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#1 2009-02-04 15:25:28


Resonance structure of nitro enol and nitro enoic acid

Answers!Yahoo chemistry wrote:

Hi people, anyone kind enough to help me with this resonance problem? I couldn't understand it despite asking around...

This is a good question. I wish I had asked it. Normally, if you have a charged intermediate, you do either a push or pull of electrons. If there is a negative charge, start with those electrons to push toward neighboring double bonds. If there is a positive charge, then you will pull neighboring electrons toward it.

With neutral structures, you can do either and there isn't a clear rule. If you have a pair of electrons, you can push toward neighboring double bonds. If you have only carbons, then you might push from the most substituted double bond as carbon substitution donate electrons. With a nitro group, there is a negative charge. Can you start with those electrons? No, that would mean breaking a sigma bond and you cannot do that.

This leave two other options. The nitro group can be used to pull electrons just as a carbonyl group to give a carbocation. Then the non-bonded electrons of the oxygen can be used to generate the third resonance structure.

Alternately, the non-bonded electrons can be used to push toward the double bond, this leaves a negative charge next to the nitro group. This can be absorbed by the nitro group to give the aci-nitro form as the third structure. These structures are all good resonacne contributors. The only structure that is a less contributor is the structure with the arrow. This structure does not have a completed octet and is thus a less contributor for that reason.

For the carboxylic acid, two possible resonance structures can be drawn. The alkene electrons can shift toward the nitro or tward the carboxylic acid. However, once it shifts, another resonance struture cannot be drawn as that would require takeing electrons away from an oxygen (the blue curved arrows). This will break two rules, a) taking electrons away from a more electron withdrawing atom and b) leaving an incomplete octet. (It isn't the electron donation from the oxygen that is bad, per se. It is because in doing so leaves the oxygen without a completed octet. That is different than the two enol structures above.

I almost didn't draw the second possible set of structures as I anticipated a nitro group to be more electron withdrawing. However, they are both possible resonacne contributors. Their net effect is to make the alkene electron deficient. The two positive charges next to each other is the least favorable structure.


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