So why in the world is CO2 acidic?

[MCAT guy]

I have a pretty good understanding of organic chemistry, but in physiology I don't really understand why CO2 is acidic.

Would it just be because it is a good electron acceptor (taking H2 and becoming carbonic acid)? I guess when I'm thinking of acids, CO2 is one where I don't automatically think acid. sometimes I've gotten questions wrong thinking it was basic.

Does anyone have a good way of thinking about this?

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[PiBond]

I have a pretty good understanding of organic chemistry, but in physiology I don't really understand why CO2 is acidic.

Would it just be because it is a good electron acceptor (taking H2 and becoming carbonic acid)? I guess when I'm thinking of acids, CO2 is one where I don't automatically think acid. sometimes I've gotten questions wrong thinking it was basic.

Does anyone have a good way of thinking about this?

CO2 is the biggest source of acid in the body because it binds to H20 in the blood and forms carbonic acid which then dissociates into H+ and HCO3-. If you keep this equation in mind it'll make sense why hypoventilation (build up of CO2) and hyperventilation (blow off too much CO2) produce respiratory acidosis and alkalosis respectively.

 

[Evergrey]

The enzyme that facilitates the blood acid equilibrium that PiBond mentioned is called carbonic anhydrase. On it's own the reaction isn't too quick. But with that enzyme, CO2 can become acidic very fast.

 

[loveoforganic]

It's a lewis acid. Carbonyl carbons are electron deficient species which are readily attacked by nucleophiles. CO2 is a double carbonyl carbon, and is thus very electron deficient. Lewis acids are electron acceptors. CO2 readily accepts electrons.

An analogous compound would be SO3, the lewis acid form of sulfuric acid.

 

[Rayhaan]

When you breathe in oxygen your body uses it up and you excrete CO2 as waste into the blood. If for some reason you had a lot of CO2 in your body then that CO2 can't just sit there (it is not very water soluble).

Converting CO2 into a more soluble form, like bicarbonate (HCO3-) makes it soluble.

CO2 + H2O ---> HCO3- + H3O

As we can see here, by Le Chatalier's Principle; if you had more CO2 then you get more H3O. So CO2 causes acidity in the blood.

 

[MCAT guy]

Thanks to each of you!

I would be lost without this forum...