EK 1001 O-Chem #73 hybridization exception

[shaybaybay91]

So for number 73 it asks for the hybridization of O3 in cocaine. O3 in the diagram is part of an ester, specifically the one oxygen that isn't in a double bond. So oxygen with its two lone pairs and two bonds one to the carbon of the functional group and another to the alpha carbon should be an sp3 hybridized oxygen, but there's an exception to the rule which is that if an atom with lone pairs is bonded to a sp2 hybridized atom then it is also sp2 hybridized since it can participate in resonance. EK has the answer as sp3. Is this right?

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[mehc012]

So for number 73 it asks for the hybridization of O3 in cocaine. O3 in the diagram is part of an ester, specifically the one oxygen that isn't in a double bond. So oxygen with its two lone pairs and two bonds one to the carbon of the functional group and another to the alpha carbon should be an sp3 hybridized oxygen, but there's an exception to the rule which is that if an atom with lone pairs is bonded to a sp2 hybridized atom then it is also sp2 hybridized since it can participate in resonance. EK has the answer as sp3. Is this right?

As an orgo major, I run into this issue frequently in MCAT stuff. From what I've seen, MCAT materials just go by 1 bond = sp3, even when the lone pairs would be happier with sp2.

 

[shaybaybay91]

I see, alright thanks for the help

 

[Czarcasm]

I see, alright thanks for the help

It's been frequently debated, but I think the correct convention to take, is when something is involved in resonance, the hybridization is sp2 because a free p-orbital is necessary for electron delocalization to occur. So it would technically be wrong to say it's sp3. Others might argue that well, it has both sp2 and sp3 characteristic and so it's technically right to say it's sp3 hybridized. If asked on the test, I personally would put sp2 as I think that's more correct given what we know about resonance. An sp3 would imply there is no free p orbital.

 

[shaybaybay91]

It's been frequently debated, but I think the correct convention to take, is when something is involved in resonance, the hybridization is sp2 because a free p-orbital is necessary for electron delocalization to occur. So it would technically be wrong to say it's sp3. Others might argue that well, it has both sp2 and sp3 characteristic and so it's technically right to say it's sp3 hybridized. If asked on the test, I personally would put sp2 as I think that's more correct given what we know about resonance. An sp3 would imply there is no free p orbital.

Yeah, that's what I answered as well, but it kind of sucks if you get marked wrong for it. I hope the MCAT takes into account the exception to the rule since it's a lot harder for me to unlearn something once I know it.

 

[mehc012]

It's been frequently debated, but I think the correct convention to take, is when something is involved in resonance, the hybridization is sp2 because a free p-orbital is necessary for electron delocalization to occur. So it would technically be wrong to say it's sp3. Others might argue that well, it has both sp2 and sp3 characteristic and so it's technically right to say it's sp3 hybridized. If asked on the test, I personally would put sp2 as I think that's more correct given what we know about resonance. An sp3 would imply there is no free p orbital.

That is absolutely right and it is more correct.
I was just pointing out that most MCAT material seems to skip that (crucial, imo) detail.

 

[Teleologist]

So for number 73 it asks for the hybridization of O3 in cocaine. O3 in the diagram is part of an ester, specifically the one oxygen that isn't in a double bond. So oxygen with its two lone pairs and two bonds one to the carbon of the functional group and another to the alpha carbon should be an sp3 hybridized oxygen, but there's an exception to the rule which is that if an atom with lone pairs is bonded to a sp2 hybridized atom then it is also sp2 hybridized since it can participate in resonance. EK has the answer as sp3. Is this right?

What exactly is this rule you speak of? I vaguely remember it but I can't remember the exact wording. Are you saying that if an atom is sp2 hybridized then anything attached to it is also sp2 hybridized? I don't think so because think about CO2. Are you instead saying that the aforementioned rule is only true when resonance is involved? In this case, are you saying that the carboxylic acid in coke can undergo resonance?

 

[premed1001]

there is no resonance in this structure?

 

[mehc012]

What exactly is this rule you speak of? I vaguely remember it but I can't remember the exact wording. Are you saying that if an atom is sp2 hybridized then anything attached to it is also sp2 hybridized? I don't think so because think about CO2. Are you instead saying that the aforementioned rule is only true when resonance is involved? In this case, are you saying that the carboxylic acid in coke can undergo resonance?

CO2 is a special, linear case because of the 2 adjacent double bonds. In most of these cases we are discussing -=-= alternating patterns, as those are the most suitable for resonance.
The point is that if the resonance structure requires a double bond somewhere, then the molecule spends at least part of its time in that form...and in order to be able to switch into that form, there would have to be a free p orbital. In other words, if you have a resonance structure with delocalized (even partially) electrons, there have to be adjacent p orbitals to support that delocalization, which leads to some atoms which would traditionally be considered sp3 to be sp2 in order to support resonance.

 

[Teleologist]

-=-=

You mean a conjugated system right?

 

[mehc012]

You mean a conjugated system right?

Yes, of course...but that word was used above and then CO2 was put forth as a counterexample, so I thought I'd go with the graphical representation so that it was clear which cases we were actually discussing.