fatty acid vs polyethylene

Both fatty acid and polyethylene are made (mostly) of carbons with hydrogens with a very similar configuration, yet, their properties are very different. What am I missing?

• Not that similar structure and not that different properties. Jun 9 '17 at 22:26
• Would you care to elaborate? Jun 9 '17 at 22:27
• Well, there are lots of solid ones and "abundance in nature" isn't exactly property. Short answer may be that fatty acids have chains short enough that presence of COOH group is still very important. Jun 9 '17 at 22:45
• @KobiT try this: en.wikipedia.org/wiki/Fatty_acid_metabolism Jun 9 '17 at 23:01
• en.wikipedia.org/wiki/Waxworm Jun 10 '17 at 16:23

I agree with @Mithoron's earlier comment that even if these two types of compounds are somewhat similar in structure (e.g., they both consist of long aliphatic chains) they are not exactly alike. If you perhaps reformulate your question it may allow us to answer it more precisely. I will make an attempt below.

A fatty acid contains a carboxylic acid group attached to an aliphatic chain. This chain can be either saturated or unsaturated and still be referred as a fatty acid. Most naturally occurring fatty acids can have somewhere between 4 up to 28 carbons in their chain.

Polyethylene (also named polyethene or poly(methylene) is a polymer with a variation on the chemical formula $$\ce{(C2H4)_n}$$ lacking a carboxyl acid group. Some important similarities between the two compounds include that the melting profile is dependent on the chain lengths. As there are greater intermolecular forces between long chains compared with smaller simple molecules.

Polyethylene is therefore a wholly non-polar compounds, while the overall polarity of fatty acids depends on which functional group is the most dominant, the head carboxyl acid group or the alkyl chain group.

In the table below I have summarized the melting temperatures of some fatty acids and polyethylenes (which are equivalent to small straight-chain alkanes) to show the difference. Observe how the melting point changes drastically by the introduction of a double bond in the saturated versus unsaturated fatty acids of the same chain length.

A characteristic property of very long chained polyethylenes (with molecular weights numbering in the millions) is that they are thermoplastics, meaning they can be heated to their melting point, cooled, and reheated again without significant degradation. Fatty acids can be derivatized to other compounds, including polymers, to gain similar properties, but are not thermoplastic in their underivatized form.

The main difference is the polarity. Alcanes and polyethylene are nearly not polar, because both $$\ce{C}$$ and $$\ce{H}$$ have nearly the same electronegativity. Fatty acids contains the group $$\ce{-COOH}$$. As the oxygen atom electronegativity is rather different from Carbon or Hydrogen, the molecules of fatty acid are polar and behave quite differently form the alcanes, whatever the length of the chain.
Or conversely, the fatty acid $$\ce{CH3(CH2)100000COOH}$$ probably has properties that are indistinguishable from the corresponding polyethylene.