Textbook Question
Stearic acid and linoleic acid each have 18 carbon atoms. Why does stearic acid melt at 69 °C but linoleic acid melts at –5 °C?
589
views
23. Lipids
Physical Properties of Fatty Acids
3:38 minutesProblem 35
Textbook Question
Which of these fatty acids has the higher melting point? Explain why.
a. Linolenic acid
b. Stearic acid
Verified step by step guidance1
Identify the structural differences between linolenic acid and stearic acid. Linolenic acid is an unsaturated fatty acid with three double bonds, while stearic acid is a saturated fatty acid with no double bonds.
Understand the relationship between saturation and melting point. Saturated fatty acids (like stearic acid) have straight chains that pack tightly together, leading to stronger intermolecular forces (van der Waals forces) and a higher melting point. Unsaturated fatty acids (like linolenic acid) have kinks in their chains due to double bonds, which prevent tight packing and result in weaker intermolecular forces.
Consider the number of double bonds in linolenic acid. The three double bonds create significant kinks in the molecule, further reducing its ability to pack tightly and lowering its melting point compared to stearic acid.
Recall that the melting point of a fatty acid is influenced by the strength of intermolecular forces. Since stearic acid has stronger intermolecular forces due to its saturated structure, it will have a higher melting point than linolenic acid.
Conclude that stearic acid has the higher melting point because it is a saturated fatty acid with no double bonds, allowing for stronger intermolecular forces and tighter molecular packing compared to the unsaturated linolenic acid.
Verified video answer for a similar problem:This video solution was recommended by our tutors as helpful for the problem above
Video duration:
3mWas this helpful?
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Fatty Acid Structure
Fatty acids are carboxylic acids with long hydrocarbon chains. Their structure can be saturated (no double bonds) or unsaturated (one or more double bonds). The presence of double bonds in unsaturated fatty acids introduces kinks in the chain, affecting how closely the molecules can pack together, which influences their melting points.
Recommended video:
Guided course
0:55
Fatty Acids Example 1
Melting Point and Saturation
The melting point of fatty acids is significantly influenced by their saturation level. Saturated fatty acids, like stearic acid, have straight chains that allow for tight packing, resulting in higher melting points. In contrast, unsaturated fatty acids, such as linolenic acid, have kinks due to double bonds, leading to looser packing and lower melting points.
Recommended video:
Guided course
03:05Boiling Point Elevation Concept 1
Hydrophobic Interactions
Fatty acids are hydrophobic molecules, meaning they do not mix well with water. The interactions between fatty acid chains, primarily through van der Waals forces, play a crucial role in determining their physical properties, including melting points. Stronger interactions in saturated fatty acids contribute to higher melting points compared to their unsaturated counterparts.
Recommended video:
Guided course
2:41Interactions within the Tertiary Structure Concept 2
:53mWatch next
Master Physical Properties of Fatty Acids Concept 1 with a bite sized video explanation from Jules
Start learningRelated Videos
Related Practice