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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?

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    $\begingroup$ Not that similar structure and not that different properties. $\endgroup$
    – Mithoron
    Jun 9 '17 at 22:26
  • $\begingroup$ Would you care to elaborate? $\endgroup$
    – Kobi T
    Jun 9 '17 at 22:27
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    $\begingroup$ 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. $\endgroup$
    – Mithoron
    Jun 9 '17 at 22:45
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    $\begingroup$ @KobiT try this: en.wikipedia.org/wiki/Fatty_acid_metabolism $\endgroup$ Jun 9 '17 at 23:01
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    $\begingroup$ en.wikipedia.org/wiki/Waxworm $\endgroup$
    – Mithoron
    Jun 10 '17 at 16:23
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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.

Showing the general structure of fatty acids retrieved from Wikimedia Commons][1]

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.

Figure showing the general structure of a polyethylene monomer

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.

Table showing difference in melting temperature between fatty acids and polyethylenes

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.

I hope this extended answer somewhat helps you to find and answer to your question.

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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.

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I disagree with the other answers that polarity is the most important. I think the degree of polymerization is the most important difference in common fatty acids and common polyethyelenes.

The DP of commercial polyethylenes is much higher than the fatty acids you are considering, north of 100,000 in some cases. For example, stearic acid is a C16 molecule. A polyethylene with the same number of carbons would be hexadecane and would have a DP of 8, only about 10,000-fold less than commercial polyethylene. Hexadecane's physical properties are reasonably similar to stearic acid in my book.

Or conversely, the fatty acid $\ce{CH3(CH2)100000COOH}$ probably has properties that are indistinguishable from the corresponding polyethylene.

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