In each pair, identify the fatty acid with the lower melting point and explain why.

Question

a. myristic acid and stearic acid

b. stearic acid and linoleic acid

c. oleic acid and linolenic acid

Answer 1

a. Myristic acid has a lower molecular weight than stearic acid (fewer carbons). Other than that they are members of a homologous series, so you should expect myristic acid to have the lower melting point.

b. Linoleic acid has the same number of carbons as stearic acid, but it is unsaturated. Since double-bond sp2 carbons have a different geometry than sp3 carbons, the shape of the molecule is weird. As a result, the chains cannot pack together as efficiently as those of the highly linear stearic acid, and the melting point of linoleic acid is lower.

c. Similarly, linolenic acid is more unsaturated than oleic acid, and has the lower melting point.

Stearic acid 70° C

Myristic acid 59° C

Oleic acid 13° C

Linoleic acid -5° C

α-Linolenic acid -11° C