Question

Three fatty acids, F1, F2, and F3 with same chain length has melting point of $45^\circ\text{C}$, $50^\circ\text{C}$ and $60^\circ\text{C}$ then the number of unsaturated carbon present in}

• A. F1 is more than F2 and F2 is more than F3
• B. F3 is more than F2 and F2 is more than F1
• C. F3 is more than F2 and F1 is more than F2
• D. F1 is more than F2 and F3 is more than F2

Hint:

Remember the rule:
Increasing double bonds $\rightarrow$ Disrupts crystalline packing $\rightarrow$ Lowers the melting point.
Hence, the lowest melting point corresponds to the highest degree of unsaturation.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The physical properties of fatty acids, such as melting point, are determined by their hydrocarbon chain length and their degree of unsaturation (the number of carbon-carbon double bonds).
Detailed Explanation:
- For fatty acids of the same carbon chain length, the melting point is inversely proportional to the degree of unsaturation (number of double bonds).
- Saturated fatty acids have straight, flexible chains that can pack tightly together through van der Waals interactions, resulting in higher melting points.
- The introduction of a double bond (typically in the cis-conformation) introduces a kink in the hydrocarbon chain.
- This kink prevents tight molecular packing, reducing intermolecular interactions and lowering the temperature required to disrupt the structure (melting point).
- Let us compare the melting points of the given fatty acids:
- $\text{F1} = 45^\circ\text{C}$ (lowest melting point)
- $\text{F2} = 50^\circ\text{C}$ (intermediate melting point)
- $\text{F3} = 60^\circ\text{C}$ (highest melting point)
- Since F1 has the lowest melting point, it must have the highest degree of unsaturation (most double bonds/unsaturated carbons).
- F3 has the highest melting point, so it must have the lowest degree of unsaturation (fewest double bonds/unsaturated carbons).
- Thus, the number of unsaturated carbons follows the order: $\text{F1} > \text{F2} > \text{F3}$.

Step 2: Final Answer:
The number of unsaturated carbons in F1 is more than F2, and F2 is more than F3. This matches option (A).