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Answer 1 · 2017-05-13T14:18:57

The systematic name tells you the number of #"C=C"# double bonds.

The common names tell you nothing about the structure of the fatty acid.

For example, consider oleic acid, linoleic acid, and linolenic acid.

If you use common names, you must memorize the number of #"C=C"# double bonds in each one.

Let's examine the systematic names for these fatty acids.

Oleic acid

Its structural formula is

#"CH"_3("CH"_2)_7"CH=CH"("CH"_2)_7"COOH"#

Its systematic name is #(Z)"-octadec-9-"bb"en""oic acid"#.

There is no multiplying prefix in front of "en", so there is only one #"C=C"# bond in the molecule.

Linoleic acid

Its structural formula is

#"CH"_3("CH"_2)_4"CH=CH-CH"_2"-CH=CH"("CH"_2)_7"COOH"#

Its systematic name is #(9Z,12Z)"-octadeca-9,12-"bb"dien""oic acid"#.

The multiplying prefix "di" in front of "en", tells you that there are two #"C=C"# bonds in the molecule.

Linolenic acid

Its structural formula is

#"CH"_3"CH"_2"CH=CH-CH"_2"CH=CH-CH"_2"CH=CH"("CH"_2)_7"COOH"#

Its systematic name is #(9Z,12Z,15Z)"-octadeca-9,12,15-"bb"trien""oic acid"#.

The multiplying prefix "tri" in front of "en", tells you that there are three #"C=C"# bonds in the molecule.