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Electron withdrawing and donating groups in electrophiles and acids
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Aleksandra_6728
#1 Posted : Monday, June 14, 2021 10:58:50 PM
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Hello,

To my surprise, oxygen atoms adjacent to a carbonyl will donate electron density to the partially positive carbon. This seems to go against trends in acidity where electronegative groups withdraw electrons, stabilizing the conjugate base through the inductive effect. How is it that the situation changes between these cases? At first I thought it depended on exactly how electronegative an adjacent atom is - an adjacent fluorine would always withdraw electrons because it is so electronegative - but then I thought about the case of oxygen. Wouldn't adjacent hydroxyl groups for example also withdraw electrons to increase acidity?

Let me know if you guys see my flaw in thinking:)

Thanks!
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INSTR_Katrina_128
#2 Posted : Tuesday, June 15, 2021 4:20:17 PM
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Hi Aleksandra,

It sounds like you're looking at carboxylic acids to me (oxygen next to a carbonyl group). Please let me know if this is NOT the case.

When carboxylic acids donate a proton to form their conjugate base, they can be stabilized through resonance. The reaction is shown here:

https://files.mtstatic.c...VVUiXzqoD6-1GUjA1ZirJLLy~-5zyY1p4jZRJewhaDBchtGIa1PfQp-ZmH4jIfqTZhU4hGRvlRPlLgIqEOE5-W1zpf2c4OrcOktE37f~gt5pzKqmlGkG3E3jKMYnC8nXiTm~c_&Key-Pair-Id=APKAJ5Y6AV4GI7A555NA

(very long link, sorry 😔)

On the left are the resonance structures for the reactant (acid). Since oxygen has formal charges in the second structure, and one of them is positive (ick!) -- this is less desirable. On the right (product, conjugate base), resonance stabilization helps to distribute the formal charge. Also, oxygen doesn't mind having a negative charge on it. It's electronegative after all!

Since the conjugate base is stabilized during electron delocalization, then the carboxylic acid --> conjugate base reaction is DESIRABLE. Thus, carboxylic acids are more reactive! 😀

Let me know if that is what you were asking about.
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