[limonade]

In the pinnacol rearrangement,. Why do we have the methanide shift to the tertiary carbon and leave the carbon on the left, a secondary carbocation? I thought a tertiary carbocation was more stable than a secondary. Or is it because the electronegative oxygen better stabilizes the positive charge? Sorry if I am missing something here, I just want to make sure I understand why this happens.

[Archer]

Resonance stabilised carbocations are always more stable, the more resonance structures which can exist (i.e. the more atoms on which the cation can reside), the more stable it is. This is particularly evident in +ve ion Mass Spectrometry.

I was taught to imagine a "hot potato", if you try to hold that in one hand you can't hold it for very long. If you throw it from one hand to the other you can hold on to it for longer.

Hope this analogy helps!

[limonade]

It does help. Thank you

[spirochete]

I was taught to imagine a "hot potato", if you try to hold that in one hand you can't hold it for very long. If you throw it from one hand to the other you can hold on to it for longer.

This is a good way to remember that resonance is stabilizing, but not the best way to explain the concept of resonance. The positive charge is not jumping back and forth between oxygen and carbon. There is partial double bond character between oxygen and carbon that allows the two atoms to share the positive charge. I can't think of an accurate real life analogy at the moment for this stabilization.

[TheOrganic]

There is more into the rearrangement, than "resonance". Simply saying that the rearrangement from tertiary to secondary carbocation is due to resonance might be incomplete.

If we recall the rules for deciding relative stability of canonical structures and contribution to resonance hybrid from our classes on General Organic Chemistry, octet-complete structures tend to be extra-ordinarily stable.
Observe that if the positive charge resides on the secondary carbon - that has the oxygen attached to it, the hybrid represents closely an octet-complete structure.

Thus in the rearrangement of , although the initial carbocation is a resonance stabilized carbocation, a pinacol-pinacolone type rearrangement still occurs - due to octet-completion.

[Archer]

I can't think of an accurate real life analogy at the moment for this stabilization.

There are no real life analogies for advanced chemistry. But some simple analogies help students to visualise theories in simplistic terms. Once this understanding is in place they can go on to read books on frontier molecular orbital theory and broaden their knowledge of mechanistic chemistry. In this case I thought best not to confuse matters by over complicating the issue.