Explain the difference in the melting points of the following fatty acids:

a. palmitic acid and stearic acid c. oleic acid and linoleic acid

b. palmitic acid and palmitoleic acid

PROBLEM 2

What products are formed when arachidonic acid reacts with excess ozone followed by

treatment with H 2 O 2 ?(Hint:See Section 20.8.)

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