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Author: Subject: Organic Bases vs Organic Acids
hodges
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[*] posted on 30-9-2004 at 15:46
Organic Bases vs Organic Acids


What determines whether an organic compound behaves as a base, an acid, or neither?

For example, ethyl alcohol is CH3CH2OH. The OH becomes an OH- ion, allowing ethyl alcohol to form organic salts (esters) as a base.

OTOH, acetic acid is CH3COOH. The last H forms an H+ ion, allowing acetic acid to form organic salts (esters) as an acid.

Why does not just the final H break off ethyl alcohol to form an acid, or conversely why does the OH not break off acetic acid to form a base?
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neutrino
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[*] posted on 30-9-2004 at 16:10


Hydroxyl groups do not become hydroxide ions, but they can react with acids, usually with the help of H<sub>2</sub>SO<sub>4</sub> as a catalyst. Organic acids protonate because of the strong electrodrawing properties of the oxygen atoms hearby. A calboxlic acid has a COOH group where one O is double bound to the carbon, while the other is held there with a single bond, the other being to the H. The last bonding site on the carbon is bound covalently to another atom, connecting the whole group to something. Both the O's draw the electron cloud toward themselves and away from the H, allowing it to come off as a proton.
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BromicAcid
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[*] posted on 30-9-2004 at 16:35


It's all relative, strong acids can force acetic acid to act as a base, strong bases can abstract a proton from ethanol and make it act acidic or strong acids can render it basic.

The hydroxide group that you speak of is by no way definitively basic or acidic by any stretch, unless you knew better you might think P(OH)3 was a base, but it's a decently strong acid. It all has to do with the electron withdrawing power of the central atom among other things such as resonance stability.

Acetic acid can loose that hydrogen and resonance stabilize itself so loosing that proton is no big deal, it's still stable. Ethanol cannot do so to any notable extent. I once read about a cyclic organic acid, upon addition to water it readily lost its hydrogen because upon loosing it one of its resonance forms obeyed the huckel rule and it became aromatic, it had an acid strength somewhere around concentrated sulfuric acid.

I don't think I clarified much but maybe I've given some more things to think about.




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[*] posted on 30-9-2004 at 20:10


IIRC resonance stabilization in the benzene ring is the reason phenol (carbolic acid) is such a releatively strong organic acid. Even though in a structural sense it is an alcohol as it has an OH group appended to a benzene ring.

There is a quantitative way of comparing acid strength for acids: pKa, the negative log of the dissociation constant, Ka. The smaller the pKa the stronger the acid. HCl's pKa is -7, methyl alcohol's is ~ 15 , and acetic acid's is 4.8. Phenol is about 10.




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[*] posted on 30-9-2004 at 20:19


I think the molecule you are thinking of is pentacyanocyclopentadiene. It has a pKa of about -11. The pKa of HCl is about -7, for reference.

Here is the reference:

J. Am. Chem. Soc. 1966, 88, 3046.
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[*] posted on 1-10-2004 at 08:12


I was actually thinking of squaric acid. It's cyclobutane with two ketone groups, one right next to the other, and on the opposite two carbons each one has a hydroxyl group. When it looses the hydrogens on those hydroxyl groups the two negative charges spread over the ring and it become aromatic like. I don't know the pKa of it though for comparison to pentacyanocyclopentadiene.

Hummm pKa of squaric acid is only 1.5, considerably higher then what I expected from the description I was thinking around -2.

Nice table of organic acids, their pKa's and ionization and such

[Edited on 10/2/2004 by BromicAcid]




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Magpie
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[*] posted on 1-10-2004 at 17:36


Bromic,

A pKa of 1.5 indicates a relatively strong organic acid. Is that not what you expected from squaric?




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[*] posted on 1-10-2004 at 19:33


This is a good question - unfortunately organic compounds simply arent as "simple" as ionic ones - hundreds of different rules and reaction types - but i would like to see someone try to come up with a general rule that determines if an organic molecule will behave as a base or acid in the presence of another - or maybe we should just look at the pka values in the crc book for the answer?

Maybe what we really should look at is what determines if there is a reaction at all between two organic compounds?

[Edited on 2-10-2004 by jimmyboy]
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BromicAcid
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[*] posted on 1-10-2004 at 19:45


Well Magpie, it's just that when I read about squaric acid it stated that its pKa was comparable to sulfuric acid (around -10 I think) so I assumed it would be a little closer then 1.5, but I do agree that it is strong for an organic acid, an interesting arrangement of molecules too. I should make some ;) Looks like squaric acid is being used in wart removers too.



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