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Author Topic: Carboxylate is electron donating?  (Read 2917 times)

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Super Newb

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Carboxylate is electron donating?
« on: August 27, 2016, 03:46:59 AM »

Hi all,

I was reading a textbook on organic chemistry and was told that carboxylate group is electron donating, but I honestly do not see how it is electron donating. Can someone kindly shed some light on this issue.

Thanks! :)

The word by word quotation is as follow:

"It can be seen that the difference between the first and second pKa values diminishes as the number of methylene groups separating the carboxyls increases,i.e. it becomes easier to lose the second proton as the other functional group is located further away. It can also be seen that since malonic acid is a stronger acid than acetic acid, then the extra carboxyl is an electron-withdrawing substituent that is stabilizing the conjugate base. Again, this effect diminishes rapidly as the chain length increases, as anticipated for an inductive effect. Of course, ionization of a carboxylic acid group to a carboxylate anion reverses the inductive effect, in that the carboxylate will be electron donating, and will destabilize the dianion. This is reflected in the pKa2 values for malonic, succinic and glutaric acids all being larger than the pKa for acetic acid. Oxalic acid appears anomalous
in this respect, and this appears to be a result of the high charge density associated with the dianion and subsequent solvation effects." (Dewick, 2006)

Dewick, P. M. (2006). Essentials of organic chemistry: For students of pharmacy, medicinal chemistry and biological chemistry. Chichester, West Sussex, England: J. Wiley.
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AdiDex

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Re: Carboxylate is electron donating?
« Reply #1 on: August 28, 2016, 10:11:34 PM »

You should know the nomenclature first .
carboxyl group is typically shown as: −COOH (neutral carboxylic acid group )
The carboxylate group is: −COO- (Anionic Carboxylic Acid group )

So what do you think ,which one is a +I group , -OH or -O- ??

Here the author wants to show that in case of Oxalic acid , second de-protonation was very tough as Carboxylate was in the vicinity so there was almost 1.5 Increase in pKa2 value from its pKa1 (Means much more harder to withdraw 2nd proton ) . See fig. 1

where as in case of second deprotonation of glutaric acid , The carboxylate group was farther away so it's electron donating nature will not affect it . (Fig.2)
There is only 0.9 increase in pKa value (Means little harder to withdraw 2nd proton )
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AdiDex

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Re: Carboxylate is electron donating?
« Reply #2 on: August 28, 2016, 10:57:25 PM »

Moreover whenever we talk about Electron withdrawing and electron releasing , It all about comparing . That this group is electron withdrawing with respect to these .
In general we have reference point Hydrogen attached to Sp3 Carbon .

Atleast in this case you can say Carboxylate is electron donating with respect to  carboxyl .
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