Chem 201. Organic Reaction Mechanisms I. Lecture 02. Molecular Orbital Theory (Pt. 1). 

The most important consideration here for determining the nucleophilicity is charge. As shown in the resonance structure, there is more electron density on the top ("carbonyl") oxygen atom, giving it a partial negative charge in comparison to the partially positive ("ether") bottom oxygen atom. Since there is a higher electron density on the carbonyl oxygen in the ester, it will be the nucleophile that attacks the electrophile. Furthermore, in order to participate in resonance (since it is a conjugated system with electrons delocalized over the entire ester (-COO-)), the ether oxygen must be sp2 hybridized, not the sp3 hybridization that we would expect from looking at the major contributing resonance structure. This is consistent with the observed planarity of ester functional groups, and is similar to the reasoning for the planarity of amides in peptide bonds that give an omega angle of ~180 degrees. When a molecule has multiple resonance structures, it is important to remember that the lowest hybridization is assigned to a given atom within the molecule. To answer your question, yes, the uncharged resonance structure is a greater contributor; however, in order to participate in resonance, the ether oxygen must have an unhybridized p orbital to allow for electron delocalization. Perhaps there is an incredibly small difference in p-character between the two oxygen atoms, though this is negligible and isn't really accounted for by VSEPR or valence bond (VB) theory; even if the ether oxygen does, in fact, have more p-character, the charge considerations are much more relevant for determining the nucleophilicity of an ester.

Roughly. P-character means elongated, directed, far from nucleus. S-character means spherical, non-directed, close to nucleus.

+PRITHWISH GHOSH Hello, after drawing the resonance structures, you can see that the upper oxygen has four points of electron densities, so it is sp3 and the bottom oxygen has three electron densities, so it is spy. hope that helps!

Double bonded C atoms form a planar structure since the double bond forces them in a fixed structure (linear).

+Rohit Sharma Hello, he calls the atomic orbital sp2 because he is separating the sigma bond from the pi bond for the argument sake. Otherwise you are right!

It is Actually sp. (other two P are making pi bonds)

dude, he did a mistake and he was propmptly told about that, if you keep watching u'll see its drawn sp instead of sp2.

He apologized and corrected it after that

Yes,I think you may complete your PhD and everything,but now I had this doubt,if nitrogen is more electronegative than carbon,why it's energy decreases,as we all know if energy is less then the molecule/atom would be stable,but proff mentioned the nitrogen is more reactive? Could you clear my doubt

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@@vikramthimaradka97 By this principle, shouldn't nitrogen be less reactive because their p orbital are lower in energy compared to carbon, so are more stable and less likely to react?

@@Minasiu This is a cation, so the Electrophile NOT the nucleophile. Interaction HOMO-LUMO is promoted when the Energy gap is small, so is promoted when the energy of the nucleophile is higher and the energy of the electrophile is lower: in this case the cation is the electrophile (the LUMO), so a lower energy because of the electronegativity means a lower gap in energy with the nuclephile (a generic one), thus a faster reaction.

Electronegativity increases from left to right B C N O F bcoz' there increasing in the number of protons

@@doctorflint2224 Hi, just to clarify my understanding, can I say: e- from nucleophile (high in energy) want to go somewhere, and Nitrogen have a empty 2p orbital with lower energy than Carbon. As e- prefers stability, i.e. lower in energy, it tends to go to 2p in N due to higher E.N. (more protons in nucleus of N so better stability)? [Lower LUMO] Conversely, e.g. in sp3 and both N and C act as nucleophile this time (1 sp3 orbital is a lone pair e-), that lone pair in C is more nucleophilic since the sp3 in C has a higher energy than N? [Higher HOMO]