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 than saturated fatty acids due to their molecular structure and intermolecular forces. The presence of at least one double bond in unsaturated fatty acids causes kinks in their hydrocarbon chains, leading to weaker van der Waals forces between the molecules and ultimately lower melting points. In contrast, saturated fatty acids have linear hydrocarbon chains allowing for close packing and stronger van der Waals forces, resulting in higher melting points.

Step-by-step solution

Define and understand fatty acids

Fatty acids are long-chain carboxylic acids, containing a carboxyl group (-COOH) and a hydrocarbon chain. There are two types of fatty acids: saturated and unsaturated. Saturated fatty acids have only single bonds between carbon atoms, while unsaturated fatty acids have at least one double bond between carbon atoms in the hydrocarbon chain.

Define melting points

The melting point is the temperature at which a solid turns into a liquid. It depends on the molecular interactions, specifically the intermolecular forces, between molecules in a substance. The stronger the intermolecular forces, the higher the melting point.

Intermolecular forces in saturated fatty acids

In saturated fatty acids, the hydrocarbon chains are linear and have a uniform structure. The primary intermolecular force acting between these straight-chain molecules is van der Waals forces or London dispersion forces. These forces arise due to the temporary dipoles that form as a result of electron cloud fluctuations. Because of the linear nature of saturated fatty acids, the molecules can pack closely together, resulting in stronger van der Waals forces and, thus, higher melting points.

Intermolecular forces in unsaturated fatty acids

Unsaturated fatty acids contain at least one double bond between carbon atoms, which causes a kink or bend in the hydrocarbon chain. This irregular shape disrupts the packing of the molecules, making it difficult for them to align closely together. As a result, the van der Waals forces between unsaturated fatty acid molecules are weaker than those in saturated fatty acids, leading to lower melting points.

Relate molecular structure to melting points

To summarize, the melting points of fatty acids depend on their molecular structure and the strength of the intermolecular forces between their molecules. Saturated fatty acids have linear hydrocarbon chains that allow for close packing and strong van der Waals forces, leading to higher melting points. In contrast, the bent hydrocarbon chains in unsaturated fatty acids result in weaker van der Waals forces and lower melting points.