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Carbonic acid formation mechanism

  1. Jan 17, 2004 #1
    I was reading through my organic chemistry book and they had a little piece on carbonic acid. So CO2 reacts with water and produces carbonic acid, very nice, I read about this before. But now that I've been enlighted with organic chemistry and the idea of mechanisms behind every reaction I was wondering how exactly CO2 reacts with water to form carbonic acid.
    I tried it but encountered some problems then tried again JUST NOW and came up with soemthing, care to verify this seemingly simple mechanism?
    Okay, one of the pi bonds in CO2 grabs a hydrogen from water, O=C=O -> H-OH ----> O=CH-O- + OH-. Next the oxygen with -ve charge in O=CH-O- takes the hydrogen from the adjacent carbon and while this is happening the OH- attacks the carbon and adds to it SN2 styles.
    Of course this implies that CO2 is a relatively strong base compared to H2O but I am sure that's correct. Double O-pi action.
    Last edited: Jan 17, 2004
  2. jcsd
  3. Jan 17, 2004 #2
    Consider the molecular formula for carbonic acid. H2CO3. This is the conjugate acid of sodium bicarbonate, NaHCO3, and calcium carbonate CaCO3. Any metal will do, my point being that the carbonate ion has a charge of -2.

    If you try to draw a structure of any of these, you'll get a carbon atom with a double to one oxygen, and two single bonds to two oxygen atoms, which will both have a negative charge or hydrogen atom, as the case may be.

    So the point is were going to make a carbon-oxygen bond.

    I'm not exactly sure what the mechanism is, but I can propose one.

    Consider a carbon dioxide molecule, linear, with two electronegative oxygen atoms withdrawing electron density from the central carbon atom. This makes it an electrophile, it wants some nucleophile to come in and attack it.

    The oxygen in water is a decent nucleophile. It's got two lone pairs. So water comes in with one of its lone pairs and attacks the central carbon atom in CO2, kicking two electrons of a carbon-oxyen pi bond up onto an oxygen, giving it a negatice charge.

    So we've got something like H2O(+)COO(-), where the new oxygen is covalently bonded to the carbon, when you count formal charges one the oxygen has a positive charge and the other has a negative charge. Do you follow? It's a pain in the ass describe this without drawing it. So now all you need to do is have proton tranfer, move one proton off of the oxygen with three bonds, onto the negatively charged oxygen. And your done, H2CO3.

    Couple of problems. CO2 isn't that great of an electrophile, and water isn't that great of a nucleophile. What probably happens is you have water donating a proton to one of the oxygens in CO2, activating the central carbon for nucleophilic attack, either by the newly formed hydroxide, or water.

    Now, can you go backwards?
  4. Jan 18, 2004 #3
    Okay, you see what I had done in organic I in the case where there were double bonds is I would use a bonding pair in the double bond to attack the electrophile. But in the case of oxygen, as it now seems obvious, the bonding pairs are more firmly held to the oxygen and gives the carbon a slight positive charge and so it acts as the electrophile. And then the oxygen in H2O attacks the carbon yadda yad.
    Okay, why do you say that CO2 isn't a good electrophile? Do you say that from experience or the fact that there are two oxygens with slight negative charge and this kind of shadows the molecule's overall electrophilic qualities.
    Going backwards, okay I'm guessing it would be exactly the reverse of the forward mechanism. I'm just not sure why the oxygen would break one of its OH bonds (in H2CO3) to form a double bond to the carbon. Is it simply because this pathway is available to the molecule and leads to more stable structures (CO2 and H2O)? But if that were true then why bother converting at all?
    That poses another question, why do equilibriums exist? If a compound can form a more stable compound then why it convert back? Is it because there are constant interactions among the molecules with collisions and stuff and this energy is enough to convert it back to its original form, then it loses that energy to its former self again and again...Is it simply a low activation energy between the two?
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