Textbook Question
Which of these fatty acids has the lower melting point? Explain why.
a. Linoleic acid
b. Stearic acid
Ch.23 Lipids
McMurry8th EditionFundamentals of GOBISBN: 9780134015187
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Table of contents
- Ch.1 Matter and Measurements27
- Ch.2 Atoms and the Periodic Table20
- Ch.3 Ionic Compounds8
- Ch.4 Molecular Compounds23
- Ch.5 Classification & Balancing of Chemical Reactions12
- Ch.6 Chemical Reactions: Mole and Mass Relationships28
- Ch.7 Chemical Reactions: Energy, Rate and Equilibrium64
- Ch.8 Gases, Liquids and Solids24
- Ch.9 Solutions52
- Ch.10 Acids and Bases25
- Ch.11 Nuclear Chemistry22
- Ch.12 Introduction to Organic Chemistry: Alkanes57
- Ch.13 Alkenes, Alkynes, and Aromatic Compounds47
- Ch.14 Some Compounds with Oxygen, Sulfur, or a Halogen50
- Ch.15 Aldehydes and Ketones45
- Ch.16 Amines42
- Ch.17 Carboxylic Acids and Their Derivatives57
- Ch.18 Amino Acids and Proteins82
- Ch.19 Enzymes and Vitamins63
- Ch.20 Carbohydrates44
- Ch.21 The Generation of Biochemical Energy65
- Ch.22 Carbohydrate Metabolism45
- Ch.23 Lipids42
- Ch.24 Lipid Metabolism33
- Ch.25 Protein and Amino Acid Metabolism24
- Ch.26 Nucleic Acids and Protein Synthesis45
Chapter 23, Problem 35
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.
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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.
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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.
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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.
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Related Practice
Textbook Question
Draw an 18-carbon unsaturated fatty acid that contains two carbon–carbon double bonds, one on carbon 6 and one on carbon 9 (count starting with the carboxyl carbon). Is this a “straight-chain” molecule or a “bent” molecule?
Textbook Question
Are the carbon–carbon double bonds in naturally occurring fatty acids primarily cis or trans?
Textbook Question
Draw the structure of glyceryl trilaurate, which is made from glycerol and three lauric acid molecules.
Textbook Question
There are two isomeric triacylglycerol molecules whose components are glycerol, one palmitic acid unit, and two stearic acid units. Draw the structures of both, and explain how they differ.
Textbook Question
Is the reaction shown here esterification, hydrogenation, hydrolysis, saponification, or substitution?