Question 23.2: Comparing Melting Points Which of these two fatty acids has ......
Comparing Melting Points
Which of these two fatty acids has the higher melting point?
(a) CH_3(CH_2)_4CH \xlongequal[]{} CHCH_2CH \xlongequal[]{} CHCH_2(CH_2)_6\overset{O}{\overset{\mid\mid}{C}}— OH
(b) CH_3(CH_2)_5CH \xlongequal[]{} CHCH_2(CH_2)_6\overset{O}{\overset{\mid\mid}{C}}— OH
ANALYSIS First determine the chain length (number of carbon atoms) and the number of unsaturated bonds present. In general, the more carbon atoms present in a molecule, the higher the melting point. However, the higher the number of unsaturated bonds, the lower the melting point. The degree of unsaturation (defined as the number of double bonds) is more important than the number of carbon atoms when the number of carbon atoms is identical or similar.
Molecule (a) has 18 carbon atoms and two unsaturated bonds. Molecule (b) has 16 carbon atoms and one unsaturated bond. Although molecule (a) is slightly larger than molecule (b) and would be expected to have a higher melting point, molecule (a) has two double bonds, whereas molecule (b) has only one double bond. Since the degree of unsaturation is more important in these similarly sized molecules, molecule (b) has the higher melting point.