I concede the point with amides being primarily sp2 because of the whole issue with the planar structure of peptides, but esters are definitely not completely sp2 hybridized.
(Btw, this isn't something I got from EK 1001. This was what I was taught in Orgo 2 from my professor.)
Here's my quick google search citation:
http://instruct.uwo.ca/chemistry/283g/Nifty Stuff/amides are flat B.htm
Where's yours?
I suggest you do a little research before coming here and chastising people who are only trying to help. The responder above made a mistake. No one's perfect. Not even you. If you want to challenge a claim, feel free to do so, but don't belittle people while you're doing it.
For the sake of the MCAT, both Oxygen and Nitrogen are sp2 hybridized. We can get into some detailed debate about all sorts of experimental evidence about it having both sp2 and sp3 characteristics, but if a question explicitly asks you whats the hybridization of that atom, you better ditch that technical thinking style and instead keep it basic.
Here are just a few sources. You can google the rest.
"Hybridizations crucial exception is this: when an atom that appears to be sp3 hybridized can participate in resonance, that atom is in fact sp2 hybridized. This is because resonance requires an open p-orbital to occur. Sp3 atoms have 4 sp3-hybridized orbitals but no open p-orbitals, and therefore cannot participate in resonance. Sp2 -hybridized atoms can. Why is it important for resonance to occur? Again, the answer lies in stability. Structures with resonance are more stable than similar structures without resonance, so whenever an atom has an opportunity to free up a p-orbital to accommodate resonance, it will do so"
Source:
http://www.chem.ucla.edu/harding/ec_tutorials/tutorial18.pdf
"Some chemists do use a "partial" hybridization model that assigns factional hybridizations, sp2.5 etc. This is a sort of more sophisticated hybridization model. We don't generally teach this because it is obviously harder to implement in a classroom. Of course, as you have heard before, both the hybridization and the partial hybridization model and just that, models. As before, we tend to use the simplest model that explains what we need to explain, we are just prepared for those situations where the model doesn't quite explain everything, as this case."
Source:
http://chm233.asu.edu/notes/resonance/S2.html
"Since hybridization state does not really change, how do you decide which is the correct hybridization state? The answer is that in cases of resonance contributing structures in which a given atom is either sp3 or sp2 hybridized, the rule is that if any significant contributing structure indicates an atom is sp2, then sp2 is the hybridization state of the atom. In the example above, this means that the partially negatively charged carbon and oxygen atoms are both sp2 hybridized, even though they appear sp3 in one contributing structure each."
Source:
http://iverson.cm.utexas.edu/courses/310M/POTD Fl12/POTD9-5-12.html
