[erhernandez 02]

I'm going over aldol condensation and am having a hard time understanding how the nucleophilicty of an enolate ion functions. I see that this ion has two resonance structures: one with a carbanion ion and carbonyl character and one with a carbon double bond and a negative charge on the oxygen. My question is, why does the electron densities around the carbon act as the nucleophile instead of the oxygen with the negative charge? To me the negative oxygen would be great nucleophile...although I know the product theoretically would produce an ether (if I'm correct)

Does it have to do with thermodynamic properties of the resonance structures? Or maybe because the alpha carbon is less stable?

[trinitrotoluene]

I think you are on the right track with it having to do with thermodynamic properties. The C=O is stronger than a C=C bond...I think (considering IR shifts of the mentioned bonds, not always the case though).

Also, O is more electronegative and can stabilize a negative charge better than C, therefore the anionic carbon will be more reactive. This logic probably makes more sense.

Just a thought, not positive.

[Archer]

If you treat a an enolate with, for example, trimethylsilyl chloride then the oxygen acts as the nucleophile.

If you are looking at condensations, even if the oxygen did act as the nucleophile the reaction would reversibly form a hemiacetal.

[discodermolide]

Try google for hard and soft nucleophiles & electrophiles.

[spirochete]

Try google for hard and soft nucleophiles & electrophiles.

Hard soft acid base theory (HSABT) is the best and most generalizable explanation that I know of.

It's also true that any reaction under thermodynamic control will form the more stable product, which is usually the ketone. So even some hard electrophiles (protons) will ultimately end up on carbon. Even though protonation of an enolate at oxygen is kinetically favored due to HSABT, the initially formed enol can tautomerize to a ketone.

[erhernandez 02]

Thanks everyone for the responses! They were very helpful.

I read up on everyone's posts and feel like I have a better understanding of what's going on (please correct me if I'm wrong):

Enolate ion s have two resonance structures both which have a negative charge on different atoms. Although a negative charge on the oxygen is nucleophile, it is considered soft nucleophile. Why? Because the oxygen is more electronegative and can stabilize the charge better. The carbanion however is more nucleophilic because the carbon can't sustain the lone pair of electrons well. Hence it will react as a stronger nucleophile than the oxygen of the enolate ion.

When looking at the resonance structures of this ion, I read (in my old o-chem text book) that the major contributing structure is the structure with the negative oxygen. Although this structure is thermodynamicly more favorable, kineticaly the carbanion will determine the product of a reaction like with aldol condensation because it reacts faster than the negative oxygen.

If this kinetic characteristic of the enolate ion were suppressed, then the oxygen would determine the product of a reaction.

Thoughts?