(a) Draw a skeletal structure of gadoleic acid, CH3(CH2)9CH CH(CH2)7COOH, a 20-carbon fatty acid obtained from fi sh 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 19.2).

Question

(a) Draw a skeletal structure of gadoleic acid, [latex]CH_{3}(CH_{2})_{9}CH= CH(CH_{2})_{7}COOH[/latex], 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 19.2).

TABLE 19.2 Common Fatty Acids
Number of C’s	Number of C=C’s	Structure	Name	Mp (°C)
Saturated Fatty Acids
12	0	[latex]CH_{3}(CH_{2})_{10}COOH [/latex]	Lauric acid	44
14	0	[latex]CH_{3}(CH_{2})_{12}COOH [/latex]	Myristic acid	58
16	0	[latex]CH_{3}(CH_{2})_{14}COOH [/latex]	Palmitic acid	63
18	0	[latex]CH_{3}(CH_{2})_{16}COOH [/latex]	Stearic acid	71
29	0	[latex]CH_{3}(CH_{2})_{18}COOH [/latex]	Arachidic acid	77
Unsaturated Fatty Acids
16	1	[latex]CH_{3}(CH_{2})_{5}CH=CH(CH_{2})_{7}COOH [/latex]	Palmitoleic acid	1
18	1	[latex]CH_{3}(CH_{2})_{7}CH=CH(CH_{2})_{7}COOH [/latex]	Oleic acid	16
18	2	[latex]CH_{3}(CH_{2})_{4}CH=CHCH_{2}CH=CH(CH_{2})_{7}COOH[/latex]	Linoleic acid	-5
18	3	[latex]CH_{3}CH_{2}CH=CHCH_{2}CH=CHCH_{2}CH=CH(CH_{2})_{7}COOH[/latex]	Linolenic acid	-11
20	4	[latex]CH_{3}(CH_{2})_{4}(CH=CHCH_{2})_{4}(CH_{2})_{2}COOH[/latex]	Arachidonic acid	-49

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.

Accepted Answer

a. and b. The skeletal structure for gadoleic acid is drawn below. The hydrophilic portion is the COOH group and the hydrophobic portion is the rest of the molecule.

c. Gadoleic acid has one cis double bond, giving it a lower melting point than arachidic acid (no double bonds), but a higher melting point than arachidonic acid (four double bonds).

Question 19.SP.2: (a) Draw a skeletal structure of gadoleic acid, CH3(CH2)9CH ...

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