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It's implied that the right structure is the more stable carbocation because it undergoes the second half of an SN1 reaction, but I'm not entirely sure why:

enter image description here

I'm also not sure why the $\ce{MeOH}$ attacks the carbon next to the carbocation and not the oxygen itself, where it has the positive charge (doesn't the nucleophile attack the carbocation, or whichever atom bears the positive charge?)

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  • $\begingroup$ the carbocation is in reality the first structure, you have an empty orbital in (left) over the right structure. the nucleophile attacks this empty orbital $\endgroup$ – Safdar Faisal May 10 at 11:14
  • $\begingroup$ @SafdarFaisal That makes a little more sense, although my professor says that the nucleophile attacks "the more stable carbocation" Also when you mean empty orbital, you mean 2p, correct? $\endgroup$ – DesperOrgo May 10 at 11:16
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    $\begingroup$ It's a mistake to think of these two as different structures. They are representations of the same thing as indicated by the double-headed arrow. Note that where the positive charge is written is only the formal charge, not the real charge. The charge here happens to be more localized on the carbon. $\endgroup$ – Zhe May 10 at 11:43
  • $\begingroup$ @Zhe I understand that thank you, I just don't know why the methanol attacks the sp2 carbon in the right structure instead of the carbon with the +1 formal charge on the left structure. An SN1 reaction has the leaving group leave, forming a carbocation, and the nucleophile "filling" in that carbocation $\endgroup$ – DesperOrgo May 10 at 11:50
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    $\begingroup$ Again, the confusion here is to think that the left structure and the right structure are different. They are not different. It's the same structure. The nucleophile reacts with the carbocation on the left for the same reason that it reacts with the oxocarbenium ion (similar to a carbonyl) on the right. They are two views of the same thing. $\endgroup$ – Zhe May 10 at 19:36
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It is important to note that the two structures shown aren't actually different. That's what resonance implies that in reality, it exists in between both of the as a 'hybrid'.

So it is not right to state that the nucleophile attacks on the left structure 'or' the right.


Coming to the carbon, (again, it's not right to take the structures to be different)

"I just don't know why the methanol attacks the $\ce{sp2}$ carbon in the right structure instead of the carbon with the $+1$ formal charge on the left structure"

It's actually the same carbon! and the hybridisation is still $\ce{sp2}$ no matter what, as the carbocation is also a planar species($\ce{sp2}$). And this is where the attack occurs.


"I'm also not sure why the MeOH attacks the carbon next to the carbocation and not the oxygen itself, where it has the positive charge"

'Carbon next to the carbocation' ? I believe you are confusing the +1 formal charge on oxygen to be a carbocation which is not true( Carbocation has the positive charge exclusively on Carbon) .
The attack is ocuring at the carbocation.


Our nucleophile here, is $\ce{OH-}$ while oxygen only has a +1 formal charge and also, the positive charge is more localised on the carbon as Zhe mentioned.

This is because oxygen being a more electronegative species, doesn't really 'like' the positive formal charge on it.

But still,

"doesn't the nucleophile attack the carbocation, or whichever atom bears the positive charge?"

Well, a nucleophile is a species that donates electrons at electron deficient sites. But oxygen still has it's lone pair! and the $\ce{OH-}$ will face repulsions from them if it tried to attack here.

The real electron deficient species here is only the carbocation and thus it is the preferred site for attack by the nucleophile.

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