Saturated vs. Unsaturated fatty Acids mp/bp

Discussion in 'DAT Discussions' started by panmit, 05.14.14.

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  1. panmit

    panmit 2+ Year Member

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    I always thought that if a molecule does not pack as tightly, it will have a lower boiling point and a lower melting point. Therefore, an unsaturated fatty acid creates a kink and decreases packing. So shouldn't the mp and bp should be lower?

    However, in ferralis bio notes, it says the mp would decrease and the bp would increase in unsaturated fatty acids. It says:

    "Double bonds increase bond polarity (area of increased electron density!), increasing boiling point due to polarity."

    But during melting and boiling point, we are NOT breaking the double bonds, but just breaking the intermolecular forces. So double bonds should have nothing to do with mp and bp. So alkanes>alkenes>alkynes in boiling point and melting point because alkanes have more saturation and higher MW due to more hydrogens. This will allow for more intermolecular force attractions.

    Also after watching Chad's videos: he says branching increases mp and decreases bp. Does this trend only apply if it is symmetrical branching? If its asymmetrical branching, mp and bp would both decrease correct?
     
    Last edited: 05.14.14
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  3. Daneosaurus

    Daneosaurus D2 2+ Year Member

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    Unsaturated fatty acids have at least one double bond which "kinks" the hydrocarbon tail. The conformation of the molecule is unable to stack as neatly and has less overall IMFs, therefore it requires less energy to dissociate from the rest. This is why unsaturated fatty acids have a lower melting point.
     
  4. panmit

    panmit 2+ Year Member

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    Thanks for the reply, does that mean ferallis notes is wrong?
     
  5. panmit

    panmit 2+ Year Member

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    Screen Shot 2014-05-14 at 12.15.56 PM.png

    I found this on masteroforganicchemistry.com

    It says that symmetrical branching increases melting point and decreases bp.

    But after comparing the branched derivatives, the linear hexane still has a higher mp and bp because it will pack together very well with the most intramolecular force attractions.

    If we are comparing the branched derivatives only, the symmetrical ones have higher mp/ lower bp (because they can pack tightly) than asymmetrical ones. So is this correct?

    For the scope of the DAT, would it just be alright to say branching decreases mp/bp?
     
  6. panmit

    panmit 2+ Year Member

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    bump...
     

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