Question

Explain why the melting points of unsaturated fatty acids are lower than those of saturated fatty acids.

Short answer

Answer: Unsaturated fatty acids have lower melting points compared to saturated fatty acids because their molecular structure includes double bonds, which create a "kink" or bend in the carbon chain. This irregular shape prevents unsaturated fatty acids from packing closely together, resulting in weaker van der Waals forces between the molecules. Consequently, less energy (lower temperature) is required to overcome these forces and transition from a solid to liquid state.

Step-by-step solution

Understand the Chemical Structures of Saturated and Unsaturated Fatty Acids

Saturated fatty acids have a straight carbon chain with no double bonds between carbon atoms. All carbon atoms are connected by single bonds, and each carbon is bonded to the maximum number of hydrogen atoms possible. Unsaturated fatty acids, on the other hand, have one or more double bonds between carbon atoms in their chain, which results in fewer hydrogen atoms being bonded to the carbon atoms surrounding the double bond.

Examine the Effects of Double Bonds on the Molecular Shape

In unsaturated fatty acids, the presence of double bonds causes a "kink" or bend in the molecule's carbon chain. This is because the double bond has a rigid planar structure, which restricts free rotation around the bond. Consequently, unsaturated fatty acids have a more irregular and "bent" shape compared to the straight chains of saturated fatty acids.

Understand the Intermolecular Forces Between Fatty Acid Molecules

The primary intermolecular force between fatty acid molecules is the van der Waals force, a type of weak attractive force that acts between nonpolar molecules. The strength of these forces depends on the surface area between the interacting molecules. In the case of saturated fatty acids, their straight carbon chains allow them to pack tightly together, maximizing the surface area in contact between the molecules and increasing the van der Waals forces between them.

Explain How the Molecular Shape Affects Melting Points

The irregular shape of unsaturated fatty acids, caused by the presence of double bonds, prevents them from packing closely together like saturated fatty acids. This leads to a decrease in the surface area between the interacting molecules, which in turn weakens the van der Waals forces between them. As a result, unsaturated fatty acids require less energy (lower temperature) to overcome these weak forces and transition from a solid to liquid state, hence their lower melting points compared to saturated fatty acids.