Which of the following fatty acids would you expect to have the lowest melting point?

• A. Palmitic acid (16:0)
• B. Oleic acid (18:1, cis-9)
• C. Stearic acid (18:0)
• D. Linoleic acid (18:2, cis-9,12)

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the relationship between fatty acid structure and melting point. Fatty acids with more double bonds (unsaturation) tend to have lower melting points because the kinks caused by cis double bonds disrupt the tight packing of molecules, reducing intermolecular forces.
2. Step 2: Analyze the given fatty acids. Palmitic acid (16:0) and Stearic acid (18:0) are saturated fatty acids, meaning they have no double bonds. Saturated fatty acids generally have higher melting points due to their ability to pack tightly.
3. Step 3: Compare Oleic acid (18:1, cis-9) and Linoleic acid (18:2, cis-9,12). Both are unsaturated fatty acids, but Linoleic acid has two cis double bonds, while Oleic acid has only one. The additional double bond in Linoleic acid further disrupts molecular packing, lowering its melting point compared to Oleic acid.
4. Step 4: Consider the chain length. Longer chain fatty acids generally have higher melting points due to increased van der Waals forces. However, the degree of unsaturation has a more significant impact on melting point than chain length in this case.
5. Step 5: Conclude that Linoleic acid (18:2, cis-9,12) has the lowest melting point among the given fatty acids due to its higher degree of unsaturation (two cis double bonds).