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Synthesis of aspirin, confused noob

  1. Oct 20, 2013 #1
    Hi,

    Take a look at this synthesis of aspirin. Because I am noob, please bare with me.
    800px-Acetylation_of_salicylic_acid%2C_mechanism.png

    On the top there you can see how the acetic anhydride gets attacked by the H plus ion.
    I cant understand why the h plus ion gets bonded there.
    I`ve been trying to find the answer to this question for quite some time now, and this is what I have come up to.

    1. There is no charge on none of the oxygen atoms. So not explanation there.
    2. There is no difference in electronegativity between the oxygen as far as I can see, but please tell me different. I would actually not mind, because I am totally stuck.
    3. There was this one step on(http://www.masterorganicchemistry.com/2012/06/18/what-makes-a-good-nucleophile/) which said something about solvent, but I didn`t understand it too well, so I`ll just move on to steric hindrance.
    4. Steric hindrance could explain why the h plus ion doesn`t bond at the bottom oxygen on the acetic anhydride, but not why the h plus ion doesn`t want to bond on the salicylic acid oxygen in the top left corner.

    Omg...
    yelp!!
     
  2. jcsd
  3. Oct 20, 2013 #2

    Ygggdrasil

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    Draw the resonance structures of the anhydride. Which oxygen carries a negative charge in the resonance structure and which carries a positive charge?
     
  4. Oct 20, 2013 #3
    I don`t see how drawing the resonance structure for the acetic anhydride could show me the answer to my question. I am pretty much only curious in why the h plus ion gets bonded on the acetic anhydride oxygen and not on the salicylic acid oxygen.
     
  5. Oct 20, 2013 #4

    Ygggdrasil

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    Ah ok, I misunderstood your question. I thought you were asking why the carbonyl oxygen of the anhydride got protonated while the bridging oxygen of the anhydride did not.

    Still, when considering whether the phenolic -OH of salicilic acid is more basic than the carbonyl oxygen of the anhydride, it is still good to write out the resonance structures for the anhydride and salicylic acid. Which oxygens carry a negative charge in the resonance structures, and which structures carry positive charges in the resonance structures?
     
  6. Oct 20, 2013 #5
    But they don`t need to be negatively charged to accept a proton, do they? What?! I dont see any reason why the carbonyl oxygen couldn`t accept an h plus ion. I just cant see it with my eyes.
     
  7. Oct 20, 2013 #6

    Ygggdrasil

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    Sure, any oxygen with a lone pair can accept a proton. But some are more likely to do so than others, and the relative amount of time they spend protonated vs unprotonated can affect the reactivity of the groups in the various reactions. For example, if the phenolic -OH of salicylic acid were more likely to be protonated than the carbonyl oxygen of the anhydride, the proposed reaction scheme would not work.

    added in edit: note that usually only catalytic amounts of phosphoric acid are added to the reaction, so there won't be enough acid around to protonate everything. Therefore, you must consider which atoms are the most basic and the most likely to be protonated under these conditions.
     
  8. Oct 20, 2013 #7
    I`ll have to read up on the ranking of basicity?
     
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