Question

Iodine number of a fat is

• A. Inversely proportional to the number of hydroxyl group present in a fatty acid
• B. Directly proportional to its molecular weight
• C. Directly proportional to its degree of unsaturation
• D. Directly proportional to the number of carboxylic acid group present in the fatty acid

Hint:

Lipid Analytical Constants:
Iodine Number $\rightarrow$ Degree of Unsaturation.
Saponification Value $\rightarrow$ Inversely proportional to Molecular Weight.
Acid Value $\rightarrow$ Free Fatty Acids.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The iodine value (iodine number) is a quantitative analytical measure of the number of carbon-carbon ethylenic double bonds in fat and oil samples.
Key Formula or Approach:
\[ \text{Iodine Number} = \text{Grams of Iodine (}\text{I}_2\text{) absorbed by } 100\text{ grams of fat} \propto \text{Number of } -\text{C}=\text{C}- \text{ Double Bonds} \]

Step 2: Detailed Explanation:
Iodine Number Iodine Value:
- Halogens (such as iodine monochloride in Wijs reagent) quantitatively add across double bonds ($-CH=CH-$) in unsaturated fatty acid hydrocarbon chains.
- Saturated fats (like butter/coconut oil) have few double bonds and low iodine values ($< 40$), whereas highly polyunsaturated drying oils (like linseed or fish oil) absorb large quantities of iodine ($> 130 - 180$).
- Thus, iodine number is directly proportional to the degree of unsaturation.

Step 3: Final Answer:
Hence, Iodine number of a fat is Directly proportional to its degree of unsaturation, matching option (C).