Why there is not a consistent pattern between the carbon chain length and the melting point of fatty acids?

Asked Jun 24 '21 at 11:50

Active Jun 24 '21 at 12:51

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I looked at the melting temperature of different saturated fatty acids and I expected to see that with a longer chain of carbon the melting point would be higher as after all the var der Waals interactions would be stronger, yet I found there is not a consistent pattern between the number of carbons in the carbon chain and the melting point:

C3H6O2: Melting point: −20.5 °C

C4H8O2: Melting point: −5.1 °C

C5H10O2: Melting point: −34.5 °C

C6H12O2: Melting point: −3.4 °C

C7H14O2: Melting point: −7.5 °C

C8H16O2: Melting point: 16.7 °C

C9H18O2: Melting point: 12.5 °C

Does anyone have an explanation for this?

edited Jun 24 '21 at 12:51

asked Jun 24 '21 at 11:50

Itay Levi

You could have added acetic acid $\ce{C2H4O2}$, melting point +$16.6$°C. This strange behavior must be related to the crystallographic structure of these acids in the solid state. – Maurice Jun 24 '21 at 12:10
And why you neglect hydrogen bonding? It doesn't simply add to other interactions, but interferes. – Mithoron Jun 24 '21 at 12:22
Based on the formulas, those are saturated fatty acids, not unsaturated – Andrew Jun 24 '21 at 12:23
@Andrew You are correct – Itay Levi Jun 24 '21 at 12:52

Why there is not a consistent pattern between the carbon chain length and the melting point of fatty acids?

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