In fatty acids, longer chains increase the melting point (and viscosity) by increasing London dispersion forces. Inversely, more double bonds decrease the melting point.

What happens when both chain length and number of double bonds increase at the same time; how much are London dispersion forces affected by each factor?

Eicosapentaenoic acid has a lower melting point (probably lower viscocity too) compared to docosahexaenoic and stearidonic acids. One carbon atom was added to the chain and one double bond at the same time.

18:4 -> 20:5 decrease in melting point
20:5 -> 22:6 increase in melting point


Intuitively bosseopentaenoic acid should have an even lower melting point (even lower viscosity). Conjugation would appear to make bonding between different molecules more difficult and therefore would also decrease London dispersion forces (and MP, BP, viscosity. etc with them)


The above are pictures of a space-filling model I found from google images supposedly generated from Accelrys DS Visualizer.

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    $\begingroup$ It's not necessarily that strange. Adding a carbon has less of an effect on how the chains pack than inserting a double bond, which completely alters the torsional freedom of the chain. You might also imagine that even numbers of double bonds allow a more linear conformation of a chain to be retained, whereas odd numbers inevitably result in a kink (edited: or vice-versa, depending on the conformation around the double bond?). $\endgroup$
    – Buck Thorn
    Oct 29, 2022 at 7:46
  • $\begingroup$ @BuckThorn It seems that EPA with 5 double bonds in more linear than DHA with 6 double bonds. How do the electrons in the molecules interact to bind together? How do DHA bind together more strongly? $\endgroup$ Oct 29, 2022 at 9:40
  • $\begingroup$ What is the origin of the figures? Are they supposed to represent conformations in the solid? Could you explain this in the post? $\endgroup$
    – Buck Thorn
    Oct 29, 2022 at 11:01
  • $\begingroup$ You may want to look at related posts such as this one: chemistry.stackexchange.com/questions/46891/… $\endgroup$
    – Buck Thorn
    Oct 29, 2022 at 11:16
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    $\begingroup$ @BuckThorn That EPA's 5 double bonds allow a more linear conformation. The kink is in DHA. $\endgroup$ Oct 30, 2022 at 12:40


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