(a) Draw a skeletal structure of gadoleic acid, CH_{3}(CH_{2})_{9}CH\overline{—}CH(CH_{2})_{7}COOH, a 20-carbon fatty acid obtained from fish oils. (b) Label the hydrophobic and hydrophilic portions. (c) Predict how its melting point compares to the melting points of arachidic acid and arachidonic acid (Table 15.2).
| Mp (°C) | Name | Structure | Number of C=C’s | ber of C’s |
| Saturated Fatty Acids | ||||
| 44 | Lauric acid | CH_{3}(CH_{2})_{10}COOH | 0 | 12 |
| 58 | Myristic acid | CH_{3}(CH_{2})_{12}COOH | 0 | 14 |
| 63 | Palmitic acid | CH_{3}(CH_{2})_{14}COOH | 0 | 16 |
| 71 | Stearic acid | CH_{3}(CH_{2})_{16}COOH | 0 | 18 |
| 77 | Arachidic acid | CH_{3}(CH_{2})_{18}COOH | 0 | 20 |
| Unsaturated Fatty Acids | ||||
| 1 | Palmitoleic acid | CH_{3}(CH_{2})_{5}CH\overline{—} CH(CH_{2})_{7}COOH | 1 | 16 |
| 16 | Oleic acid | CH_{3}(CH_{2})_{7}CH\overline{—} CH(CH_{2})_{7}COOH | 1 | 18 |
| -5 | Linoleic acid | CH_{3}(CH_{2})_{4}CH\overline{—}CHCH_{2}CH\overline{—} CH(CH_{2})_{7}COOH | 2 | 18 |
| -11 | Linolenic acid | CH_{3}CH_{2}CH\overline{—}CHCH_{2}CH\overline{—}CHCH_{2}CH\overline{—}CH(CH_{2})_{7}COOH | 3 | 18 |
| -49 | Arachidonic acid | CH_{3}(CH_{2})_{4}(CH\overline{—} CHCH_{2})_{4}(CH_{2})_{2}COOH | 4 | 20 |
Analysis
• Skeletal structures have a carbon at the intersection of two lines and at the end of every line.
The double bond must have the cis arrangement in an unsaturated fatty acid.
• The nonpolar C—C and C—H bonds comprise the hydrophobic portion of a molecule and the
polar bonds comprise the hydrophilic portion.
• For the same number of carbons, increasing the number of double bonds decreases the melting
point of a fatty acid.
