In free fatty acid form, the chemical structure of stearidonic acid is:

Examples
Polyunsaturated fatty acids with 16- and 18-carbon chains are sometimes classified as short chain polyunsaturated fatty acids (SC-PUFA), as opposed to long-chain polyunsaturated fatty acids (LC-PUFA), which have more than 18 carbon atoms.Both the essential fatty acids are SC-PUFA with an 18-carbon chain:

ω-3 fatty acid:
α-linolenic acid or ALA (18:3n-3)
ω-6 fatty acid:
linoleic acid or LA (18:2n-6)These two fatty acids cannot be synthesized by humans because humans lack the desaturase enzymes required for their production. They form the starting point for the creation of more desaturated fatty acids, most of which also have a longer carbon chain:

ω-3 fatty acids:
eicosapentaenoic acid or EPA (20:5n-3)
docosahexaenoic acid or DHA (22:6n-3)
ω-6 fatty acids:
gamma-linolenic acid or GLA (18:3n-6)
dihomo-gamma-linolenic acid or DGLA (20:3n-6)
arachidonic acid or AA (20:4n-6)Except for GLA, which has a short 18-carbon chain, these fatty acids have more than 18 carbon atoms and are typically classified as LC-PUFA.ω-9 fatty acids are not essential in humans because they can be synthesized from carbohydrates or other fatty acids. Essentiality in human diet
Mammals lack the ability to introduce double bonds in fatty acids beyond carbon 9 and 10, hence the omega-6 linoleic acid (18:2n-6; LA) and the omega-3 Alpha-linolenic acid (18:3n-3; ALA) are essential for humans in the diet. However, humans can convert both LA and ALA to fatty acids with longer carbon chains and a larger number of double bonds, by alternative desaturation and chain elongation. In humans, arachidonic acid (20:4n-6; AA) can be synthesized from LA.
In turn, AA can be converted to an even longer fatty acid, the docosapentaenoic acid (22:5n-6; DPA). Similarly, ALA can be converted to docosahexaenoic acid (22:6n-3; DHA), although the latter conversion is limited, resulting in lower blood levels of DHA than through direct ingestion. This is illustrated by studies in vegans and vegetarians. If there is relatively more LA than ALA in the diet it favors the formation of DPA from LA rather than DHA from ALA. This effect can be altered by changing the relative ratio of LA:ALA, but is more effective when total intake of polyunsaturated fatty acids is low. In preterm infants, the capacity to convert LA to AA and ALA to DHA is limited, and preformed AA and DHA may be required to meet the needs of the developing brain. Both AA and DHA are present in breastmilk and contribute along with the parent fatty acids LA and ALA to meeting the requirements of the newborn infant. Many infant formulas have AA and DHA added to them with an aim to make them more equivalent to human milk. Essential nutrients are defined as those that cannot be synthesized de novo in sufficient quantities for normal physiological function. This definition is met for LA and ALA but not the longer chain derivatives in adults. The longer chain derivatives particularly, however, have pharmacological properties that can modulate disease processes, but this should not be confused with dietary essentiality.