Question

Trans isomer of oleic acid is known as

• A. Elaidic acid
• B. Palmitic acid
• C. Myristic acid
• D. Propionic acid

Hint:

Fatty Acid Isomers:
Oleic Acid = cis-9-Octadecenoic acid ($18:1, \text{cis}$).
ELAIDIC ACID = trans-9-Octadecenoic acid ($18:1, \text{trans}$).
Vaccenic Acid = trans-11-Octadecenoic acid.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Stereoisomerism of unsaturated fatty acids: the trans geometrical isomer of cis-9-octadecenoic acid (oleic acid) is trans-9-octadecenoic acid (elaidic acid).
Key Formula or Approach:
\[ \underbrace{\text{Oleic Acid (cis-9-18:1)}}_{\text{Bent Chain, } T_m = 13.4^\circ\text{C}} \xrightleftharpoons{\text{Geometrical Isomerization}} \underbrace{\mathbf{Elaidic \text{ } Acid \text{ (trans-9-18:1)}}}_{\text{Linear Chain, } T_m = 45^\circ\text{C}} \]

Step 2: Detailed Explanation:
In lipid biochemistry and fatty acid stereochemistry:
- Oleic Acid is the predominant monounsaturated fatty acid in milk and edible fats, having the chemical structure cis-9-octadecenoic acid ($18:1, \text{cis}-9$).
- The Trans Isomer of Oleic Acid (having the double bond in the trans geometrical configuration at the 9th carbon) is Elaidic Acid (trans-9-octadecenoic acid) (A).
- Due to the linear, straight-chain conformation of the trans bond, elaidic acid packs tightly into crystal lattices, possessing a much higher melting point ($45^\circ ext{C}$) than oleic acid ($13.4^\circ ext{C}$). It is formed during industrial partial hydrogenation of vegetable oils (vanaspati) and ruminal biohydrogenation (vaccenic acid $18:1, \text{trans}-11$).

Step 3: Final Answer:
Therefore, the trans isomer of oleic acid is Elaidic acid, matching option (A).