Which of these fatty acids has the lower melting point? Explain why.
a. Linoleic acid
b. Stearic acid

Verified step by step guidance

1. Step 1: Understand the structure of the two fatty acids. Linoleic acid is an unsaturated fatty acid with two double bonds in its hydrocarbon chain, while stearic acid is a saturated fatty acid with no double bonds in its hydrocarbon chain.
2. Step 2: Recall 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 higher melting points. Unsaturated fatty acids (like linoleic acid) have kinks in their chains due to double bonds, which prevent tight packing and result in weaker intermolecular forces and lower melting points.
3. Step 3: Analyze the specific structures. Linoleic acid's two double bonds create more kinks in its structure compared to a single double bond or no double bonds, further reducing its ability to pack tightly and lowering its melting point.
4. Step 4: Compare the two fatty acids. Since stearic acid is fully saturated and has no double bonds, it will have a higher melting point compared to linoleic acid, which is unsaturated and has two double bonds.
5. Step 5: Conclude that linoleic acid has the lower melting point due to its unsaturated structure with double bonds, which disrupts tight packing and weakens intermolecular forces.

Key Concepts

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, preventing tight packing and lowering the melting point.

Melting Point and Saturation

The melting point of fatty acids is influenced by their saturation level. Saturated fatty acids, like stearic acid, have higher melting points due to their straight chains that pack closely together. In contrast, unsaturated fatty acids, such as linoleic acid, have lower melting points because their kinks disrupt packing, resulting in a more fluid structure.

Physical Properties of Fatty Acids

The physical properties of fatty acids, including melting point, are critical for understanding their behavior in biological systems. Lower melting point fatty acids tend to be liquid at room temperature, while higher melting point fatty acids are solid. This property affects their roles in cell membranes and energy storage.