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Topic: Are esters electron-withdrawing or electron-donating?  (Read 9647 times)

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Offline maydengar

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Are esters electron-withdrawing or electron-donating?
« on: January 08, 2016, 05:46:05 PM »
In question 13, chapter 8 of Clayden, the answers say that replacing COOH with COOEt results in a greater electron-withdrawing effect.

On the other hand, the book also says that COOEt can act as an electron-donor.



I'm wondering, is this just a case of induction vs resonance? If this is the case, how would you draw the resonance of COOEt acting as an electron-donor? Thanks.





Offline orgopete

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Re: Are esters electron-withdrawing or electron-donating?
« Reply #1 on: January 08, 2016, 08:39:29 PM »
Can you draw the resonance structure of acetamide? One can draw a resonance structure of ethyl acetate similarly.
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Offline maydengar

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Re: Are esters electron-withdrawing or electron-donating?
« Reply #2 on: January 09, 2016, 04:37:38 PM »
Yes, you delocalise the charge from Nitrogen and simultaneously push the negative charge onto Oxygen. But i'm wondering how this would help. 

Offline orgopete

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Re: Are esters electron-withdrawing or electron-donating?
« Reply #3 on: January 13, 2016, 05:54:58 PM »
The process of donating electrons from nitrogen to carbon reduces the electron withdrawing character of the carbonyl. If an ester is a weaker electron withdrawer than a ketone, what might thus suggest that the ester oxygen might also do?
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Offline maydengar

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Re: Are esters electron-withdrawing or electron-donating?
« Reply #4 on: January 23, 2016, 06:58:28 PM »
Thanks. I guess I already knew the answer to the second part.

But, for the first part, I'm not sure why substituting the H for an alkyl group would increase electron withdrawing capacity.

Offline orgopete

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Re: Are esters electron-withdrawing or electron-donating?
« Reply #5 on: January 25, 2016, 09:05:30 AM »
. I'm not sure why substituting the H for an alkyl group would increase electron withdrawing capacity.

You have discovered the 'electronegativity paradox' (also known as why Pauling's electronegativity scale is wrong). Another scale of electronic effects is the Hammett scale for aromatic substituent effects. Although this does not measure electron withdrawing properties, it can reveal an ability to withdraw or donate charge. If you look at it, you will find that a methyl group is electron donating compared to hydrogen. In that case, you should expect hydrogen to be more electron withdrawing.

Re electronegativity
I have many posts on this subject including a challenge for an example of fluorine being the most electron withdrawing element, it isn't. You may find it helpful to think that electronegativity is the property your professor is asking about on the test and electron withdrawing is the property you will find in the lab. You know I ~ Br > Cl >> F, F>O>N>C, & H>C.

This entire subject is a bit complicated, but I believe I have a good explanation for the data Pauling was attempting to explain. My explanation relies upon the inverse square law (Coulomb's Law) and can explain the energy values and acidities that Pauling was attempting to explain.
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