Which of the following is NOT described by a fatty acid designated C18:3 n-3?

Question

BlackTom AI · Accepted Answer

C18:3 n-3 denotes an 18-carbon fatty acid with three double bonds and an omega-3 configuration.

Relevant concepts:
- “C18” = 18 carbons in the chain.
- “:3” = three double bonds total.
- “n-3” (omega-3) = the first double bond counting from the methyl (omega) end is between the 3rd and 4th carbons from that end; equivalently, one double bond is located three carbons from the omega end. In delta (Δ) notation from the carboxyl end, common positions for C18:3 n-3 (alpha‑linolenic acid) are Δ9,12,15.

Step-by-step reasoning:
1. Carbon count: The prefix C18 explicitly states there are 18 carbons — so "It has 18 carbons in the carbon chain" is correct.
2. Number of double bonds: “:3” means three double bonds in total — so "It has 3 double bonds in total" is correct.
3. Omega position meaning: “n-3” means the terminal (closest to the methyl end) double bond is at the 3rd carbon from the omega end, but this does not imply there is only one double bond. It only specifies the position of the nearest double bond to the omega end.
4. Identity: C18:3 n-3 corresponds to linolenic acid (specifically alpha‑linolenic acid in many contexts), so "It is linolenic acid" is correct.

Why the incorrect option is wrong:
- "It has only 1 double bond located at the third carbon from the omega side" misreads the notation. C18:3 n-3 indicates three double bonds, not one. A molecule with a single double bond at that omega-3 position would be written C18:1 n-3 (one double bond), not C18:3 n-3.

Therefore the statement that is NOT described by C18:3 n-3 is: "It has only 1 double bond located at the third carbon from the omega side." This matches the provided answer.

Which of the following is NOT described by a fatty acid designated C18:3 n-3?单项选择题

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