Which of the following fatty acids is generally solid at room temperature?

• A. Oleic acid (C_{18}:1)
• B. Linoleic acid (C_{18}:2)
• C. Linolenic acid (C_{18}:3)
• D. Stearic acid (C_{18}:0)

Answer: (D)

Verified step by step guidance

1. Understand the structural differences between the fatty acids listed. Fatty acids are characterized by their chain length and the number of double bonds. For example, Stearic acid (C_{18}:0) has 18 carbons and no double bonds, while Oleic acid (C_{18}:1), Linoleic acid (C_{18}:2), and Linolenic acid (C_{18}:3) have 1, 2, and 3 double bonds, respectively.
2. Recall the relationship between the number of double bonds and the melting point of fatty acids. Saturated fatty acids (no double bonds) have higher melting points because their straight chains pack tightly together, forming solids at room temperature. Unsaturated fatty acids (with one or more double bonds) have kinks in their structure, preventing tight packing and resulting in lower melting points.
3. Analyze the given fatty acids: Stearic acid (C_{18}:0) is saturated, while Oleic acid (C_{18}:1), Linoleic acid (C_{18}:2), and Linolenic acid (C_{18}:3) are unsaturated. The presence of double bonds in the unsaturated fatty acids lowers their melting points, making them liquid at room temperature.
4. Conclude that Stearic acid (C_{18}:0), being a saturated fatty acid with no double bonds, is generally solid at room temperature due to its ability to pack tightly and form a stable structure.
5. Verify your understanding by comparing the physical states of these fatty acids in real-world contexts. For example, Stearic acid is commonly found in solid fats like animal fat, while unsaturated fatty acids like Oleic acid are found in liquid oils such as olive oil.