The numbers carry different pieces of information
In a shorthand such as 18:2, 18 is the carbon count and 2 is the number of double bonds. Those two numbers alone do not identify where the bonds sit or their geometry. [1]
Linoleic acid has double bonds at positions 9 and 12 when counted from the carboxyl end. Its omega-6 designation describes the first double bond encountered from the opposite, methyl end; it does not mean that the molecule contains six double bonds. [1]
A family name is not a complete structural name
Alpha-linolenic acid is 18:3 in the omega-3 family, with double bonds at carboxyl-end positions 9, 12 and 15. Linoleic acid and alpha-linolenic acid therefore differ in both bond count and family. [1]
Systematic names can also specify geometry. Retain that information when identifying an exact molecule: a broad omega family or carbon count does not fully distinguish every possible isomer. [1]
Convert the end of the chain being counted
For a hypothetical 18-carbon chain whose double bond closest to the methyl end begins at carboxyl-end position 12, subtract 12 from 18. The result, 6, identifies the omega-6 position. Other bonds still need their own positions.
| Detail | Interpretation |
|---|---|
| 18:2 | 18 carbons, two double bonds |
| Delta 9,12 | Carboxyl-end positions |
| Omega-6 | First bond from the methyl end |
Sources for this article
- 7304x0685
Source consulted: 2026-09-19. This is a reference resource, not a claim that we conducted the studies or tested a product.