Pinacol-Pinacolone Rearrangement

[sleek whale]

Could anyone please provide the mechanism for the reaction when this reacts with H+ ?

[upper wave]

oh yeah forgot loss of H2O too

after step 2

like this i think

we want to create pinacolone with most stable carbocation being supported by OH and attacking with that OH's lone pair to make the double bond O

if red path is taken

We will get + on left side

which is good

stability wise

but there is no OH there now

so we try the other one (blue)

and after rearrangement (phenyl shift)

u get a stable carbocation with OH attached

now just shift the lone pair of O and ur done

[sleek whale]

why does only the phenyl shift happen and not maybe the benzene with OH or NO2 attached??

[upper wave]

u mean why doesnt the benzene with OH / NO2 do shift?

first the one with NO2 cant shift cuz its already attached to the carbocation. even if it could be, its -M. we wanna stabalize the charge by giving it e- but we instead take from it.

now the phenol shift is actually possible but since the conditions are acidic, it would prefer getting protonated than participate in that shift.

for more clarity revise alkene hydrolysis and carbocation stability

[sleek whale]

ohhhh okay, thank you

[sleek whale]

even if we went with the red path, the benzene with NO2 won't shift because if it did, the resulting carbocation would have lesser stability, so in any case, only the phenyl ring will shift.

got it, thank you

[upper wave]

yes

its just trial and error

look at most cases and check if it works

[sleek whale]

hmm

[sturdy fox]

<@&1227988359910260737>

Note for OP

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[sleek whale]

+solved @upper wave