guy
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Carboxylic acid strengths
Why do carboxylic acids decrease in acidity as they increase the number of carbons? Example, formic acid is stronger than acetic acid. It just seems
that more carbon would increase the electronegativity and make it stronger.
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12AX7
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Seems to me it "dilutes" the strength. More like a charge or electronegativity per molecular weight thing.
Tim
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Magpie
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I looked it up. Alkyl groups are better electron donating groups (via induction) than is hydrogen.
This is the same phenomenon that provides stability to a carbocation. Stability decreases as: (CH3)3C+ >(CH3)2CH+ > CH3CH2+ > CH3+
The single most important condition for a successful synthesis is good mixing - Nicodem
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The_Davster
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Yeah, anything electron donating, whether by induction or resonance will destabilize the conjugate base of the carboxylic acid, therby reducing the
strength of the carboxylic acid. Electron withdrawing groups will increase the strenght of the carboxylic acid.
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guy
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Can anyone explain why alkyl groups are more electron donating than hydrogen? Is it because the carbon's attraction for electrons decreased becuase
of the bonds to the hydrogens?
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Magpie
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From my organic class notes:
Carbon has slightly more electronegativity than hydrogen (2.5 vs 2.2). This makes the carbon in a methyl group very slightly negative. This allows
the group to be weakly "electon donating" via induction.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Nick F
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Here's an interesting question, let's see if anyone gets it  :
Why is trichloroacetic acid about ten thousand times more acidic than acetic acid?
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The_Davster
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Because fluorine is so damn electronegative, that the negative charge 'on' the oxygen is sucked through the molecule to the fluorines, with the net
effect being the negative charge delocalized throughout the fluoroacetate ion, and as delocalized charges are more stable the trifluoro acetate ion is
much moe stable than acetate. Greater stability of conjugate base= stronger acid.
EDIT: Cmon, that was easy
[Edited on 6-7-2006 by rogue chemist]
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12AX7
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Edit 2: he said chloroacetic, not fluoroacetic.  But same idea.
Tim
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The_Davster
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Yeah, hehe, oops, got so used to seeing TFAA in the usual examples that I did it for that. Oh well, everything I said still holds.
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Nick F
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Rogue: that has an effect, but it is not the major one .
Not so easy!!
Edit: your clue is the letter "S".
[Edited on 6-7-2006 by Nick F]
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Darkblade48
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| Quote: | Originally posted by Nick F
Rogue: that has an effect, but it is not the major one .
Not so easy!!
Edit: your clue is the letter "S".
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Really? I've always thought that the electron withdrawing chlorines (or fluorines ) cause the proton to be easily abstracted, leading to the increased acidity of the acid.
I'm not sure what the clue is supposed to represent though, I can't think of anything that would make a compound more acidic that starts with an "S"
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Nerro
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The more stable the conjugate base, the stronger the acid. This is why HClO4 is a stronger acid than HCl, the free electron can occupy more space in
the ion thus spreading the charge and lessening its disturbing effects on the molecules' structure.
In Cl3C-COOH the chlorines will draw the electron towards them thus lowering internal stresses due to the differences in electronegativity. Which
makes the Cl3C-COO- more stable than the H3C-COO- ion. The trichloroacetate ion is more Stable
#261501 +(11351)- [X]
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The_Davster
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Yeah, I am confused about what the reason could be now, what I said above got me full marks on a question on an o-chem final question, I also looked
in my text, and it said the same.
Now you got me all curious
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Nerro
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Actually it's not either or, its both. I merely explained why electron-suckers like halogens increase acidity 
#261501 +(11351)- [X]
the \"bishop\" came to our church today
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The_Davster
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Minus the stresses caused by electronegativity that you mentioned, the reasons seem very similar. I had not known that differences in
electronegativity causes stresses in a molecule, but it makes sense.
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Nick F
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Hehehe .
To be fair, it would have taken me a very long time to come up with the answer if I had not been taught it.
S - entropy!! Not the most obvious of clues, I admit...
When acetic acid is deprotonated, the negative charge is delocalised onto only two oxygen atoms, with little effect from the CH3. Thus that part of
the molecule has a high charge density, and so water molecules are very strongly attracted to it. So, when acetic acid is deprotonated, lots of water
molecules are suddenly bound quite strongly to the COO- end of the molecule, which leads to a large decrease in entropy.
When trichloroacetic acid (trifluoro, etc) is deprotonated, the inductive effect from the halogens causes the negative charge to be less localised. In
turn, this means that the surrounding water molecules are less strongly attracted, and so the decrease in entropy is much smaller, and so
deprotonation of trihaloacetic acids is more favourable.
The inductive effect does of course also mean that the proton is more labile, but that alone is not nearly enough to account for the ten thousand-fold
increase in acidity.
Told you it was interesting .
Organic chemistry - you've gotta love it!
Edit: I said "water atoms" by mistake. But it's late and I'm drunk .
[Edited on 6-7-2006 by Nick F]
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Nerro
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stresses may not be the best word, the lowered electron density on the C's next to the halogens slightly compensates for the increase in electron
density when the proton is released. The "extra" electron just has more places to go.
-edit- and the entropy thing makes some sense. I must admit I'd never heard it before. But the energies involved with S are very small, is the
influence really that substancial?
[Edited on Thu/Jul/2006 by Nerro]
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Nick F
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I actually used to hate physical organic chemsitry, but the more I learn, the more interesting it becomes. It also gets you major brownie points in
exams if, for example, you can justify organic mechanisms by giving approximate Hammett rho values for reactions. Examiners love that stuff!
Edit: yeah, apparently entropy is the major contributing factor! It was mentioned very briefly in one of my physical organic lectures, I'll have a
look in my notes tomorrow to see if it says any more....
[Edited on 6-7-2006 by Nick F]
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The_Davster
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Heh, interesting, thanks for the little lesson, I take physical chemistry next year. Kinda looking forward to it, a bit worried though as I seem to be
the weakest in it out of all the subclasses of chemistry.
(I never really liked entropy, and I absolutly despise gibbs free energy)
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Nerro
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Once you get the hang of the delta G it's really not to hard
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guy
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Hey how come HClO is weaker than HCl. I think it is because the O is less withdrawing because it also withdrawing from the Cl, am I right?
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12AX7
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OCl- is analogous to OH-, but with a Cl substituted. Note that Cl is in the +1 state and O is -2.
Tim
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pantone159
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| Quote: | Originally posted by Nick F
Why is trichloroacetic acid about ten thousand times more acidic than acetic acid? |
There is a good discussion of this in the book:
Why chemical reactions happen, by James Keeler & Peter Wothers, Chapter 12.4.
The energy difference between the different anions is completely insignificant in determining the acid strengths, it is indeed completely due to less
entropy decrease for water surrounding CCl3COO- vs CH3COO- etc. It is even energy *unfavorable* for CCl3COOH to lose its proton.
That book, btw, is really good, and I'd highly recommend it to somebody who wants to understand physical chemistry.
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Nick F
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Yeah, I've had both Keeler and Wothers as lecturers, they are very good at explaining things and I imagine that the book would be a good read.
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