Organic Chemistry MCAT Practice Exam

Which type of fatty acids tend to pack better and often form solids?

1. Unsaturated fatty acids

2. Saturated fatty acids

3. Polyunsaturated fatty acids

4. Bifunctional fatty acids

The correct answer is: Saturated fatty acids

Saturated fatty acids possess no double bonds between their carbon atoms, which allows the fatty acid chains to lie straight and pack closely together. This close packing is facilitated by strong van der Waals interactions among the linear alkane-like structures of saturated fatty acids. As a result, they have higher melting points compared to unsaturated varieties, often resulting in solid forms at room temperature, such as butter or lard. In contrast, unsaturated and polyunsaturated fatty acids contain one or more double bonds, which introduce kinks in their chains. These kinks prevent the fatty acids from packing tightly, leading to lower melting points and keeping them in liquid form at room temperature, as seen with oils. Bifunctional fatty acids, while they may refer to fatty acids with additional functional groups, do not inherently relate to the solid versus liquid state in the way that saturated fatty acids do, making them less relevant in this context. Thus, the characteristic structure and bonding of saturated fatty acids are key to their ability to form solids.