Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: JennLyn6 on April 13, 2004, 06:58:46 PM
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why are carboxylic acids considered weak acids? is it because they contain the cooh group?
Edit: edited title for better indexing. Mitch
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Weak does not to the degree of non-acidic. Carboxylic acids are fairly acidic, partly due to the resonance stabilization of the conjugate base.
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Weak and strong refer to the degree of dissociation of the hydrogen ion. For example:
HCl --> H+ + Cl-
proceeds near to 100% completion in water. The hydrogen ion and chloride ion dissociate from each other, therefore the term dissociation. This means that if you put 1 mol of HCl into water, you get one mol of hydrogen ions and one mol of chloride ions.
If you have an acid that dissociates to a very high degree (99%+ usually), it is considered a strong acid. If it doesn't, then it's a weak acid. Most acids out there are considered weak acids. When I took HS chemistry, we had to memorize the strong acids. I'll see if I can list them here:
HCl
HBr
HI
HClO4
HBrO4
HIO4
H2SO4
HNO3
HClO3
HBrO3
HIO3
There's probably some that I missed, but that's what I can remember off the top of my head.
Now, for the rest of the acids, they don't fully dissociate. That is to say, if you put 1 mol of acetic acid in water, it doesn't give one mole of hydrogen ions and one mole of acetate ions. The amount that it does dissociate is given by the equilibrium constant, Ka.
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This is much complex problem, eg CCl3COOH or especially CF3COOH are quite strong acids. Generally, acidity or basicity depends on "easiness of loosing" H+ or OH- and almost all carboxylic acids or organic amines fall in these weak categories.
Most inorganic acids with hight contents of oxygen are strong as showed by hmx9123
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It is complex, but I was trying to make some generalizations; once you get into the world of chemistry, you realize that the rules presented in general chemistry are just guidelines, not hard facts. You can find inorganic acids that are weak. You can find organic acids that are extremely acidic, and possibly some that dissociate as much as inorganic acids. Some highly nitrated acids jump to mind (picric acid, nitroform). Just bear in mind that the stronger an acid, the more easily it dissociates to give H+.
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Yet another thing you might want to memorize (but just in case I do memorize my chemistry lessons correctly ;) ) is that the longer the C-chain linked to the C carrying the carboxylic group the lower the acidity due to the inductional effects of the long chain on the carboxylic group (the long chain "pushes" the electron clouds towards the carboxylic group) -> this rules works vice versa too, that is the shorter the chain the higher the acidity.
Other factors changing the acidity of carboxylic acids are: Electronegativity of the substituents, the number of carboxylic groups and their distance from each other and the grade of "branching" (I don't know how to put this correctly in English) of the substituents.
I hope at least some of the things I wrote down above are correct
:P
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;D ;D ;D
is it because they contain the cooh group?
I find this rather amusing.. cuz -COOH is known as the carboxylic acid group!
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the strength of acid is assessed via its dissosiation equilibrium, that means it's a thermodynamic property. Once you want to explain this, you should pay attention on the stability of each particle in the equilibrium, especially the conjugated base.
If the conjugated base is stable, the equilibrium favors to the direction of proton formation, and reverse. In case of carboxylic acid, the nagative charge delocalize over two oxygen atoms. In case of inorganic acid in the electronagative atom or group of electronagative atoms: Cl, NO3, SO4...
As you can see the radius of Cl>>O the negative charge is more delocalized on Cl- as a result it's more stable than COO-
NO3- has a large amount of electronegative atoms, the negative charge is really favors on it. As a result it's more stable than COO-...
Of course, the substituant can affect this result due to its electroeffects
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Thermodynamic considerations like this can be quite complicated. For example, why is HF a relatively weak acid (pKa 3.17), while HCl, HBr and HI are much stronger? A possible explanation is that HF in water forms a complex of the form F-*H3O+, which is very stable. So in the equilibrium
HF + H2O <--> F-*H3O+ <--> F- + H3O+
the complex in the middle is the preferred form. Thus, the full dissociation of HF is less favourable because HF in water is stabilised.
(Source: http://dx.doi.org/10.1021%2Fja00537a008)
Anhydrous HF, on the other hand, is very acidic, similar to sulfuric acid!
What I want to say with this is that often there is no simple explanation, or simple rules are often misleading because you have to take *both* sides of the equilibrium into account.